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A three-dimensional spectral boundary element method has been employed to compute for the dynamics of the droplet motion driven by shear flow near a single solid substrate with a rough surface. The droplet size is comparable with the surface features of the substrate. This is a problem that has barely been explored but with applications in biomedical research and heat management. This work numerically investigated the influences of surface roughness features, such as the roughness amplitude and wavelength, on the droplet deformation and velocities. We observe that a greater amplitude or wavelength leads to larger variations in droplet velocity perpendicular to the substrate. The droplet velocity along the substrate increases when the amplitude is reduced or when the wavelength increases. The effects of capillary number and viscosity ratios have also been studied. The droplet deformation and its velocity increases as we increase the capillary number, while the viscosity ratio shows a non-monotonic influence on the droplet behavior. The predicted droplet behaviors, including deformation, velocities, and trajectories, can provide physical insight, help to understand the droplet behavior in microfluidic devices without a perfectly smooth surface, and contribute in the design and operation of those devices. [ABSTRACT FROM AUTHOR]

Continuous-wave lasers generated bubbles in microfluidic channels are proposed for applications such as needle-free jet injection due to their small size and affordable price of these lasers. However, water is transparent in the visible and near-IR regime, where the affordable diode lasers operate. Therefore, a dye is required for absorption, which is often unwanted in thermocavitation applications, such as vaccines or cosmetics. In this work, we explore a different mechanism of the absorption of optical energy. The microfluidic channel wall is partially covered with a thin gold layer, which absorbs light from a blue laser diode. This surface absorption is compared with the conventional volumetric absorption by a red dye. The results show that this surface absorption can be used to generate bubbles without the requirement of a dye. However, the generated bubbles are smaller and grow slower when compared to the dye-generated bubbles. Furthermore, heat dissipation in the glass channel walls affects the overall efficiency. Finally, degradation of the gold layer over time reduces the reproducibility and limits its lifetime. Further experiments and simulations are proposed to potentially solve these problems and optimize the bubble formation. Our findings can inform the design and operation of microfluidic devices used in phase transition experiments and other cavitation phenomena, such as jet injectors or liquid dispensing for bio-engineering. [ABSTRACT FROM AUTHOR]

The study of heat and mass transfer in a Hele-Shaw cell rotating around a perpendicular axis has various advanced technological applications. These include the design of microfluidic devices and continuous-flow chemical microreactors, to name a couple. In this setup configuration, the quasi-two-dimensional design allows for recording the density field using optical methods, and the rotation enables control of this field through spatially distributed inertial forces. As is known, in the limit of an infinitely thin layer, the Coriolis force vanishes within a standard mathematical model. However, experimental observations of fluid flow in a rotating Hele-Shaw cell indicate the opposite. In this paper, we show that the correct derivation of the equation of motion under the Hele-Shaw approximation leads to the appearance of a Boussinesq-type term for the Coriolis force. To study the effect of the Coriolis buoyancy, we consider the problem of fluid stability during the internal generation of a transfer component, which can be either the concentration of the dissolved substance or the temperature of the medium. The careful study of system dynamics involves linear stability analysis, weakly nonlinear analysis, and direct numerical simulation. The general properties of the disturbance spectrum are analyzed. The branching of solutions near the first bifurcation is studied using the technique of multiple time scales. A stationary convection is replaced by an oscillatory one under the action of the Coriolis force, as demonstrated by weakly nonlinear analysis. Finally, we investigate the nonlinear dynamics using direct numerical simulation. [ABSTRACT FROM AUTHOR]

This study introduces the first experimental analysis of shear cavitation in a microscale backward-facing step (BFS) configuration. It explores shear layer cavitation under various flow conditions in a microfluidic device with a depth of 60 μm and a step height of 400 μm. The BFS configuration, with its unique characteristics of upstream turbulence and post-reattachment pressure recovery, provides a controlled environment for studying shear-induced cavitation without the complexities of other microfluidic geometries. Experiments were conducted across four flow patterns: inception, developing, shedding, and intense shedding, by varying upstream pressure and the Reynolds number. The study highlights key differences between microscale and macroscale shear cavitation, such as the dominant role of surface forces on nuclei distribution, vapor formation, and distinct timescales for phenomena like shedding and shockwave propagation. It is hypothesized that vortex strength in the shear layer plays a significant role in cavity shedding during upstream shockwave propagation. Results indicate that increased pressure notably elevates the mean thickness, length, and intensity within the shear layer. Instantaneous data analysis identified two vortex modes (shedding and wake modes) at the reattachment zone, which significantly affect cavitation shedding frequency and downstream penetration. The wake mode, characterized by stronger and lower-frequency vortices, transports cavities deeper into the channel compared to the shedding mode. Additionally, vortex strength, proportional to the Reynolds number, affects condensation caused by shockwaves. The study confirms that nuclei concentration peaks in the latter half of the shear layer during cavitation inception, aligning with the peak void fraction region. [ABSTRACT FROM AUTHOR]

The present study focuses on the unsteady flow of a viscoelastic Maxwell nanofluid with couple stress in a circular microchannel under the combined action of periodic pressure and magnetic field. The Green's function method is applied to the unsteady Cauchy momentum equation to derive the velocity field. We strive to check out the analytical solutions of the current model by validating them with existing results. In addition, the effects of several dimensionless parameters (such as the couple stress parameter γ, the Deborah number De, and the dimensionless frequency ω) on the streaming potential and the electrokinetic energy conversion (EKEC) efficiency of the three waveforms (cosine, square, and triangular) are portrayed via graphical illustrations. Within the range of parameters chosen in this study, the results demonstrate that increasing the value of the Deborah number or decreasing the dimensionless frequency can effectively enhance the streaming potential. The velocity field and EKEC efficiency are improved with increasing couple stress parameters. Furthermore, our investigation is extended to compare the EKEC efficiency for square and triangular waveforms when the couple stress parameters are set to 20 and 40, respectively. The results also indicate that increasing the couple stress parameter significantly improves the EKEC efficiency, particularly in the context of the square waveform. These findings will provide valuable assistance in the design of periodic pressure-driven microfluidic devices. [ABSTRACT FROM AUTHOR]

In this study, a novel approach for enhancing the rapid spreading of free liquid film on lithium niobate (LiNbO3) substrate under megahertz (MHz) surface acoustic wave (SAW) excitation is presented by treating it with surfactants. Through the design of a specific interdigital transducer structure, it was discovered that exciting the SAW at a frequency of 32.3 MHz can achieve optimal spreading performance for water droplets on the surface of surfactant-treated LiNbO3 substrate. The maximum average velocity reaches 1.76 mm/s at position P2 = 1250 μm in the water film front, and the stable film spreading speed shows a 204.9% increase compared to the existing research. Simultaneously, through the investigation of the spreading experiment phenomenon of silicone oil and de-ionized water droplets at varying frequencies, we have discovered the dynamic mechanism of "reverse phase" propagation in liquid film for the first time. This entails that the advancing edge of the wetting film demonstrates a spreading motion law that is opposite to the traditional spreading phenomena, with the spreading velocity in the central exceeding that on both sides. Our research demonstrates that this microfluidic device developed by SAWs enhances the spreading efficiency of the free films, enabling rapid expansion of the target liquid to form a high-surface area film layer. This advancement holds promise for overcoming the limitations of low sensitivity and short response time in the field of rapid pathological diagnosis in contemporary medicine. [ABSTRACT FROM AUTHOR]

