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This article presents an approach of microfluidic flow cytometry capable of continuously characterizing cytoplasmic viscosities of single cells. The microfluidic system consists of a major constriction channel and a side constriction channel perpendicularly crossing each other. Cells are forced to rapidly travel through the major channel and are partially aspirated into the side channel when passing the channel junction. Numerical simulations were conducted to model the time dependence of the aspiration length into the side channel, which enables the measurement of cytoplasmic viscosity by fitting the model results to experimental data. As a demonstration for high-throughput measurement, the cytoplasmic viscosities of HL-60 cells that were native or treated by N-Formylmethionine-leucyl-phenylalanine (fMLP) were quantified with sample sizes as large as thousands of cells. Both the average and median cytoplasmic viscosities of native HL-60 cells were found to be about 10% smaller than those of fMLP-treated HL-60 cells, consistent with previous observations that fMLP treatment can increase the rigidity of white blood cells. Furthermore, the microfluidic system was used to process granulocytes from three donors (sample size >1,000 cells for each donor). The results revealed that the cytoplasmic viscosity of granulocytes from one donor was significantly higher than the other two, which may result from the fact that this donor just recovered from an inflammation. In summary, the developed microfluidic system can collect cytoplasmic viscosities from thousands of cells and may function as an enabling tool in the field of single-cell analysis. © 2019 International Society for Advancement of Cytometry., (© 2019 International Society for Advancement of Cytometry.)

The development of artificial spider silk with properties similar to native silk has been a challenging task in materials science. In this study, we use a microfluidic device to create continuous fibers based on recombinant MaSp2 spidroin. The strategy incorporates ion-induced liquid-liquid phase separation, pH-driven fibrillation, and shear-dependent induction of β-sheet formation. We find that a threshold shear stress of approximately 72 Pa is required for fiber formation, and that β-sheet formation is dependent on the presence of polyalanine blocks in the repetitive sequence. The MaSp2 fiber formed has a β-sheet content (29.2%) comparable to that of native dragline with a shear stress requirement of 111 Pa. Interestingly, the polyalanine blocks have limited influence on the occurrence of liquid-liquid phase separation and hierarchical structure. These results offer insights into the shear-induced crystallization and sequence-structure relationship of spider silk and have significant implications for the rational design of artificially spun fibers. Native spider silk has desirable mechanical properties, but these are challenging to replicate in an artificial material. Here, the authors report the use of a microfluidic system to create continuous fibers based on recombinant spidroin. [ABSTRACT FROM AUTHOR]

Digital microfluidics, which relies on the movement of drops, is relatively immune to clogging problems, making it suited for micro-reactor applications. Here, graphene oxide paper of 100 μm thickness, fabricated by blade coating sedimented dispersions onto roughened substrates, followed by drying and mechanical exfoliation, was found to be relatively free of cracks and curling. It also exhibited high wettability and elasto-capillary characteristics. Possessing low enough stiffness, it could rapidly and totally self-wrap water drops of 20 μL volume placed 2 mm from its edge when oriented between 0 and 60° to the horizontal. This complete wrapping behavior allowed drops to be translated via movement of the paper over long distances without dislodgement notwithstanding accelerations and decelerations. An amount of 2 drops that were wrapped with separate papers, when collided with each other at speeds up to 0.64 m/s, were found to eschew coalescence. This portends the development of robust digital microfluidic approaches for micro-reactors. [ABSTRACT FROM AUTHOR]

Capillary rise within rough structures is a wetting phenomenon that is fundamental to survival in biological organisms, deterioration of our built environment, and performance of numerous innovations, from 3D microfluidics to carbon capture. Here, to accurately predict rough capillary rise, we must couple two wetting phenomena: capillary rise and hemiwicking. Experiments, simulations, and theory demonstrate how this coupling challenges our conventional understanding and intuitions of wetting and roughness. Firstly, the critical contact angle for hemiwicking becomes separation-dependent so that hemiwicking can vanish for even highly wetting liquids. Secondly, the rise heights for perfectly wetting liquids can differ between smooth and rough systems, even with the same 0∘ contact angle. Finally, the raised liquid volumes are substantially increased in rough compared to smooth systems. To explain and predict all rise heights and volumes with quantitative accuracy, we present the Dual-Rise model that is valid for general roughness, liquids, and surface wettabilities. Capillary rise is a process whereby a liquid spontaneously rises against gravity within a narrow space due to capillary forces; but even though it is a well-understood phenomenon in smooth channels, predictive models to account for surface roughness are lacking. Here, the authors develop a theory of capillary rise in textured channels, supported by simulations and experiments, demonstrating the complex interplay between channel width, texture and wettability. [ABSTRACT FROM AUTHOR]

