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[Display omitted] Water wetting induced corrosion is the core issue for uncovering the corrosion mechanism in multiphase flow environments, relevant to many industrial applications. Here, we experimentally investigated the dynamic failure of an oil film attached on the pre-wetted model surfaces by the electrochemical current detection using an "Alternate Wetting Cell" and the direct visualization of near-wall fluid states. The oil pre-wetted surface performed a superior corrosion mitigation efficiency, exhibiting a protective oil film with a duration time at least 5 times longer than the water pre-wetted surface. It confirms that the oil film rupture is a combined process of the local penetration and pinning of micro-droplets and the phase redistribution of the near-wall fluids. Corrosion finally initiates and propagates on the surface once the droplets pin there or damage the oil film. The result suggests new control strategies for materials corrosion in complex systems by surface modification and fluid management. [ABSTRACT FROM AUTHOR]

Microdroplets are a class of soft matter that has been extensively employed for chemical, biochemical, and industrial applications. However, fabricating microdroplets with largely controllable contact-area shape and apparent contact angle, a key prerequisite for their applications, is still a challenge. Here, by engineering a type of surface with homocentric closed-loop microwalls/microchannels, we can achieve facile size, shape, and contact-angle tunability of microdroplets on the textured surfaces by design. More importantly, this class of surface topologies (with universal genus value = 1) allows us to reveal that the conventional Gibbs equation (widely used for assessing the edge effect on the apparent contact angle of macrodroplets) seems no longer applicable for water microdroplets or nanodroplets (evidenced by independent molecular dynamics simulations). Notably, for the flat surface with the intrinsic contact angle ~0°, we find that the critical contact angle on the microtextured counterparts (at edge angle 90°) can be as large as >130°, rather than 90° according to the Gibbs equation. Experiments show that the breakdown of the Gibbs equation occurs for microdroplets of different types of liquids including alcohol and hydrocarbon oils. Overall, the microtextured surface design and topological wetting states not only offer opportunities for diverse applications of microdroplets such as controllable chemical reactions and low-cost circuit fabrications but also provide testbeds for advancing the fundamental surface science of wetting beyond the Gibbs equation. [ABSTRACT FROM AUTHOR]

Open microfluidic channels, which capture and transport liquid on flat substrate surfaces through wettability contrast, offer a promising platform for the analysis, mixing, separation, etc. of small‐volume liquid samples. This study focuses on the development of open microfluidic channels featuring oleophilic/oleophobic micropatterning in a hyperbranched, space‐filling structure, thereby creating high‐density channels. These newly designed channels allow for the swift collection of microdroplets with low surface tension within less than 10 s through hierarchical transport and merging of the microdroplets within the channel. The collection efficiency reaches 66% in the channel with the optimized channel geometry. These results underscore the effectiveness of this method for the collection of low surface tension microdroplets, paving the way for the advancement in the domain of open microfluidics of oil, a field that has yet to be fully explored. [ABSTRACT FROM AUTHOR]

Digital microfluidics (DMF) is a technique for actively dispensing and manipulating discrete microdroplets on hydrophobic surfaces. DMF devices based on electrowetting on medium (EWOD) effect are widely used due to their advantages of fast response speed, simple system and high accuracy. Microfluidics is important for sweat sensing because it minimizes sweat pollution and evaporation of sweat on the skin. However, the current microfluidic channel has some problems, such as the old and new sweat is easy to mix in the channel, the self‐driving efficiency is too low, and the small sweat volume cannot be detected in time. Digital microfluidics can accurately transmit small sweats, avoid sweat residue in the channel, and have high voltage drive efficiency. Therefore, in this paper, we will use DMF digital microfluidic array technology to realize the collection of trace sweat, and realize the collection and detection of micro‐nano fabrication technology—photolithography. In this way, it is a microfluidic sweat sensor that realizes efficient transmission, trace detection, and intelligent monitoring, and is expected to be applied in wearable sweat sensing. [ABSTRACT FROM AUTHOR]