This study provides a critical understanding of controlling solute distribution in microfluidic systems by examining the effects of symmetric and asymmetric zeta potentials under magnetohydrodynamic (MHD) pulsatile electroosmotic flow. These findings are vital for enhancing the efficiency of microfluidic devices used in lab-on-a-chip applications. The aim of this study is to explore the modulation of solute transport in MHD pulsatile electroosmotic microchannel flow, focusing on both symmetric and asymmetric wall zeta potentials. Using the Debye–Hückel approximation, the Poisson–Boltzmann equation is obtained. Subsequently, the convection–diffusion equation is solved using the velocity profile, with Aris's method of moments to derive the moment equations. These equations are then solved using a finite difference scheme. The mean concentration is calculated employing Hermite polynomials. We examined the effects of key parameters such as the electroosmotic parameter (κ), the Hartmann number (Ha), and zeta potentials on the dispersion coefficient (DT), mean concentration distribution (Cm), skewness, and kurtosis. We consider three situations: symmetric ( ζ 1 = ζ 2 ), partially asymmetric ( ζ 1 = 1.0 , ζ 2 = 0.0), and fully asymmetric ( ζ 1 = 1.0 , ζ 2 = − 1.0) zeta potentials. Our results reveal that asymmetric zeta potentials lead to faster dispersion, with DT decreasing with increasing κ for symmetric potentials and increasing for asymmetric ones. As the Hartmann number increases, dispersion decreases for both symmetric and asymmetric zeta potentials while the peak of mean concentration rises. The mean concentration profile exhibits Gaussian behavior at both small and large times, with anomalous behavior in the intermediate region. For symmetric zeta potentials, Gaussianity is observed at t = 0.001 in the diffusive regime and at t = 10.0 in Taylor's regime, while for asymmetric potentials, Gaussianity occurs at t = 0.03 and t = 3.0, indicating that large-time Gaussian behavior is approximately 3.33 times faster and dispersion becomes transient for asymmetric potentials. [ABSTRACT FROM AUTHOR]

A theoretical and numerical investigation of electrokinetic flow is performed in a nanochannel with the charged symmetric corrugated surfaces. The perturbation and numerical solutions of electrokinetic flow variables are given, and the effects of corrugation geometry, such as wave amplitude and wave number, on the electrokinetic flow characteristics are systematically examined. The results show that the electrokinetic flow recirculation may occur easily at wave crest due to the electroviscous effect. The velocity profile is strongly dependent on wave number, but the maximum or minimum velocity may be insusceptible to wave number. Furthermore, the distributions of streaming potential and energy conversion efficiency are also investigated. We find that, for some special geometry of corrugations, the streaming current and conversion efficiency obtained from the present corrugated nanochannel are higher than that from the smooth nanochannel. Specially, when the dimensionless wave number is 0.5/π, the magnitude of streaming potential is enhanced about 29% at δ = 0.5 and the peak value of conversion efficiency is enhanced about 2% at δ = 0.1. We believe that the optimal corrugation geometry parameters can be of benefit in designing a microfluidic device with higher streaming current and conversion efficiency. [ABSTRACT FROM AUTHOR]

In this article, the role of slip-dependent (SD) zeta potential in the hydrodynamic characteristics of mixed electromagnetohydrodynamic (EMHD) and electroosmotic driven flow of couple stress fluid within a rotating microfluidic channel is theoretically investigated. This work is the first to analyze the hydrodynamic characteristics of slip-independent (SI) and slip-dependent (SD) zeta potentials in a rotating microchannel including a detailed analysis of Ekmann spirals in the microchannel. Ekmann spirals show the effect of rotational flow caused by different parameters, particularly, the slip parameter and the Hartmann number being the most significant ones. Ekmann plot variations, observed under both SI and SD model cases, show a significant effect on rotating flow dynamics. The effect of pertinent parameters on the rotational flow velocity, centerline velocity, and volumetric flow rate is graphically depicted. The findings of this research reveal that the SD zeta potential plays a crucial role in determining the rotating flow velocity and volume flow transport. The normalized transverse centerline in the magnitude flow velocity increases with the couple stress parameter and decreases with the slip parameter for both SI and SD model cases. Notably, the magnitude of the normalized transverse flow rate increases with rotational parameter values. In contrast, it decreases with an increase in the slip parameter under the SD model case. The outcomes of this study can be directly used in applications like transportation of biofluid models in Lab-On-a-Chip (LOC) devices and microfluidic systems under certain conditions. [ABSTRACT FROM AUTHOR]

As an advanced flow-drive technology, micro-electrohydrodynamic (EHD) conduction pumping has become a new prospect in many micro-scale industrial applications, including lab-on-chip devices and microfluidic cooling systems. Under micro-scale conditions, the effect of the electric double layer (EDL) has to be considered. Zeta potential is an adjustable and measurable experimental value and has been proposed to estimate the strength of EDL in simulations. In this work, the effect of zeta potential on the performance of micro-EHD conduction pumping has been numerically investigated. A method to estimate the surface charge density without the Debye–Hückel approximation was introduced. A two-dimensional flush electrode configuration with a typical size of 50 μm was considered. The coupled series of governing equations was implemented in the finite-volume framework of OpenFOAM® and solved based on the PIMPLE algorithm. The results show that zeta potential can enhance the asymmetry of the electric field and change the distribution of the Coulomb force. For the construction considered in this work, negative zeta potential can reduce the size and strength of the vortex in the flow field and improve the pump's net flow rate and static pressure. In contrast, positive zeta potential has the opposite effect. Maximum performance enhancement up to 94.8%–115.1% has been observed for different electrode length ratios within the parameters studied in this paper. The results guide the zeta potential optimization of micro-EHD conduction pumping. By matching the pairs of solid and liquid materials, researchers can adjust zeta potential to an optimal value, thereby improving the pump performance. [ABSTRACT FROM AUTHOR]

Thermo-electro-convective modes induced by a dielectrophoretic force in a differentially heated horizontal rectangular cavity have been investigated using direct numerical simulations in stable and unstable thermal stratifications. The variation of the electric tension applied to the plates of the cavity leads to multiple modes under microgravity as well as under both stable and unstable stratifications in terrestrial conditions. An effective electric Rayleigh number incorporating the effects of both the electric potential and the thermal stratification has been introduced in order to analyze the heat transfer induced by thermoelectric convection, leading to a unique curve of the variation of the Nusselt number with the effective electric Rayleigh number. The results can be used for modeling the heat transfer in microfluidic devices where the Archimedean buoyancy is very weak or to simulate natural convection at any planet using experiments performed on the Earth. [ABSTRACT FROM AUTHOR]

Many computational studies on hotspot microfluidic cooling devices found in the literature rely on simplified assumptions and conventions that do not capture the full complexity of the conjugate thermal problem, such as constant thermophysical fluid properties, radiation, and free air convection on the external walls. These assumptions are generally applied to typical microfluidic devices with a large number of microchannels and operating at Reynolds numbers between 100 and 1000. A one microchannel chip is a suitable starting point to analyze more systematically the implications of these assumptions, in particular at lower Reynolds numbers. Although it is a simpler system, it has been studied experimentally and numerically as a basic block of a thermal microfluidic device. In this work, we analyze the modeling of the overall heat transfer from a hotspot to a microfluidic heat sink, focusing on the effect of the different thermal transfer mechanisms (conduction, convection, and radiation), and temperature-dependent thermophysical properties of the fluid and the chip material. The study is developed as a function of the pressure difference applied to the system based on simulations performed using a finite volume method. Analyzing and comparing the different contributions to the energy losses, this work provides a critical discussion of the usually considered approximations, to make a reliable modeling of the overall thermal performance of a single rectangular straight channel embedded in a polydimethylsiloxane microfluidic chip. [ABSTRACT FROM AUTHOR]