Many natural creatures have demonstrated unique abilities in directional liquid transport (DLT) for better adapting to the local environment, which, for a long time, have inspired the material fabrication for applications in microfluidics, self-cleaning, water collection, etc. Recently, DLTs aroused by the corner effect have been witnessed in various natural organisms, where liquid transports/spreads spontaneously along the corner structures in microgrooves, wedges or conical structures driven by micro-/nano- scaled capillary forces without external energy input. Particularly, these DLTs show advantages of ultrahigh speed, continuous proceeding, and/or external controllability. Here, we reviewed recent research advances on the bioinspired DLTs induced by the corner effect, as well as the involved mechanisms and the artificial counterpart materials with various applications. We also introduced some bioinspired materials that are capable of stimulus-responsive DLT under external fields. Finally, we suggested perspectives of the bioinspired DLTs in liquid manipulations. [ABSTRACT FROM AUTHOR]

We explore air invasion in a dead-end compliant water-filled microchannel containing a constriction. The phenomenon is driven by the pervaporation of the liquid present in the channel through the surrounding medium. The penetration is intermittent, jerky and characterised by a stop-and-go dynamics as the bubble escapes the constriction. We demonstrate that this sequence of arrest and jump of the bubble is due to an elasto-capillary coupling between the air-liquid interface and the elastic medium. When the interface enters the constriction, its curvature increases strongly, leading to a depressurisation within the liquid-filled channel that drives a compression of the channel. As the interface is forced to leave the constriction at a given threshold pressure, due to the ongoing loss of liquid content by pervaporation, the pressure is suddenly released, which gives rise to a rapid propagation of the air bubble away from the constriction, and a restoration of the rest shape of the channel. Combining macroscopic observations and confocal imaging, we present a comprehensive experimental study of this phenomenon. In particular, the effect of the channel geometry on the time of arrest in the constriction and the jump length is investigated. Our novel microfluidic design succeeds in mimicking the role of inter-vessel pits in plants, which transiently stop the propagation of air embolisms during long and severe droughts. It is expected to serve as a building block for further biomimetic studies in more complex leaf-like architectures, in order to recover this universal phenomenon of intermittent propagation reported in real leaves. [ABSTRACT FROM AUTHOR]

The capillary action is a unique feature of micro-grooves with numerous applications. This spontaneous flow eliminates the need for an extra pumping device to deliver a liquid. Capillary action depends on physical properties and features of the solid surface, as well as on thermophysical properties of the liquid. In this study, our previously proposed unifying capillary rise model is extended to include the effect of surface roughness. A new characteristic length scale is proposed that includes salient geometrical parameters, such as micro-grooves height, width, and surface roughness. Furthermore, it is shown that by using the proposed characteristic length scale, it can be determined whether the capillary action would occur in a given micro-groove and liquid. Various metallic and polymeric surfaces with a wide range of surface roughness are fabricated from aluminum, stainless-steel, natural graphite sheet, and 3D-printed stainless-steel and a polymer. A profilometer and sessile drop method are used to measure surface roughness and the contact angles, respectively. The present unifying model is compared against our measured data, and it is shown that it can predict the capillary rise in rough micro-grooves with less than a 10% relative difference. It is observed that the capillary height can be increased for a wetting surface by introducing surface roughness and by using optimal micro-groove cross-sections that are triangular as opposed to rectangular. The proposed compact, unifying model can be used to predict the capillary rise for any given micro-groove cross-section, and as a design tool for numerous industrial and biomedical applications, such as heat pipes, power electronic cooling solutions, sorption systems, medicine delivery devices, and microfluidics that utilize capillary micro-grooves. [ABSTRACT FROM AUTHOR]