[Display omitted] Droplet spreading governs various daily phenomena and industrial processes. Insights about microdroplet spreading are limited due to experimental difficulties arising from microdroplet manipulation and substrate wettability control. For droplet sizes approaching the capillary length scale, the gravitational force plays an important role in spreading. In contrast, capillary and viscous forces dominate as the droplet size reduces to smaller length scales. We hypothesize that the dynamic spreading behavior of microdroplets whose radius is far lower than the capillary length differs substantially from established and well understood dynamics. To systematically investigate the spreading dynamics of microdroplets, we develop contact-initiated wetting techniques combined with structuring-independent wettability control to achieve microdroplet (<500 μm) spreading on arbitrary surfaces while eliminating parasitic pinning effects (pining force ∼ 0) and initial impact momentum effects (Weber number ∼ 0). Our experiments reveal that the capillary-driven initial spreading of microdroplets is shorter, with significantly reduced oscillation dampening, when compared to millimeter-scale droplets. Furthermore, spreading along with capillary wave propagation results in coupling between the spreading velocity and dynamic contact angle at the contact line. These findings, along with our proposed microdroplet manipulation platform, may find application in microscale heat transfer, advanced manufacturing, and aerosol transmission studies. [ABSTRACT FROM AUTHOR]

This study experimentally investigated the evaporation and wetting transition behavior of fakir droplets on five different microstructured surfaces. Diamond-like carbon was introduced as the substrate, and the influence of varying the width, height, and pitch of the micropillars was assessed. The experimental results showed that the interfacial properties of the surfaces change the evaporation behavior and the starting point of the wetting transition. An important result of this study is the demonstration of a slippery superhydrophobic surface with low depinning force that suppresses the transition from the Cassie–Baxter state to the Wenzel state for microdroplets less than 0.37 mm in diameter, without employing large pillar height or multiscale roughness. By selecting an appropriate pillar pitch and employing tapered micropillars with small pillar widths, the solid–liquid contact at the three-phase contact line was reduced and low depinning forces were obtained. The underlying mechanism by which slippery superhydrophobic surfaces suppress wetting transitions is also discussed. The accuracy of the theoretical models for predicting the critical transition parameters was assessed, and a numerical model was developed in the surface evolver to compute the penetration of the droplet bottom meniscus within the micropillars. [ABSTRACT FROM AUTHOR]

Major advances in molecular detection are being driven by goals associated with the development of methods that are amenable to miniaturization and automation, and that have high sensitivity and low interference. The new detection methods are confronted by many interfacial issues, which when properly addressed can lead to improved performance. One interfacial property, special wettability, can facilitate precise delivery and local enrichment of molecules to sensing elements. This review summarizes applications of unique features of special wettability in molecular detection including (1) chemical and electrochemical reactions in anchored microdroplets on superwetting surfaces, (2) enrichment of analytes and active materials at low contact areas between droplets and superwetting surfaces, (3) complete opposite affinities of superwetting surfaces toward nonpolar/polar solutes and oil/water phases, and (4) directional droplet transportation on asymmetric superwetting surfaces. The challenges and opportunities that exist in design and applications of special wettability in interfacial delivery and enrichment for detection of a few molecules are also discussed. [ABSTRACT FROM AUTHOR]

We develop a numerical framework for simulating the coalescence and jumping of microdroplets on superhydrophobic surfaces. The framework combines the volume of fluid (VOF) method with models for advancing and receding contact angles on a number of superhydrophobic surfaces. We demonstrate the temporal and spatial convergence of the framework and show agreement between our numerical results and other experimental studies. The capillary‐inertial scaling is investigated together with the existence of a cut‐off behaviour frequently observed in the lower size‐range of that regime. We investigate findings in some of the previous studies that the cut‐off behaviour can be attributed to viscosity effects and dissipation due to interaction with surface microstructures. We exemplify specific features related to the jumping process and the corresponding energy budget analysis when microdroplets coalesce and jump. We have tested droplets of a radius as small as 0.5 μm that are still jumping but recorded a decrease in the jumping velocity and the degree of energy conversion compared to the jumping of larger droplets. We argue and prove that strong capillary forces originating from the high curvature oscillations dissipate the energy of the system significantly faster in the case of microdroplets. [ABSTRACT FROM AUTHOR]