In the advancement of paper-based microfluidic devices, it is reported that the two-ply channel transports fluid noticeably faster than traditional single-ply channels. In this work, the capillary flows in two-ply channels, consisting of a gap of width w between two porous sheets with porosity ε and thickness d, are investigated through many-body dissipative particle dynamics simulations. The advancing meniscus varies with position, characterized by the penetration lengths in the gap (Lg), the porous sheet (Lp), and the maximum value (Lmax). Lmax is always located within the porous sheet but near the gap. The time evolution of the penetration lengths can be described by Washburn's expression, L2 = (S)t, and the imbibition rates Sp, Sg, and Smax depend on ε, d, and w, differing from each other. Two distinct imbibition characteristics are identified: Sg > Sp for low porosities and Sg < Sp for high porosities. Both Sg and Sp decrease with d but increase with w. As ε increases, a minimum of Smax occurs due to the synergistic competition between Sp and Sg. Compared to the single-ply channel, which consists of a single porous sheet, the imbibition rate of the two-ply channel is significantly enhanced by at least four times due to side-imbibition from the gap (acting as a reservoir) toward the porous sheet. [ABSTRACT FROM AUTHOR]

The electroviscous effects are relevant in controlling and manipulating the fluid, thermal, and mass transport microfluidic processes. The existing research has mainly focused on the fixed contraction ratio ( d c , i.e., the area ratio of contraction to expansion) concerning the widely used contraction–expansion geometrical arrangement. This study has explored the influence of the contraction ratio ( d c ) on the electroviscous flow of electrolyte liquids through the charged non-uniform microfluidic device. The numerical solution of the mathematical model (Poisson's, Nernst–Planck, and Navier–Stokes equations) using a finite element method yields the local flow fields. In general, the contraction ratio significantly affects the hydrodynamic characteristics of microfluidic devices. The total electrical potential and pressure drop maximally change by 1785% (from −0.2118 to −3.9929) and 2300% (from −0.0450 to −1.0815), respectively, as the contraction ratio ( d c ) varies from 1 to 0.25. Furthermore, an electroviscous correction factor (Y, i.e., the ratio of apparent to physical viscosity) maximally enhances by 11.24% (at K = 8, S = 16 for 0.25 ≤ d c ≤ 1), 46.62% (at S = 16, d c = 0.75 for 20 ≥ K ≥ 2), 22.89% (at K = 2, d c = 0.5 for 4 ≤ S ≤ 16), and 46.99% (at K = 2, d c = 0.75 for 0 ≤ S ≤ 16). Thus, the electroviscous effect is obtained maximum at d c = 0.75 for the considered ranges of conditions. Finally, a pseudo-analytical model has been developed for a charged microfluidic device with variable contraction size (0.25 ≤ d c ≤ 1), based on the Hagen–Poiseuille flow in the uniform slit, which calculated the pressure drop within ±3% of the numerical results. The present numerical results may provide valuable guidelines for the performance optimization and design of reliable and essential microfluidic devices. [ABSTRACT FROM AUTHOR]

This study addresses the critical need to enhance mixing quality and cost efficiency in electroosmotic micromixers, crucial for various applications, such as chemical synthesis, medical diagnostics, and biotechnology, utilizing the precision of microfluidic devices. The intricate dynamics of time-dependent electroosmotic vortices induced by microelectrodes are investigated, exploring the nonlinear physics principles driving mixing enhancement. Specifically, an examination is made of how nonlinear phenomena, such as convective flow instabilities, chaotic advection, and nonlinear interactions between fluid flow and channel geometry, contribute to observed improvements in mixing performance. Through comprehensive numerical simulations employing finite element-based solvers, the impact of relevant parameters, such as voltage amplitude (V0), frequency (f), Reynolds number (Re), and Debye parameter (k), on mixing performance is systematically analyzed. Findings reveal that optimizing these parameters, coupled with the strategic design of micromixers featuring offset inlets and outlets, leads to a remarkable mixing quality of 98.44%. Furthermore, a methodology is proposed for selecting the optimal micromixer configuration (MM1), balancing mixing quality, and cost efficiency. This study advances the understanding of electroosmotic micromixers and provides practical guidelines for optimizing microfluidic device performance in diverse applications. [ABSTRACT FROM AUTHOR]

In this work, we develop a unique and efficient metamaterial device known as the "hydrodynamic evener," which can stabilize a flow field rapidly when a transition through channels with constrictions or expansions occurs. The hydrodynamic metadevice is designed from the theory of scattering cancelation for fluid flow in a porous medium. Its precise theoretical formulation furnishes it with an almost flawless capability to guarantee uniform flow, and thus we named it as the hydrodynamic evener. This hydrodynamic evener opens up new avenues for fluid manipulation and control across numerous industrial and scientific domains, including enhancing the design of microfluidic reactors and optimization of fluid flow in microfluidic devices and the structural design of various industrial equipment. [ABSTRACT FROM AUTHOR]

Surface wettability influences the droplet impact characteristics, especially for a droplet impacting with low inertia. The present work reports an experimental investigation of droplet impact on homogeneous and heterogeneous wettability surfaces for different Weber numbers. Droplet impact characteristics on surfaces with three homogeneous surface wettabilities, i.e., hydrophilic, hydrophobic, and superhydrophobic, and two heterogeneous surface wettabilities, i.e., hydrophilic–hydrophobic and hydrophilic–superhydrophobic, have been analyzed. The symmetric deposition, spreading, and recoiling on homogeneous surfaces are affected by the surface wettability gradient across the droplet on heterogeneous surfaces resulting in asymmetric behavior. Furthermore, hybrid wettability surfaces suppress the partial rebound, complete rebound, and complete rebound with droplet breakup observed in the homogeneous hydrophobic and superhydrophobic surfaces. The initial inertia force of the droplet significantly affects the asymmetric and droplet migration behavior. The average recoiling velocity of the droplet increases with the inertia of the droplet. The rate of increase in droplet migration is maximum for a Weber number of 12 for both surfaces with hybrid wettability. The analysis of asymmetric spreading and migration of impacting droplets on heterogeneous surfaces is important in enormous applications, such as microfluidic devices, self-transport of liquid, and water harvesting. [ABSTRACT FROM AUTHOR]

To precisely control the size of droplets is of great importance for the applications of the droplet microfluidics. In a flow-focusing microdroplet generator, the pressure-driven microfluidic device is designed to control the flow rates of the fluids. For a specific geometry of the flow-focusing microchannel, a mathematical model of droplet formation is established, and the nonlinear relation between the droplet length and the driven-pressure ratio can be described by our model. For pressure-driven microfluidic flows, the nonlinear relation between the droplet length and the driving-pressure ratio is measured experimentally in the flow-focusing microchannel. Particularly, by using the closed-loop control method of droplet generation, good agreements are shown between the measured size of droplets and the predicted size of the droplets. As a result, the control precision of the droplet size can be increased drastically by the closed-loop control method of droplet generation. Consequently, monodisperse droplets of extremely small size can be produced in the flow-focusing microdroplet generator. [ABSTRACT FROM AUTHOR]

The current work focuses on the basic principle of voltage-induced electrowetting and relevant movement of the microfluidic droplets. The prototype of microfluidic devices are fabricated on the Teflon-on-flexible substrate. Three different liquid droplets, namely, the de-ionized (DI) water, sucrose (aq.) solution, and eugenol, have been studied for such purpose within the voltage range of 1–16 V. Electrowetting and subsequent changes in contact angle are extensively investigated with the modification of "work of adhesion" and "work of cohesion" upon application of external voltage. The liquid droplet is positioned on the dielectric-hydrophobic layer which also separates it from the metal electrodes. Eugenol exhibits more susceptibility to electrowetting compared to sucrose solution and DI water. Consequently, sucrose (aq.) solution and DI water show comparatively more droplet displacement. The "work of spreading" for the liquids under test on Teflon surface is obtained. The spreading of eugenol starts at relatively low voltages than sucrose (aq.) solution and DI water. Eugenol follows the Young–Lippmann equation, i.e., linear relation between {cos(θv) − cos(θ0)} with voltage2 (V2); however, sucrose (aq.) solution and DI water deviate from such nature. Here, θ0 and θv are the initial and voltage modified contact angles, respectively. Thus, the current study provides an accurate approach to analyze the interaction of solid–liquid surfaces and its consequent effect upon application of external voltages. [ABSTRACT FROM AUTHOR]