Superhydrophilic–superhydrophobic patterned surfaces constitute a branch of surface chemistry involving the two extreme states of superhydrophilicity and superhydrophobicity combined on the same surface in precise patterns. Such surfaces have many advantages, including controllable wettability, enrichment ability, accessibility, and the ability to manipulate and pattern water droplets, and they offer new functionalities and possibilities for a wide variety of emerging applications, such as microarrays, biomedical assays, microfluidics, and environmental protection. This review presents the basic theory, simplified fabrication, and emerging applications of superhydrophilic–superhydrophobic patterned surfaces. First, the fundamental theories of wettability that explain the spreading of a droplet on a solid surface are described. Then, the fabrication methods for preparing superhydrophilic–superhydrophobic patterned surfaces are introduced, and the emerging applications of such surfaces that are currently being explored are highlighted. Finally, the remaining challenges of constructing such surfaces and future applications that would benefit from their use are discussed. HIGHLIGHTS: •The fundamental theories of surface wettability are introduced to explain the spreading of a droplet on a solid surface. •The methods for constructing superhydrophilic–superhydrophobic patterned surfaces are described regarding hierarchical structure fabrication, chemical modification, and wettability patterning. •The emerging applications of such surfaces are highlighted, including droplet arrays, cell arrays, biosensors, microfluidic systems, and environmental protection. • The remaining challenges of preparing such surfaces and future applications that would benefit from their use are discussed. [ABSTRACT FROM AUTHOR]

Spontaneous capillary imbibition is a classical problem in interfacial fluid dynamics with a broad range of applications, from microfluidics to agriculture. Here we study the duration of the cross-over between an initial linear growth of the imbibition front to the diffusive-like growth limit of Washburn's law. We show that local-resistance sources, such as the inertial resistance and the friction caused by the advancing meniscus, always limit the motion of an imbibing front. Both effects give rise to a cross-over of the growth exponent between the linear and the diffusive-like regimes. We show how this cross-over is much longer than previously thought - even longer than the time it takes the liquid to fill the porous medium. Such slowly slowing-down dynamics is likely to cause similar long cross-over phenomena in processes governed by wetting. [ABSTRACT FROM AUTHOR]

In the fields of microgripping and microassembly, the self-alignment motion of a solid micro-object linked by a liquid meniscus to a substrate or a tool is an inexpensive way to overcome the current limitations of the assembly processes at microscale by getting rid of the positioning actuators. Original models providing a dynamical description of the capillary self-alignment of an L × D × d chip are reported, as well as experimental results as evidence of their validity. The first two models describe the liquid and the solid physics in two dimensions. Both include nonlinearities and describe the coupling between a laminar flow and a solid structure. The fluid–solid coupling is ensured by the boundary conditions at their surface of contact and by the forces the liquid and the solid apply on each other. Both models yield the shift, lift and tilt modes of deformation of the liquid meniscus. Equations are first numerically solved by using a finite element method (model 1). By approximating the menisci with spherical caps, a geometrical model is then presented (model 2). Next, for small oscillations and thin liquid layers, the equations are linearised. The solution to the semianalytical three degrees of freedom (3-DOF) modal analysis is thus obtained (model 3). Finally, a semianalytical 1-DOF model is presented and numerically solved by considering a one-dimensional motion for the solid object (model 4). Solutions for models 1, 3 and 4 are computed and show good agreement with the experimental measurements. Yet, the remaining deviations are investigated to identify their origin. [ABSTRACT FROM AUTHOR]

Surface roughness is ubiquitous in subsurface formations due to weathering and diagenesis. Yet micromodel studies of porous media have not sufficiently addressed the impact of pore‐wall roughness on multiphase fluid displacement. We investigate the influence of pore‐wall roughness on capillary trapping by implementing surface roughness into glass micromodels and conducting imbibition experiments. Time‐lapse fluorescence microscopy, micromolding in capillaries, and scanning electron microscopy are used to examine flow behavior during spontaneous and forced imbibition experiments with capillary numbers between 10−6 and 10−4. It is found that surface roughness causes interface pinning and irregular displacement fronts within a single pore, promotes latent pore filling following imbibition, enhances snap‐offs, and contributes to interface relaxation. Furthermore, we verify a cooperative effect between corner flow and surface roughness that causes pore filling to occur at over 10 times the channel width ahead of the main bulk front. Consequently, a significant increase in trapped air saturation of up to 60% is observed, especially at lower capillary numbers and narrower pores. Effects of surface roughness also decrease the rate of spontaneous imbibition, which is slower than the prediction from Washburn's equation. Our results emphasize the importance of surface roughness in building conceptual models for capillary trapping when the ratio of surface roughness‐to‐pore size reaches a critical value of about 0.1. Above this critical threshold value, subpore‐scale trapping mechanisms contribute significantly to capillary trapping, which theoretical and numerical models should consider for accurate quantification of fluid‐transport processes. Key Points: Pore‐wall roughness causes interface pinning and irregular displacement fronts, promotes latent pore filling, and enhances snap‐offsCooperative effect of boundary flow and surface roughness causes pore filling beyond the primary meniscus and increases capillary trappingSurface roughness decreases the rate of spontaneous imbibition due to additional friction caused by corner and film flows [ABSTRACT FROM AUTHOR]