The interplay between phase separation and wetting of multicomponent mixtures is ubiquitous in nature and technology and recently gained significant attention across scientific disciplines, due to the discovery of biomolecular condensates. It is well understood that sessile droplets, undergoing phase separation in a static wetting configuration, exhibit microdroplet nucleation at their contact lines, forming an oil ring during later stages. However, very little is known about the dynamic counterpart, when phase separation occurs in a nonequilibrium wetting configuration, i.e., spreading droplets. Here we show that liquid–liquid phase separation strongly couples to the spreading motion of three-phase contact lines. Thus, the classical Cox–Voinov law is not applicable anymore, because phase separation adds an active spreading force beyond the capillary driving. Intriguingly, we observe that spreading starts well before any visible nucleation of microdroplets in the main droplet. Using high-speed ellipsometry, we further demonstrate that the evaporation-induced enrichment, together with surface forces, causes an even earlier nucleation in the wetting precursor film around the droplet, initiating the observed wetting transition. We expect our findings to improve the fundamental understanding of phase separation processes that involve dynamical contact lines and/or surface forces, with implications in a wide range of applications, from oil recovery or inkjet printing to material synthesis and biomolecular condensates. [ABSTRACT FROM AUTHOR]

1. Research is conducted on the central and non-central impact of micron droplets. 2. The CLSVOF method is adopted to track the morphology of micro droplets. 3. There is gas retention phenomenon at the interface between liquid and dust. 4. Spreading-retraction-oscillation phenomenon appears after droplet impact. 5. Contact angle increases, droplets escape from the coal dust surface and move upward. The removal of respirable coal dust particles has been the focus of domestic and foreign research, and the spray dust suppression technology has been adopted for the prevention and control process; it is based on the mechanism whereby liquid droplets impact dust particles. To further study the wetting properties of microdroplets impacting respirable coal dust particles based on kinetics, this study focused on the droplet morphology, and mathematical and physical models of a microdroplet impacting a respirable coal dust particle were established to explore the variation laws of the droplet spreading and wetting under central and non-central impacts, for which the CLSVOF numerical simulation approach was adopted. The results showed the occurrence of gas retention at the phase interface, which affected the droplet wetting characteristics on the coal dust surface, after the contact between the liquid and dust particles. The influence of the impact velocity, particle size ratio, contact angle, and offset distance on the droplet spreading behavior was investigated by comparative analysis of 12 working conditions with different parameters. The droplets can completely wrap the coal dust particles under only one working condition parameter (V = 5m/s,Φ = 2, θ = 90°, L = 0 μm). The microdroplets are extremely difficult to wet respirable coal dust particles and the spreading behavior of the final droplet could be divided into three states: static adsorption, complete encapsulation and permanent escape. Our results can help better understand the wetting relationship between microdroplets and respirable coal dust particles, serving as a basis for dust prevention and control. [Display omitted] [ABSTRACT FROM AUTHOR]

Magnetically responsive surfaces have received increasing attention because of their convenience, immediate response, remote controllability, and well biocompatibility. A facile way is designed to prepare magnetically responsive superhydrophobic surfaces by one‐step spray coating coupled with magnetic induction. These surfaces with responsive properties can find applications in liquids manipulation. The influences of different Fe3O4 content on the surface morphology and wettability are investigated. The unidirectional, bidirectional, random‐path, and inclined surfaces microdroplet manipulation in an active, noncontact, and fast manner is demonstrated on the prepared surfaces. The model of microdroplet manipulation is established, and its mechanism is explained. Moreover, manipulation of different volumes and different types of microdroplets including water droplets and normal saline is realized. Such magnetically responsive superhydrophobic surfaces not only provide new approach for low‐loss droplet manipulation and transportation but also develop new ways for the realization of programmable and intelligent microfluidic systems. [ABSTRACT FROM AUTHOR]

[Display omitted] Roughness is an important parameter in applications where wetting needs to be characterized. Micro-computed tomography is commonly used to characterize wetting in porous media but the main limitation of this approach is the incapacity to identify nanoscale roughness. Atomic force microscopy, AFM, however, has been used to characterize the topography of surfaces down to the molecular scale. Here we investigate the potential of using AFM to characterize wetting behavior at the nanoscale. Droplets of water on cleaved calcite under decane were imaged using quantitative imaging QI atomic force microscopy where a force-distance curve is obtained at every pixel. When the AFM tip passed through the water droplet surface, an attraction was observed due to capillary effects, such that the thickness of the water film was estimated and hence the profile of the droplet obtained. This enables parameters such as the contact angle and contact angle distribution to be obtained at a nanometer scale. The contact angles around the 3-phase contact line are found to be quasi-symmetrically distributed between 10–30°. A correlation between the height profile of the surface and contact angle distribution demonstrates a quasi-proportional relationship between roughness on the calcite surface and contact angle. [ABSTRACT FROM AUTHOR]