Droplet manipulation on a substrate by electrical signals is instrumental to the automation and miniaturization of labor-intensive assays in life science and chemistry. Current techniques are primarily based on either electrowetting or a more recent ionic-surfactant-mediated electro-dewetting effect. Here, we report that the two effects can occur simultaneously on the same substrate. Using a dope silicon substrate and an aqueous droplet with a cationic surfactant, the surface exhibits dewetting at positive biases and wetting at negative. Such a polarity-dependent wetting–dewetting transition enables a more significant wettability change (>60° contact angle change between ±3 V), which preserves after multiple wetting–dewetting cycles. We also find that the transition does not experience contact angle hysteresis that sole electrowetting commonly suffers from. Benefitting from these features, we experimentally show that droplet manipulation on a digital microfluidic device is more efficient and robust using this joint mechanism. [ABSTRACT FROM AUTHOR]

Accuracy and temporal resolution of flow meters are often unacceptable below the microliter per minute scale, limiting their ability to evaluate the real-time performance of many microfluidic devices. For conventional flow meters, this problem arises from uncertainties that depend on physical effects, such as evaporation, whose relative impacts scale inversely with flow rate. More advanced techniques that can measure nanoliter per minute flows are often not dynamic and require specialized equipment. Herein, we report on new experimental and theoretical results that overcome both limitations using an optofluidic flow meter. Previously, we showed that this device can measure flow rates as low as 1 nl/min with roughly 5% relative uncertainty by leveraging the photobleaching rate of a fluorescent dye. We now extend that work by determining the flow meter's relaxation time over a wide range of flow rates and incident irradiances. Using a simplified analytical model, we deduce that this time constant arises from the interplay between the photobleaching rate and transit time of the dye through the optical interrogation region. This motivates us to consider a more general model of the device, which, surprisingly, implies that all time constants are related by a simple scaling relationship depending only on the flow rate and optical irradiance. We experimentally validate this relationship to within 5% uncertainty down to 1 nl/min. Additionally, we measure a relaxation time of the flow meter on the order of 100 ms for 1 nl/min flows, demonstrating the ability to make dynamic measurements of small flows with unprecedented accuracy. [ABSTRACT FROM AUTHOR]

We explore the dynamics of droplet propagation and subsequent disintegration in a symmetric bifurcating Y-microchannel by varying the wettability characteristics of one of the daughter channels while maintaining the wettability of the other constant. The temporal evolution of the droplet is numerically investigated using the phase-field method. Based on the neck-width evolution, the droplet bifurcation phenomenon has been divided into three separate stages, namely, squeezing, transition, and pinch-off. During the squeezing stage, the rate of change of neck width increases as the wettability angle decreases, while an opposite trend is observed at the pinch-off stage, leading to almost identical breakup time for the droplet regardless of the wettability angle. We identify pertinent regimes of droplet breakup, such as symmetric breakup, asymmetric breakup, no-breakup upper channel, no-breakup lower channel, and spreading regime, over wide ranges of capillary numbers (Ca) and viscosity ratio ( μ r ). Our study indicates that an increase in the relative influence of viscous force (high Ca) reduces the droplet's wettability effect. The same pattern is obtained when the viscosity of the droplet is increased in relation to the viscosity of the carrier fluid. In contrast, for low Ca flows, the relatively strong interfacial tension favors the wettability characteristics of the surface, resulting in a dominance of non-breakup regimes. The regime plots proposed in this paper depict the roles of Ca and μ r on various breakup regimes in detail. Such regime diagrams may emerge as fundamental design basis of microfluidic devices in diverse applications, such as biopharmaceuticals, microreactors, and food processing. [ABSTRACT FROM AUTHOR]

The manipulation and control of microparticles through non-intrusive methods is pivotal in biomedical applications such as cell sorting and cell focusing. Although several experimental and numerical studies have been dedicated to single suspended particles or clusters of rigid spheres, analogous cases with deformable particles have not been as thoroughly studied, especially when the suspending liquid exhibits relevant viscoelastic properties. With the goal of expanding the current knowledge concerning these systems, we perform a computational study on the hydrodynamic interactions between two neutrally buoyant initially spherical elastic particles suspended in Newtonian and shear-thinning viscoelastic matrices subjected to pressure-driven flow in a cylindrical microchannel. Due to the well-known focusing mechanism induced by both particle deformability and fluid elasticity, the two particles are assumed to flow at the axis of the tube. The rheological behavior of the viscoelastic continuous phase is modeled via the Giesekus constitutive equation, whereas the particles are assumed to behave as neo-Hookean solids. The problem is tackled by employing a mixed finite-element method. The effects of particle deformability, fluid elasticity, confinement ratio, and initial interparticle separation distance on the pair dynamics are investigated. The main outcome of this study is a quantitative indication of the flow conditions and spatial configurations (initial distances) under which the particles will spontaneously form organized structures. Such results are helpful to design efficient microfluidic devices with the aim of promoting particle ordering. [ABSTRACT FROM AUTHOR]

In this paper, the flow and particle trajectories, induced by standing surface acoustic waves (SSAWs) in a poly-dimethylsiloxane microchannel, are investigated by establishing a two-dimensional cross-sectional model with the finite element method and improved boundary conditions. Extensive parametric studies are conducted regarding the channel height, ranging from 0.2 to 4.0 times the spacing of the repetitive vertical interference pattern, to investigate its influences on the flow field and microparticle aggregation. The first-order flow field is found to be related to the channel height, exhibiting a periodic spatial distribution and oscillatory variation in its amplitude as the height changes. We theoretically analyze the propagation mechanism of the acoustic waves in the vertical direction and thus determine the periodicity of the wave interference pattern. Furthermore, we find that the speed of the particle aggregation is a function of the channel height, so the channel height can be optimized to maximize the strength of the first-order flow field and thus minimize the time of particle aggregation. The optimum heights can reduce the aggregation time by up to 76%. In addition, the acoustophoretic motions of microparticles exhibit a spatially dependent pattern when the channel height becomes larger than a quarter of the wavelength of the SAW, which can be explained by the change in the ratio between the radiation force and the streaming drag force from position to position. Our findings provide guidelines to the design and optimization of SSAW-based acoustofluidic devices. [ABSTRACT FROM AUTHOR]

Micromixers play a crucial role in mixing different fluids within microfluidic systems. Therefore, it is essential to analyze parameters, such as dimensional characteristics, mixing length, micromixer efficiency, and the mixing process, to enhance their performance. In this study, we examine various T-shaped micromixer designs, including triangular, rectangular, and trapezius configurations, to evaluate their mixing performance and compare them with a corresponding circular micromixer. Additionally, we investigate the effects of obstacles, varying their angles and distances, in the circular micromixer to determine trends in mixing improvement across cases. The micromixers have minimal dimensions, resulting in laminar flow. By comparing the outcomes of the proposed cases with those without obstacles, we find that the triangular micromixer exhibits the highest mixing performance with 8.3% improvement with respect to the circular case. Furthermore, while the rectangular case initially displayed the weakest performance at lower Reynolds numbers, a discernible enhancement was observed as Reynolds numbers increased. This improvement was attributed to the emergence of vortices at Re = 20. The performance showed a substantial increase, reaching a coefficient of 0.98 at Re = 40, a value closely approaching that of the triangular case. Among the three obstacles, one obstacle is varied at four different angles (0°, 60°, 90°, and 120°), while the other two obstacles remain fixed at distances of 150 and 200 μm. In cases involving obstacles, a noteworthy enhancement was evident when compared to cases without obstacles. In these cases, the introduction of obstacles resulted in a remarkable 34% improvement in the mixing index compared to obstacle-free scenarios. This improvement can be attributed to the observed flow behavior, where the formation of vortices, even at low Reynolds numbers, emerges as a key factor contributing to this enhancement. In addition, we assess the mixing enhancement to identify the most efficient arrangement of obstacles. The results indicate angles of 90° and 120° are more effective than others in improving mixing proficiency. [ABSTRACT FROM AUTHOR]