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels. [ABSTRACT FROM AUTHOR]

We present a combined experimental and theoretical investigation of the dynamics of water imbibition through paper with swelling. The Washburn equation has been widely used to describe the dynamics of the liquid absorption in paper, but its prediction of liquid imbibition speed has been reported to be inaccurate. Our recent study (Chang et al., J. Fluid Mech., vol. 845, 2018, pp. 36–50) demonstrated that the internal cavity of cellulose fibres composing the paper is partially responsible for the limited accuracy of the Washburn equation based on oil imbibition experiments. Here we extend the investigation to water absorption through paper with swelling. We demonstrate that the swelling of the cellulose fibre network in addition to the internal voids of the cellulose fibres crucially affects the imbibition dynamics. Based on the microscopic observation that paper swelling is caused by the expansion of inter-fibre space, we suggest a mathematical model for water imbibition which considers both intra-fibre voids and swelling. By introducing parameters that characterize the swelling speed and volume of paper, our model markedly improves prediction of the water imbibition speed. The results provide not only a theoretical background for designing paper-based microfluidic systems, but also new insights into capillary flow through expandable porous media. [ABSTRACT FROM AUTHOR]

The flow of liquids through paper is challenging to model due to the complexity and disordered layout of the fibre matrix. The expanding use and capability of microfluidic paper-based analytical devices (μPADs) and their requirement for precision has increased the need to accurately predict the flow of liquids through paper. Many studies have developed models and revealed some of the physical mechanisms responsible for the flow behaviour, but we still lack a complete understanding, particularly in relation to how the fluid fills the various voids with a wide range of shapes and sizes in the fibre matrix of paper. In the featured article, Chang et al. (J. Fluid Mech., vol. 845, 2018, 36-50) used a combined experimental and theoretical approach to uncover the importance of the liquid filling the intra-fibre pores, showed that this results in deviation from the flow behaviour predicted by the Lucas--Washburn equation and developed a model which accounts for this effect. [ABSTRACT FROM AUTHOR]

We present a combined experimental and theoretical investigation of the dynamics of liquid imbibition through paper. The Washburn equation is widely used to describe the dynamics of capillary flow through paper, but this classical model has limited accuracy, which often makes it difficult to use in developing analytic systems such as paper-based microfluidic devices. We here report that the internal cavity of the cellulose fibres composing paper is significantly responsible for the limited accuracy of the Washburn equation. Our experiments demonstrated that liquid can be absorbed in the internal cavity of the cellulose fibres as well as in the inter-fibre pores formed by the fibre network. We developed a mathematical model for liquid imbibition by considering the flow through the intra-fibre pores based on experimental measurements of the intra-structure of cellulose fibres. The model markedly improves the prediction of the liquid absorption length, compared with the results of the Washburn equation, thus revealing the physics behind the limits of the Washburn equation. This study suggests that the accurate description of capillary imbibition through paper require parameters characterizing the internal pores of the cellulose fibres comprising the paper. Our results not only provide a new insight into porous media flows with different sized pores, but also provide a theoretical background for flow control in paper-based microfluidic systems. [ABSTRACT FROM AUTHOR]

In this paper, we investigate both experimentally and theoretically the dynamics of a liquid plug driven by a cyclic periodic forcing inside a cylindrical rigid capillary tube. First, it is shown that, depending on the type of forcing (flow rate or pressure cycle), the dynamics of the liquid plug can either be stable and periodic, or conversely accelerative and eventually leading to plug rupture. In the latter case, we identify the sources of the instability as: (i) the cyclic diminution of the plug viscous resistance to motion due to the decrease in the plug length and (ii) a cyclic reduction of the plug interfacial resistance due to a lubrication effect. Since the flow is quasi-static and the forcing periodic, this cyclic evolution of the resistances relies on the existence of flow memories stored in the length of the plug and the thickness of the trailing film. Second, we show that, contrary to unidirectional pressure forcing, cyclic forcing enables breaking of large plugs in a confined space although it requires longer times. All the experimentally observed tendencies are quantitatively recovered from an analytical model. This study not only reveals the underlying physics but also opens up the prospect for the simulation of 'breathing' of liquid plugs in complex geometries and the determination of optimal cycles for obstructed airways reopening. [ABSTRACT FROM AUTHOR]