This study presents droplet dynamics in a rough capillary channel. Prior studies investigating the effect of roughness on fluid flow have mainly considered a continuous phase whose behavior is different from a discontinuous phase, i.e., an oil slug. To explore the dynamic behavior of droplet motion across a rough channel, a direct numerical simulation of in a three-dimensional channel is performed. Three models have been considered: model A had a rough surface only on the bottom walls, model B on both the bottom and top walls, and model C on all walls. The results show that in contrast with common observations, roughness promotes droplet mobility in comparison with smooth walls. The presence of roughness results to an additional energy required to move the droplet, and the degree of confinement increases with the roughness; thus, the difficult of mobilization increases with the increase in roughness. Different roughness parameter effects have been investigated. The results have shown that the critical pressure increases with the increase in the pillar's height and decreases with the pillars spacing. The offset leads to a decrease in flow resistance for larger contact angles. We noted also that it is more difficult to mobilize a discontinuous phase in a neutral-wet surface condition. Furthermore, discontinuous pillars in the lateral direction led to much higher resistance. Through our comprehensive numerical study, we provide valuable insights into the impact of roughness in capillary channels. These findings can be used as guidelines for designing droplet flow on complex and rough surfaces, such as microfluidic devices, and hold significant relevance in the optimization of droplet control strategies in enhanced oil recovery methods. [ABSTRACT FROM AUTHOR]

Over the past few decades, there has been a significant increase in the use of paper-based microfluidic devices in various fields, including environmental monitoring, food safety analysis, and medical diagnostics. As a result, flow through paper-based substrates has gained much attention in the research community. Liquid flows through a paper substrate due to the inherent capillary suction pressure. In order to predict the flow through a paper substrate, we used macro- and microscopic methodologies to construct an analytical and numerical model. We have considered the effect of different factors, e.g., roughness, swelling, dynamic contact angle, and evaporation simultaneously to predict liquid wicking. A modified capillary radius technique is used to incorporate the effects of roughness and swelling into the numerical model, while a sink factor in Darcy's equation is used to model the evaporation. Experiments are performed to validate the developed models, and it is found that both models are in good agreement with the experiments, with a maximum error of 5%. The proposed analytical and numerical models can be used to forecast the capillary rise in a paper-based substrate, which has implications for paper-based microfluidic devices. [ABSTRACT FROM AUTHOR]

The oscillation characteristics of a bubble in a confined space have important implications for various applications, including liquid pumping and mixing and particle conveyance in microfluidic devices. In this study, analytical solution with second-order accuracy and numerical solution are derived for the free oscillation of a single bubble in a narrow gap between parallel plates, and the applicability to dimensionless initial values of the analytical solutions is clarified. Moreover, the free-oscillation characteristics of the bubble within the gap are explored and described and are compared to those of a bubble in an infinite liquid. The primary conclusions are as follows: (1) The inherent nature of bubble oscillation in a gap is significantly influenced by the bubble equilibrium radius, and the oscillation amplitude of different orders of the analytical solution is significantly influenced by the dimensionless initial radius. (2) The difference between the natural frequency and acoustic damping constant during bubble oscillation in a gap and those in an infinite liquid decreases with increasing equilibrium radius, and the value of the difference is not less than 50%. (3) Within the gap, the bubble radius, wall velocity, and wall acceleration of a bubble in a narrow gap predicted by the bubble equation dramatically differ from those of a bubble in an infinite liquid, with the differences increasing with the dimensionless initial radius, where the values of the differences in the acceleration can be as high as the order of 10 4 %. [ABSTRACT FROM AUTHOR]

Viscoelastic focusing has emerged as a promising method for label-free and passive manipulation of micro and nanoscale bioparticles. However, the design of microfluidic devices for viscoelastic particle focusing requires a thorough comprehensive understanding of the flow condition and operational parameters that lead to the desired behavior of microparticles. While recent advancements have been made, viscoelastic focusing is not fully understood, particularly in straight microchannels with rectangular cross sections. In this work, we delve into inertial, elastic, and viscoelastic focusing of biological cells in rectangular cross-section microchannels. By systematically varying degrees of fluid elasticity and inertia, we investigate the underlying mechanisms behind cell focusing. Our approach involves injecting cells into devices with a fixed, non-unity aspect ratio and capturing their images from two orientations, enabling the extrapolation of cross-sectional equilibrium positions from two dimensional (2D) projections. We characterized the changes in hydrodynamic focusing behaviors of cells based on factors, such as cell size, flow rate, and fluid characteristics. These findings provide insights into the flow characteristics driving changes in equilibrium positions. Furthermore, they indicate that viscoelastic focusing can enhance the detection accuracy in flow cytometry and the sorting resolution for size-based particle sorting applications. By contributing to the advancement of understanding viscoelastic focusing in rectangular microchannels, this work provides valuable insight and design guidelines for the development of devices that harness viscoelastic focusing. The knowledge gained from this study can aid in the advancement of viscoelastic particle manipulation technique and their application in various fields. [ABSTRACT FROM AUTHOR]

The physics of drop motion in microchannels is fundamental to provide insights when designing applications of drop-based microfluidics. In this paper, we develop a boundary-integral method to simulate the motion of drops in microchannels of finite depth with flat walls and fixed depth but otherwise arbitrary geometries. To reduce computational time, we use a moving frame that follows the droplet throughout its motion. We provide a full description of the method, including our channel-meshing algorithm, which is a combination of Monte Carlo techniques and Delaunay triangulation, and compare our results to infinite-depth simulations. For regular geometries of uniform cross section, the infinite-depth limit is approached slowly with increasing depth, though we show much faster convergence by scaling with maximum vs average velocities. For non-regular channel geometries, features such as different branch heights can affect drop partitioning, breaking the symmetric behavior usually observed in regular geometries. Moreover, non-regular geometries also present challenges when comparing the results for deep and infinite-depth channels. To probe inertial effects on drop motion, the full Navier–Stokes equations are first solved for the entire channel, and the tabulated solution is then used as a boundary condition at the moving-frame surface for the Stokes flow inside the moving frame. For moderate Reynolds numbers up to Re = 5, inertial effects on the undisturbed flow are small even for more complex geometries, suggesting that inertial contributions in this range are likely small. This work provides an important tool for the design and analysis of three-dimensional droplet-based microfluidic devices. [ABSTRACT FROM AUTHOR]

Machine learning has recently been introduced in the context of droplet microfluidics to simplify the process of droplet formation, which is usually controlled by a variety of parameters. However, the studies introduced so far have mainly focused on droplet size control using water and mineral oil in microfluidic devices fabricated using soft lithography or rapid prototyping. This approach negated the applicability of machine learning results to other types of fluids more relevant to biomedical applications, while also preventing users that do not have access to microfluidic fabrication facilities to take advantage of previous findings. There are a number of different algorithms that could be used as part of a data driven approach, and no clear comparison has been previously offered among multiple machine learning architectures with respect to the predictions of flow rate values and generation rate. We here employed machine learning to predict the experimental parameters required for droplet generation in three commercialized microfluidic flow-focusing devices using phosphate buffer saline and biocompatible fluorinated oil as dispersed and continuous liquid phases, respectively. We compared three different machine learning architectures and established the one leading to more accurate predictions. We also compared the predictions with a new set of experiments performed at a different day to account for experimental variability. Finally, we provided a proof of concept related to algae encapsulation and designed a simple app that can be used to generate accurate predictions for a given droplet size and generation rate across the three commercial devices. [ABSTRACT FROM AUTHOR]

High-precision and high-purity acquisition of tumor cells from whole blood is vital for early disease detection and diagnosis. Here, we investigated a cascaded inertial and acoustic microfluidic device for sheathless and label-free separation of circulating tumor cells (CTCs) from the blood through numerical methods. We introduced a spiral microfluidics channel in the first stage (1st) for cell focusing and rough sorting to improve chip integration and reduce the dependence on sheath flow and extra syringe pumps. In the 1st, we simulated the spiral microfluidic with a rectangular cross section to determine the key parameters affecting the migration kinetics of blood cells and tumor cells. Under the influence of Dean drag force and inertial lift force, blood cells migrate toward the inner side of the channel, while CTCs flow out close to the outer side. A flow rate of 400 μl/min was optimized for the operating flow rate. To improve and further enhance the 1st sorting efficiency and purity, we introduced tilted angle standing surface acoustic wave (SSAW) in the second stage (2st). Based on the parametric study, the SSAW with 33.3 MHz, tilted angle with 5°, and acoustic pressure amplitude with 0.7 MPa was selected as the operating parameter. The product of the 1st is used as input for the 2st acoustofluidic unit, enabling a more accurate separation process to obtain CTCs. The simulation results show that the inertial microfluidic units arranged in the first stage help to improve throughput and assist in 2st acoustofluidic separation, and the cascaded chip has accomplished a separation performance of nearly 100% in terms of purity and efficiency. [ABSTRACT FROM AUTHOR]

Microfluidic devices, which enable precise control and manipulation of fluids at the microscale, have revolutionized various fields, including chemical synthesis and space technology. A comprehensive understanding of fluid behavior under diverse conditions, particularly in microgravity, is essential for optimizing the design and performance of these devices. This paper aims to investigate the effects of discontinuous wettability on droplet breakup structures under microgravity conditions using a microchannel wall. The approach we adopt is underpinned by the volume-of-fluid methodology, an efficient technique renowned for its accurate resolution of the fluid interface in a two-phase flow. Furthermore, a modified dynamic contact angle model is employed to precisely predict the shape of the droplet interface at and near the wall. Our comprehensive model considers influential parameters such as slug length and droplet generation frequency, thereby providing crucial insights into their impact on the two-phase interface velocity. Validated against existing literature data, our model explores the impact of various configurations of discontinuous wettability on breakup morphology. Our findings highlight the significance of employing a dynamic contact angle methodology for making accurate predictions of droplet shape, which is influenced by the wall contact angle. Emphasis is placed particularly on the effects of slug length and droplet generation frequency. Notably, we demonstrate that the use of a hybrid surface at the junction section allows for precise control over the shape and size of the daughter droplets, contrasting with the symmetrical division observed on uniformly hydrophilic or superhydrophobic surfaces. This study contributes valuable insights into the complex dynamics of the droplet breakup process, which has profound implications for the design and optimization of microfluidic devices operating under microgravity conditions. Such insights are further poised to augment applications in space exploration, microreactors, and more. [ABSTRACT FROM AUTHOR]

A Hele–Shaw quadratic flow in the form of a six-port microfluidic device is employed to study the deformation of a single channel-spanning or Hele–Shaw Newtonian drop suspended in a Newtonian medium. An initially circular drop in a quadratic flow deforms into a regular triarcle, i.e., a rounded-corner triangle. Theoretically, the deformation is calculated in the limit of small capillary number, which is defined as C a = C μ a 4 / (4 γ b 2) , where C is the quadratic flow rate, μ is the suspending fluid viscosity, a is the drop radius, γ is the interfacial tension, and b is the channel depth. The theory matches reasonably well with the experiment for small capillary numbers. This is the new way of deforming drops on a Hele–Shaw quadratic platform experimentally and may lead to measurements such as complex interfacial properties and breakup. [ABSTRACT FROM AUTHOR]

We conducted laboratory experiments and numerical simulations to investigate the formation and evolution of drops formed by partially miscible two-phase fluid, n-butanol (the continuous phase) and water (the dispersed phase), in a flow focusing microfluidic system. We carefully calibrated the numerical model to obtain good agreement with experimental data in drop velocity and mass transfer, demonstrating the model's capability to capture realistic drop dynamics. Our detailed investigation of the numerical results allowed us to determine the mechanism of drop formation and obtain a relevant criterion in terms of the disperse-to-continuous flow ratio beyond which the tubing patterns would occur. Additionally, we found that the mass transfer between the two phases, specifically at the drop interface, strongly depends on the local distribution of dissolved concentration of the dispersed phase. To enhance mass transfer, we conducted numerical simulations on alternating curved channels, which allows for the lateral advection of the dispersed phase concentration in the continuous phase at the curved section. We found that this lateral movement enhances mass transfer at the drop interface. Through detailed investigation of numerical results, we addressed mechanisms of mass transfer enhancement in the curved channel. Overall, our findings provide insight into the mechanisms of drop formation and mass transfer in partially miscible two-phase fluids in microfluidic systems, which could be useful in designing and optimizing such systems for various applications. [ABSTRACT FROM AUTHOR]

Droplet generation in microscale has gained enormous attention in recent years especially in the pharmaceutical industry due to their application in targeted drug delivery into droplets. In most of these applications, monodispersity and uniformity of droplets are essential. Microfluidic devices can generate droplets at high throughput, enabling thousands of droplet compound encapsulation per second. The monodispersity of the droplets is ensured hydrodynamically through the dripping regime and their uniformity is controlled by active and passive microflow control methods. Here, we study numerically a microfluidic chip that uses a non-embedded co-flow-focusing geometry, so that the droplet generation throughput can take advantage of the flow-focusing devices while the non-embedded co-flow geometry forecloses the surfactant addition necessity. The continuous and dispersed phases were light mineral oil and water, respectively. We investigated the formation of droplets and studied how changing the external diameter of the chip affects the transition between the dripping regime (which corresponds to monodispersity) and the jetting regime. The number of parameters to be taken into account for the optimization of the device is enormous; therefore, in order to account for the effect of many geometrical and hydrodynamical parameters, we trained an artificial neural network based on our simulation data. Using this neural network, we evaluated droplet formation in 3240 different cases. This approach resulted in a remarkable reduction of computation time, from months to seconds. Examining numerous cases in such a short period lets us choose the optimum geometry and flow rate based on the application. The optimization was able to find the best geometry to extend the region of dripping regime in the flow rate map. Finally, to harness the droplet generation frequency, we also simulated a periodically switched laser and we were able to predict the generation of droplets with the same frequency as the switching frequency. Therefore, altering and controlling the frequency and dimensions of the droplets for a given flow rate ratio could be achieved with this technique, even without satellite droplets. [ABSTRACT FROM AUTHOR]

In this work, a series of experiments and numerical simulations performed using a volume-of-fluid approach were carried out to investigate the flow of miscible viscous fluid systems through microfluidic flow focusing devices with one central inlet stream (with "fluid 1") and two lateral inlet streams (with "fluid 2"). The combined effect of the fluid viscosity ratio and the inlet velocity ratio on the characteristics of the central focused outlet stream was assessed in microfluidic channels with different aspect ratios. An analytical expression for the two-dimensional case, relating the width of the central focused stream in the outlet channel with the velocity ratio and the viscosity ratio, was also derived from first principles. The analytical results are in excellent agreement with the two-dimensional numerical results, and the expression is also able to represent well the experimental findings for the configuration with an aspect ratio of 0.84. The width of the central focused outlet stream at the center plane is seen to decrease with both the velocity ratio and the viscosity ratio. The results of the three-dimensional numerical simulations and experimental measurements are in good agreement, producing further insight into the curved interface known to exist when high viscosity contrasts are present in parallel flow systems. It was observed that the interface curvature across the depth of the channel cross section is strongly dependent on the ratio of inlet viscosities and microchannel aspect ratio, highlighting the three-dimensional nature of the flow, in which confinement plays a significant role. [ABSTRACT FROM AUTHOR]