Multiphase flow in porous media is important in many natural and industrial processes, including geologic CO2 sequestration, enhanced oil recovery, and water infiltration into soil. Although it is well known that the wetting properties of porous media can vary drastically depending on the type of media and pore fluids, the effect of wettability on multiphase flow continues to challenge our microscopic and macroscopic descriptions. Here, we study the impact of wettability on viscously unfavorable fluid-fluid displacement in disordered media by means of high-resolution imaging in microfluidic flow cells patterned with vertical posts. By systematically varying the wettability of the flow cell over a wide range of contact angles, we find that increasing the substrate's affinity to the invading fluid results in more efficient displacement of the defending fluid up to a critical wetting transition, beyond which the trend is reversed. We identify the pore-scale mechanisms--cooperative pore filling (increasing displacement efficiency) and corner flow (decreasing displacement efficiency)--responsible for this macroscale behavior, and show that they rely on the inherent 3D nature of interfacial flows, even in quasi- 2D media. Our results demonstrate the powerful control of wettability on multiphase flow in porous media, and show that the markedly different invasion protocols that emerge--from pore filling to postbridging-- are determined by physical mechanisms that are missing from current pore-scale and continuum-scale descriptions. [ABSTRACT FROM AUTHOR]

This article discusses how to derive a closed-form analytical solution describing the profile of unidirectional flow through a class of noncircular tubes. The cross sections of the conduits are a slightly distorted version of regular shapes, like triangles or trapeziums or rectangles, that are commonly seen in microfluidic or heat pipe applications. Hence, the study leads to approximate but useful expression for velocity profiles as well as resistivity for the aforementioned channels. Moreover, the paper shows how to extend the fundamental idea to other arbitrary geometries, providing simple expressions for flow fields that can be used subsequently in various hydrodynamic calculations. The analysis also gives an estimate of error in such a mathematical treatment that is found to be adequately accurate. Thus, the findings will be especially relevant in a number of fields, including microfluidic separation, heat pipe design, and capillary transport optimization. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive review of the development of the optical stretcher, a powerful optofluidic device for single cell mechanical study by using optical force induced cell stretching. The different techniques and the different materials for the fabrication of the optical stretcher are first summarized. A short description of the optical-stretching mechanism is then given, highlighting the optical force calculation and the cell optical deformability characterization. Subsequently, the implementations of the optical stretcher in various cell-mechanics studies are shown on different types of cells. Afterwards, two new advancements on optical stretcher applications are also introduced: the active cell sorting based on cell mechanical characterization and the temperature effect on cell stretching measurement from laser-induced heating. Two examples of new functionalities developed with the optical stretcher are also included. Finally, the current major limitation and the future development possibilities are discussed. [ABSTRACT FROM AUTHOR]

This mini-review presents recent progresses in the development of microfluidic constriction channels enabling high-throughputmechanical property characterization of single cells. We first summarized the applications of the constriction channel design in quantifying mechanical properties of various types of cells including red blood cells, white blood cells, and tumor cells. Then we highlighted the efforts in modeling the cellular entry process into the constriction channel, enabling the translation of raw mechanical data (e.g., cellular entry time into the constriction channel) into intrinsic cellular mechanical properties such as cortical tension or Young's modulus. In the end, current limitations and future research opportunities of the microfluidic constriction channels were discussed. [ABSTRACT FROM AUTHOR]

We report a simple method to make hybrid or pure silica micropatterns at the surface of a substrate based on the combination of sol–gel process and nano-imprint lithography. The silica patterns can be easily designed during the photolithographic step and functionalized with a vapor phase deposition of fluorosilane molecules to obtain superhydrophobic surfaces. Benefiting from the properties of silica, our superhydrophobic patterns can withstand elevated temperatures and show interesting optical properties. These surfaces can be used for thermal transfer applications or microfluidic devices for example to limit noise in fluorescence measurements for biological applications. In connection to the fabrication of superhydrophobic surfaces, the organization of patterns (period of grating) and height of patterns were tested, and the stability of the Cassie–Baxter state studied. The transition can be described on a wide range of tested parameters by the sliding threshold where the control of side wall angle of patterns and chemistry of surface is essential. [ABSTRACT FROM AUTHOR]