The equilibrium positions of red blood cells (RBCs) and their steady motions in microchannel affect the hemodynamics in vivo and microfluidic applications on a cellular scale. However, the dynamic behavior of a single RBC in three-dimensional cylindrical microchannels still needs to be classified systematically. Here, with an immersed boundary method, the phase diagrams of the profiles and positions of RBCs under equilibrium states are illustrated in a wide range of Capillary numbers. The effects of initial positions are explored as well. Numerical results present that the profiles of RBCs at equilibrium states transform from snaking, tumbling to slipper, or parachute with the increase in flow rates, and whether RBCs finally approach slipper or parachute motion under large shear rates is dependent on their initial positions. With the increase in tube diameters, the equilibrium positions of RBCs are closer to tube walls relatively. Although both the increase in membrane shear modulus and the viscosity ratio are regarded as the stiffening of RBCs, the change of membrane property does not affect the dependence of the profiles and positions of RBCs at equilibrium states on the shear rates of the flow obviously, but with the increase in viscosity ratio, RBCs move further away from the centerline of the tube associating with more asymmetric characteristics in their stable profiles. The present results not only contribute to a better understanding of the dynamic behavior and multiple profiles of single RBC in microcirculation, but also provide fundamentals in a large range of Capillary numbers for cell sorting with microfluidic devices. [ABSTRACT FROM AUTHOR]

Bubble removal from a solid surface is of significant importance to many technical processes and applications. In addition to the conventional buoyancy-aided bubble removal, there is also a passive strategy to remove bubbles from a solid surface via coalescence. However, likewise several processes, the coalescence-induced removal of bubbles from the solid surface is masked by the dominant buoyancy, hence, difficult to observe in terrestrial conditions. Microgravity condition offers a unique opportunity to investigate such phenomenon in great detail that can significantly improve our fundamental understanding. In this work, we report coalescence-induced jumping of isolated vapor bubbles from the heated substrate during shear flow in microgravity condition. We show that, similar to the coalescence-induced jumping droplets, when two bubbles coalesce, the resulting big coalesced bubble jumps from the substrate due to the conversion of excess surface energy into the translational kinetic energy, which provides the requisite initial velocity for jumping. Jumping of bubbles over a wide range of bubble size (post-coalescence radius ≈ 0.9 – 3.4 mm) is observed. Bubbles oscillate continuously while rising through certain height post-coalescence. We perform force balance and scaling analysis to develop a model to predict the maximum jumping height of bubbles. We show that the jumping height is strongly related to the bubble size and the non-dimensional Ohnesorge number, which captures the role of fluid properties governing the coalescence. The physical insight presented in this work has implication for the design of energy systems and microfluidic devices for the earth and space-based applications. [ABSTRACT FROM AUTHOR]

The chaotic state of microfluidic devices such as electroosmotic micromixers has received extensive attention. Its unsteady flow and multi-physics mask low-dimensional structure and potential attractors. Based on the dynamic mode decomposition and the sparse identification of nonlinear dynamics, this study aims to construct a manifold equation with the minimum degree of freedom, reveal the mixing mechanism of micromixers, and discover the evolution of chaotic states. The attenuation degree of freedom was introduced to force the modal coefficients to be pure oscillations. The six, four, and two-dimensional minimum reduced-order models (ROMs) were constructed under different mixing conditions. The nonlinear dynamics evolves on attractors resembling a six-petal structure based on the amplitude-phase method. The attractor periodicity and decay map the evolution of the periodic oscillation and limit cycle of the active modes and are related to the appearance of the low-energy dominant non-axisymmetric modes. These results emphasize the significance of ROM technology in revealing the low-dimensional structure and attractor of the electroosmotic micromixer. [ABSTRACT FROM AUTHOR]

Precise manipulation of fluids and objects on the microscale is seldom a simple task, but, nevertheless, crucial for many applications in life sciences and chemical engineering. We present a microfluidic chip fabricated in silicon–glass, featuring one or several pairs of acoustically excited sharp edges at side channels that drive a pumping flow throughout the chip and produce a strong mixing flow in their vicinity. The chip is simultaneously capable of focusing cells and microparticles that are suspended in the flow. The multifunctional micropump provides a continuous flow across a wide range of excitation frequencies (80 kHz–2 MHz), with flow rates ranging from nl min−1 to μl min−1, depending on the excitation parameters. In the low-voltage regime, the flow rate depends quadratically on the voltage applied to the piezoelectric transducer, making the pump programmable. The behavior in the system is elucidated with finite element method simulations, which are in good agreement with experimentally observed behavior. The acoustic radiation force arising due to a fluidic channel resonance is responsible for the focusing of cells and microparticles, while the streaming produced by the pair of sharp edges generates the pumping and the mixing flow. If cell focusing is detrimental for a certain application, it can also be avoided by exciting the system away from the resonance frequency of the fluidic channel. The device, with its unique bundle of functionalities, displays great potential for various biochemical applications. [ABSTRACT FROM AUTHOR]

Experiments were performed to investigate the collapse dynamics of a cavitating bubble generated between a pair of symmetrically arranged oblique plates. A 2.0 mm gap was left at the converging end of the two plates, which were inclined at an angle of 10°. A focused laser beam generated a cavitation bubble of about 4.0 mm in diameter, at four different locations that were placed on the centerline between the glass plates. A high-speed camera captured the bubble's cavitating dynamics at a frame rate of 75 kHz. The initial position of the bubble and, thus, the boundary conditions significantly influenced the bubble's dynamics. The bubble's first collapses showed a distinct unidirectional extended jetting but without notch formation on the bubble's left surface. Subsequent collapses led to intense nucleation, a feature useful in microfluidic devices. Further on, we observed vertical pillar-shaped cavities, floating toroids, etc., shapes that were rarely mentioned in previous investigations. To support our experimental results, we performed numerical simulations based on solving the Navier–Stokes equations, to replicate similar bubble dynamics. Our results provided insight into bubble dynamics generated between oblique plates, thereby potentially contributing to an improved understanding of microfluidic pumping techniques, surface cleaning devices, fouling of complex shapes, biomedical devices employing cavitation-based methods, and micromixing of fluids. Results of these experiments may serve also as benchmark data to validate numerical methods. [ABSTRACT FROM AUTHOR]

When two fluids of different electrical conductivities are transported under the influence of an electric field, the electrokinetic instability (EKI) phenomenon often triggers in a microfluidic device once the electric field strength and conductivity gradient exceed some critical values. This study presents a detailed numerical investigation of how the rheological behavior of a fluid obeyed by the non-Newtonian power-law constitutive relation could influence this EKI phenomenon in a microfluidic T-junction. We find that as the fluid rheological behavior changes from shear-thickening (n >1) to shear-thinning (n <1), the EKI phenomenon is significantly influenced under the same conditions. In particular, the intensity of this EKI phenomenon is found to be significantly higher in shear-thinning fluids than in Newtonian and shear-thickening fluids. Also, the critical value of the applied electric field strength for the inception of this EKI phenomenon gradually increases as the fluid rheological behavior progressively moves from shear-thinning to shear-thickening. The corresponding mixing phenomenon, often achieved using this EKI phenomenon, is also notably higher in shear-thinning fluids compared to Newtonian and shear-thickening fluids. A detailed analysis of both the flow dynamics and mixing phenomena inside the microdevice is presented and discussed in this study. To perform so, we also employ the data-driven dynamic mode decomposition technique, considered one of the widely used reduced-order models to analyze a dynamical system. This analysis facilitates a better understanding of the EKI-induced chaotic convection and mixing phenomena inside the microdevice. We observe that the spatial expanse and intensity of the coherent flow structures differ significantly as the power-law index changes, thereby providing valuable insight into certain aspects of the underlying flow dynamics that, otherwise, are not apparent from other analyses. [ABSTRACT FROM AUTHOR]