Extensional flows and the underlying stability/instability mechanisms are of extreme relevance to the efficient operation of inkjet printing, coating processes and drug delivery systems, as well as for the generation of micro droplets. The development of an extensional rheometer to characterize the extensional properties of low viscosity fluids has therefore stimulated great interest of researchers, particularly in the last decade. Microfluidics has proven to be an extraordinary working platform and different configurations of potential extensional microrheometers have been proposed. In this review, we present an overview of several successful designs, together with a critical assessment of their capabilities and limitations. [ABSTRACT FROM AUTHOR]

In nature, rice leaves exhibit special anisotropic sliding capabilities. Although researchers have succeeded in fabricating artificial rice leaf structures and realizing the wettability function of the leaf surface, these methods used to date are complex and do not allow the fabrication of surfaces with large area. Herein, we adopted a simple technology - two steps soft transfer to fabricate biomimetic rice leaf. The fabricated surface well reproduced the structures of the rice leaf surface and exhibited a static superhydrophobic property similar to that of the real rice leaf surface. In terms of its dynamic wettability, it clearly exhibited an anisotropic sliding property. Systematic measurements showed that the sliding angles parallel and perpendicular with the vein direction were 25° and 40°, respectively. The method was simple and reliable, without the need for expensive instruments and complex technologies, which could be used for the rapid fabrication of large-area artificial rice leaf surfaces. We believe that the artificial rice leaf surface fabricated by this method has great potential applications in biomimetic functional surfaces, microfluidics, and so on. [ABSTRACT FROM AUTHOR]

Contact angle measurements on micropillar arrays were used to determine the conditions that trigger spontaneous penetration of liquids into surface structures. Square micropillars (20 μm) were fabricated in photoresist or quartz and modified chemically to alter the inherent contact angle (i.e., for a flat surface). The lattice spacing of the pillar array and pillar height was also adjusted to investigate the influence of geometry on the wetting behavior. A critical inherent contact angle, θ, was observed below 90°, at which enhanced hydrophobicity switches to enhanced hydrophilicity. This differs from Wenzel's prediction of θ= 90°. The transition is not a Cassie-Wenzel state transition. Above the critical angle, the static advancing contact angle increased with pillar coverage due to pinning. Below the critical angle, liquid spreads ahead of the droplet between the pillars to form a stable film. An example of chemical detection and the implications for multiphase microfluidics is discussed. [ABSTRACT FROM AUTHOR]

Polymers with their tunable functionalities offer the ability to rationally design micro- and nano-engineered materials. Their synthesis as thin films have significant advantages due to the reduced amounts of materials used, faster processing times and the ability to modify the surface while preserving the structural properties of the bulk. Furthermore, their low cost, ease of fabrication and the ability to be easily integrated into processing lines, make them attractive alternatives to their inorganic thin film counterparts. Chemical vapor deposition (CVD) as a polymer thin-film deposition technique offers a versatile platform for fabrication of a wide range of polymer thin films preserving all the functionalities. Solventless, vapor-phase deposition enable the integration of polymer thin films or nanostructures into micro- and nanodevices for improved performance. In this review, CVD of functional polymer thin films and the polymerization mechanisms are introduced. The properties of the polymer thin films that determine their behavior are discussed and their technological advances and applications are reviewed. [ABSTRACT FROM AUTHOR]

In the absence of significant body forces the passive manipulation of fluid interfacial flows is naturally achieved by control of the specific geometry and wetting properties of the system. Numerous ‘microfluidic’ systems on Earth and ‘macrofluidic’ systems aboard spacecraft routinely exploit such methods and the term ‘capillary fluidics’ is used to describe both length-scale limits. In this work a collection of analytic solutions is offered for passive and weakly forced flows where a bulk capillary liquid is slowly drained or supplied by a faster capillary flow along at least one interior edge of the container. The solutions are enabled by an assumed known pressure (or known height) dynamical boundary condition. Following a series of assumptions this boundary condition can be in part determined a priori from the container dimensions and further quantitative experimental evidence, but not proof, is provided in support of its expanded use herein. In general, a small parameter arises in the scaling of the problems permitting a decoupling of the edge flow from the global bulk meniscus flow. The quasi-steady asymptotic system of equations that results may then be easily solved in closed form for a useful variety of geometries including uniform and tapered sections possessing at least one critically wetted interior edge. Draining, filling, bubble displacement and other imbibing flows are studied. Cursory terrestrial and drop tower experiments agree well with the solutions. The solutions are valued for the facility they provide in computing designs for selected capillary fluidics problems by way of passive transport rates and meniscus displacement. Because geometric permutations of any given design are myriad, such analytic tools are capable of efficiently identifying and comparing critical design criteria (i.e. shape and size) and the impact of various wetting conditions resulting from the fluid properties and surface conditions. Sample optimizations are performed to demonstrate the utility of the method. [ABSTRACT FROM AUTHOR]

A fluid droplet located on a superhydrophobic surface makes contact with the surface only at small isolated regions, and is mostly in contact with the surrounding air. As a result, a fluid in motion near such a surface experiences very low friction, and superhydrophobic surfaces display strong drag reduction in the laminar regime. Here we consider theoretically a superhydrophobic surface composed of circular posts (so-called fakir geometry) located on a planar rectangular lattice. Using a superposition of point forces with suitably spatially dependent strength, we derive the effective surface-slip length for a planar shear flow on such a fakir-like surface as the solution to an infinite series of linear equations. In the asymptotic limit of small surface coverage by the posts, the series can be interpreted as Riemann sums, and the slip length can be obtained analytically. For posts on a square lattice, our analytical prediction of the dimensionless slip length, in the low surface coverage limit, is in excellent quantitative agreement with previous numerical computations. [ABSTRACT FROM AUTHOR]

This article considers Couette and Poiseuille flows past superhydrophobic surfaces containing alternating micro-grooves and ribs aligned longitudinally to the flow. The effects of interface curvature on the effective slip length are quantified for different shear-free fractions and groove–rib spatial periods normalized using the channel height. The numerical results obtained demonstrate the importance of considering interface curvature effects in ascertaining the effective slip length. The effective slip length and performance of longitudinal grooves are compared with those corresponding to transverse grooves, for which analytical results are available for small shear-free fractions and normalized groove–rib periodic spacing. For the same shear-free fraction and interface protrusion angle, the effective slip length corresponding to the Poiseuille flow is found to be strongly affected by the normalized groove–rib spacing, in contrast to the Couette flow. For the Poiseuille flow, when the interface deforms by large protrusion angles into the liquid phase, the effective slip length approaches zero or becomes negative for large values of shear-free fraction and normalized groove–rib spacing due to significant flow blockage effects. [ABSTRACT FROM AUTHOR]

Surfaces exhibiting complex topographies, such as those encountered in biology, give rise to an enormously rich variety of interfacial morphologies of a liquid to which they are exposed. In the present article, we elaborate on some basic mechanisms involved in the statics and dynamics of such morphologies, focusing on a few simple paradigm topographies. We demonstrate that different liquid interface morphologies on the same sample frequently coexist. To exemplify the impact of the dynamics on the final droplet morphology, we discuss die shape instability of filamentous liquid structures in wedge geometries. We finally show that some side effects that may dominate on a larger scale, such as contact line pinning and contact angle hysteresis, seem to play a minor role on the microscopic scale under study. This establishes the validity of simple theoretical concepts of wetting as a starting point for describing liquids at substrate surfaces of high complexity. [ABSTRACT FROM AUTHOR]

Soft organic surfaces with more and more complex topologies are required daily to engineer appropriate microstructures for many different applications such as DNA array technology, biological optics for advanced photonic systems and microfluidics. Complementarily to conventional lithographic processes, several pioneering methods have been developed recently, by controlling phase separation of polymer blends, spinodal decomposition of homopolymers or by using the action of additional external forces driving diverse instabilities. Here we present a method that not only provides original concepts towards the three-dimensional (3D) structuring of liquids, on the basis of the synergistic effects of molecular diffusion and confined nucleation, but also suggests original solutions for the transport, mixing and filtering of small volumes of liquid. Through the intrinsic destabilization of a liquid–liquid bilayer, the 2D pattern of a chemically structured surface with ‘hydrophilic’ and ‘hydrophobic’ domains is transferred to a solid/liquid interface as a 3D topography with either ‘positive’ or ‘negative’ replication. This easy-to-use process has potential applications in various technological realms requiring a specific topography at interfaces such as microfluidics or biosensors. [ABSTRACT FROM AUTHOR]

Development and optimization of multifunctional devices for fluidic manipulation of films, drops, and bubbles require detailed understanding of interfacial phenomena and microhydrodynamic flows. Systems are distinguished by a large surface to volume ratio and flow at small Reynolds, capillary, and Bond numbers are strongly influenced by boundary effects and therefore amenable to control by a variety of surface treatments and surface forces. We review the principles underlying common techniques for actuation of droplets and films on homogeneous, chemically patterned, and topologically textured surfaces by modulation of normal or shear stresses. [ABSTRACT FROM AUTHOR]