We experimentally study the repulsion of charged microscopic particles with the channel walls during electrophoresis in microfluidic devices. For low frequencies of the electric fields (< 10 kHz), this repulsion is mainly due to the hydrodynamic interaction caused by the flow vortices that arise from the slip velocity induced by the electric field on the particle surface, as shown in a recent publication [Fernandez-Mateo et al., Phys. Rev. Lett. 128, 074501 (2022)]. The maximum slip velocity on the particle surface is inferred from measurements of wall-particle separation. Importantly, this procedure allows us to infer very small slip velocities that, otherwise, are too weak to be measured directly. Data at small electric field amplitudes ( E 0 ) agree with theoretical predictions using the model of Concentration Polarization Electro-osmosis (CPEO), which has recently been proposed as the mechanism behind the flow vortices on the surface of the particles. Data for higher electric fields show that the predictions of the CPEO theory for weak electric fields are not valid beyond E 0 ∼ 60 kV/m. Additionally, we also show that, for sufficiently strong electric fields, the quadrupolar flow structures become disrupted, leading to a weaker wall repulsion. [ABSTRACT FROM AUTHOR]

With the recent great interest in microfluidic devices, a better understanding of preconcentration technology has become increasingly important. Herein, concentration enrichment of charged samples is achieved by using field-amplified sample stacking (FASS) technology in the microchannel. This paper aims to develop a fundamental understanding of FASS and to propose a method to enhance the enrichment quality of FASS. First, numerical investigations are carried out to systematically study the effects of various parameters including the applied voltage, the charged properties of the sample, the buffer concentration ratio, the injection length, and the microchannel width on FASS enrichment performance. The results show that reducing the width of the microchannel is an effective way to improve the enrichment quality. The maximum enrichment ratio can be improved by 67.35% by reducing the width of the microchannel to less than 10 μm due to the inhibition of background buffer diffusion. Second, to improve traditional FASS performance, a high-conductance gradient boundary is established by photoinitiating fabrication of a porous plug at the enriched interface position. This structure provides a region that reduces the local size of the internal channel to less than 5 μm and has high flow resistance, but allows the electromigration of the charged analyte. Experimental results show that an electropherogram signal increases by a maximum factor of 329 in electrophoretic enrichment of fluorescein–Na with 5 × 10−7 M initial concentrations, and the enrichment quality of traditional FASS is greatly improved. [ABSTRACT FROM AUTHOR]

The concept of "hydrodynamic cavitation on a chip" offers facile generation of cavitating flows in microdomains, which can be easily scaled up by arranging short microchannels (micro-orifices) in cascade formations. In this regard, microscale cavitation in an energy-efficient test rig has the potential of increasing utilization possibilities of cavitation in a wide range of applications such as liquid-phase exfoliation. In this study, a new experimental test rig was constructed to generate microscale hydrodynamic cavitation. This setup enables cavitation bubble generation at low upstream pressures through the control of the downstream pressure of the device. Particular attention was directed to the classification of flow patterns, scale effects, and cavitating flow evolutions with an in-depth categorization of underlying mechanisms such as Kelvin–Helmholtz instability. Cavitation inception appeared in the form of a single bubble. The appearance of different attached cavitating flow patterns within the microfluidic device was accompanied by new physics, which revealed that cavitation generation and development are affected by the existence of various fluid flow phenomena, particularly the jet flow. The outcome of this study makes hydrodynamic cavitation on a chip attractive for applications, where the cavitation effects are sought in the presence of multiphase fluid flows. [ABSTRACT FROM AUTHOR]

We have developed a coupled level set and volume of fluid-based computational fluid dynamics model to analyze the droplet formation mechanism in a square flow-focusing microchannel. We demonstrate a flexible manipulation of droplet formation and flow regime based on the modified flow-focusing microchannel with a constricted orifice. Furthermore, we have systematically studied the influence of geometrical confinement, flow rate, and interfacial tension on the droplet formation regime, length, volume, velocity, and shape. Three different flow regimes, namely squeezing, dripping, and jetting, are observed, and the flow regime maps are formulated based on the Reynolds and capillary numbers. After an extensive numerical investigation, we described the boundaries between the different regimes. Droplet shape is also quantified based on the deformation index value. Plug-shaped droplets are observed in the squeezing regime, and near spherical droplets are found in the dripping and jetting regimes. Our study provides insights into the transition of a regime under various geometrical confinement and fluid properties. The results reveal that the modified flow-focusing microchannel can substantially enhance dripping while decreasing the squeezing regime, which is of paramount importance from the standpoint of producing high throughput stable and monodisperse microdroplets. Eventually, this work emphasizes the importance of geometrical confinement, fluid properties, and flow conditions on the droplet formation process in a flow-focusing microchannel that can effectively provide helpful guidelines on the design and operations of such droplet-based microfluidic systems. [ABSTRACT FROM AUTHOR]

Directional and self-propelled flow in open channels has a variety of applications, including microfluidic and medical devices, industrial filtration processes, fog-harvesting, and condensing apparatuses. Here, we present versatile three-dimensional-printed liquid diodes that enable spontaneous unidirectional flow over long distances for a wide range of liquid contact angles (CAs). Typically, we can achieve average flow velocities of several millimeters per second over a distance of tens to hundreds millimeters. The diodes have two key design principles. First, a sudden widening in the channels' width, in combination with a small bump, the pitch, ensure pinning of the liquid in the backward direction. Second, an adjustable reservoir with differing expansion angles, the bulga, is introduced to manipulate the liquid velocity. Using a combination of experiments and lattice Boltzmann simulations, we provide a comprehensive analysis of the flow behavior and speed within the channels depending on CAs, pitch heights, and bulga angles. This provides guidelines for the fabrication of bespoke liquid diodes with optimal design for their potential applications. As a feasibility investigation, we test our design for condensation of water from fog and subsequent transport uphill. [ABSTRACT FROM AUTHOR]

We proposed herein a unique method of insulator-decorated bipolar electrochemistry (IDBE), for realizing large-scale separation of bioparticles in microchannels driven by AC dielectrophoresis (DEP). In IDBE, a pair of planar driving electrodes is placed at the bottom of channel sidewalls, between which an array of the rectangular floating electrode (FE) strips without external Ohmic contact are evenly spaced along transversal direction, and a series of insulating dielectric blocks are periodically deposited above all the inter-electrode gaps and in full contact with the channel bottom surface. By creating local field maximum and minimum at multiple sites, IDBE extends well the actuating range of DEP force field from the immediate vicinity of electrode tips in traditional bipolar electrochemistry to current fluid bulk. Considering DEP force plays the dominant role around 1 MHz, we utilize Lagrange particle tracing algorithm to calculate motion trajectories of incoming samples for testing the feasibility of microchip in continuous separation of live and dead yeast cells. By applying suitable voltage parameters, highly efficient DEP sorting is theoretically achievable under a moderate inlet flow rate, where most of the viable yeasts are trapped by positive-DEP to sharp dielectric edges, while all the incoming nonviable yeasts are repelled by negative-DEP to the top surface of both FE and insulating block to form multiple thin beams co-flowing into the channel outlet. The microfluidic device exploiting insulators on bipolar FE effectively expands the actuating range of nonlinear electrodynamics and provides invaluable guidelines for developing flexible electrokinetic frameworks in modern microfluidic systems. [ABSTRACT FROM AUTHOR]

The use of three-dimensional (3D) printing for fabrication of master molds for microfluidic devices is very attractive due to its availability and simplicity and replaces the standard methods of soft lithography. However, the commercially available photopolymer resins inhibit the curing of polydimethylsiloxane (PDMS), preventing reliable replication of 3D printed master mold structures. Here, we present a simple and safe method to post-process 3D printed photopolymer master molds for PDMS microfluidic devices. This approach expands the possibilities of prototyping microfluidic PDMS devices for a wider research community without complex post-processing tools currently required for fabrication of 3D photopolymer master molds. [ABSTRACT FROM AUTHOR]