This review focuses on self-cleaning surfaces, from passive bio-inspired surface modification including superhydrophobic, superomniphobic, and superhydrophilic surfaces, to active micro-electro-mechanical systems (MEMS) and digital microfluidic systems. We describe models and designs for nature-inspired self-cleaning schemes as well as novel engineering approaches, and we discuss examples of how MEMS/microfluidic systems integrate with functional surfaces to dislodge dust or undesired liquid residues. Meanwhile, we also examine "waterless" surface cleaning systems including electrodynamic screens and gecko seta-inspired tapes. The paper summarizes the state of the art in self-cleaning surfaces, introduces available cleaning mechanisms, describes established fabrication processes and provides practical application examples. [ABSTRACT FROM AUTHOR]

Abstract.: The phenomena on the capillarity-driven migration of small objects are full of interest for both scientific and engineering communities, and a critical review is thereby presented. The small objects mentioned here deal with the non-deformable objects, such as particles, rods, disks and metal sheets; and besides them, the soft objects are considered, such as droplets and bubbles. Two types of interfaces are analyzed, i.e., the solid-fluid interface and the fluid-fluid interface. Due to the easily deformable properties of the soft objects and distorted interfacial shapes induced by small objects, a more convenient way to obtain the driving force is through the potential energy of the system. The asymmetric factors causing the object migration include the asymmetric configuration of the interface, and the difference between the interfacial tensions. Finally, a simple outlook on the potential applications of small object migration is made. These behaviors may cast new light on the design of microfluidics and new devices, environment cleaning, oil and gas displacement and mineral industries.Graphical abstract: [ABSTRACT FROM AUTHOR]

Background: Leukocyte-mediated pulmonary inflammation is a key pathophysiological mechanism involved in acute respiratory distress syndrome (ARDS). Massive sequestration of leukocytes in the pulmonary microvasculature is a major triggering event of the syndrome. We therefore investigated the potential role of leukocyte stiffness and adhesiveness in the sequestration of leukocytes in microvessels.Methods: This study was based on in vitro microfluidic assays using patient sera. Cell stiffness was assessed by measuring the entry time (ET) of a single cell into a microchannel with a 6 × 9-μm cross-section under a constant pressure drop (ΔP = 160 Pa). Primary neutrophils and monocytes, as well as the monocytic THP-1 cell line, were used. Cellular adhesiveness to human umbilical vein endothelial cells was examined using the laminar flow chamber method. We compared the properties of cells incubated with the sera of healthy volunteers (n = 5), patients presenting with acute cardiogenic pulmonary edema (ACPE; n = 6), and patients with ARDS (n = 22), of whom 13 were classified as having moderate to severe disease and the remaining 9 as having mild disease.Results: Rapid and strong stiffening of primary neutrophils and monocytes was induced within 30 minutes (mean ET >50 seconds) by sera from the ARDS group compared with both the healthy subjects and the ACPE groups (mean ET <1 second) (p < 0.05). Systematic measurements with the THP-1 cell line allowed for the establishment of a strong correlation between stiffening and the severity of respiratory status (mean ET 0.82 ± 0.08 seconds for healthy subjects, 1.6 ± 1.0 seconds for ACPE groups, 10.5 ± 6.1 seconds for mild ARDS, and 20.0 ± 8.1 seconds for moderate to severe ARDS; p < 0.05). Stiffening correlated with the cytokines interleukin IL-1β, IL-8, tumor necrosis factor TNF-α, and IL-10 but not with interferon-γ, transforming growth factor-β, IL-6, or IL-17. Strong stiffening was induced by IL-1β, IL-8, and TNF-α but not by IL-10, and incubations with sera and blocking antibodies against IL-1β, IL-8, or TNF-α significantly diminished the stiffening effect of serum. In contrast, the measurements of integrin expression (CD11b, CD11a, CD18, CD49d) and leukocyte-endothelium adhesion showed a weak and slow response after incubation with the sera of patients with ARDS (several hours), suggesting a lesser role of leukocyte adhesiveness compared with leukocyte stiffness in early ARDS.Conclusions: The leukocyte stiffening induced by cytokines in the sera of patients might play a role in the sequestration of leukocytes in the lung capillary beds during early ARDS. The inhibition of leukocyte stiffening with blocking antibodies might inspire future therapeutic strategies. [ABSTRACT FROM AUTHOR]