Inspired by liquid super‐repellent skin of springtail, air pockets with tiny openings are fabricated on the surface of polydimethylsiloxane (PDMS) to achieve a stable existence of gas between liquid and micro‐nanostructure. After further low‐energy grafting, the PDMS with air pocket microstructures shows a well hydrophobic and oleophobic property. When the liquid droplet fell on the fabricated surface, the positive pressure generated by the air pocket promotes the wetting state to be much fitting with the Cassie–Baxter model. Based on the super‐repellent microstructures, the flexible material shows properties of anti‐icing and anti‐fog. Additionally, the positive pressure of the air pocket to liquid droplet changed to be negative when stretching stress is applied to the PDMS elastomer. Based on the changed pressure, wetting state of the liquid droplet reversed from super‐low adhesion to high adhesion. The stimulus‐responsive material with reversible wettability has great potential in the applications of microfluidic chip and controlled releasing liquid micro‐droplets. [ABSTRACT FROM AUTHOR]

Abstract: Micropatterning of hydrogel has brought innovative outcomes in fundamental and applied material sciences. Previous approaches have mainly been dedicated to fabricate arrays of bulk hydrogel beads, which have inherent challenges including loading ability, scalability, specificity, and versatility. Here, a methodology is presented to create hollow microcapsule arrays from sessile microdroplets. The difference in wettability between hydrophilic and hydrophobic surfaces enables self‐partitioning of liquid into microdroplet arrays, serving as microreservoirs to load complementarily functionalized host–guest polymers, cucurbit[8]uril‐threaded highly branched polyrotaxanes (HBP‐CB[8]) and naphthyl‐functionalized hydroxyethyl cellulose (HEC‐Np). The interfacial dynamic complexation between positively charged HBP‐CB[8] and HEC‐Np occurs in the presence of negatively charged surfactants, resulting in condensed supramolecular hydrogel skins. The hydrogel microcapsules are uniform in size and are developed to encapsulate target cargos in a robust and well‐defined manner. Moreover, the microcapsule substrates are further used for surface enhanced Raman spectroscopy sensing upon loading of gold nanoparticles. This facile assembly of microcapsule arrays has potential applications in controlled cargo delivery, bio‐sensing, high‐throughput analysis, and sorting. [ABSTRACT FROM AUTHOR]

Abstract: Recently, reversible surface superwettability has attracted enormous interest, and methods to shorten the cycle time of transition have also garnered the attention of researchers. Herein, a superhydrophobic, open‐cell graphene network (OCGN) is fabricated via self‐assembly of graphene oxide and vapor ejection. Owing to the special open‐cell microstructure, the OCGNs can be transformed to be superhydrophilic rapidly within only 1 s by air plasma treatment. Moreover, the OCGNs with pure graphene composition have a high conductivity and show an ultrafast Joule heating rate of up to 20 °C s−1 at a voltage of 20 V. By means of this property, for the first time an ultrafast recovery of the superhydrophobicity for OCGNs by self‐induced Joule heating with the shortest time of 1 min is reported. The mechanism of ultrafast, reversible transition is also explored specifically in this study. In addition, the superhydrophilic OCGNs show superoleophobicity in water and their underwater adhesion for oil droplets can be controlled by plasma treatment. Finally, the OCGNs with different oil adhesion properties are fabricated and the underwater oil microdroplet transportation is realized using OCGNs. Therefore, the OCGNs with smart surface can be an excellent candidate for achieving multifunctional superwettability of surfaces. [ABSTRACT FROM AUTHOR]

Binary nanoscale interfacial materials are fundamental issues in many applications for smart surfaces. A binary nanoscale interface with binary surface morphology and binary wetting behaviors has been prepared by a facile wet-chemical method. The prepared surface presents superhydrophobicity and high adhesion with the droplet at the same time. The composition, surface morphology, and wetting behaviors of the prepared surface have been systematic studied. The special wetting behaviors can be contributed to the binary nanoscale effect. The stability of the prepared surface was also investigated. As a primary application, a facile device based on the prepared binary nanoscale interface with superhydrophobicity and high adhesion was constructed for microdroplet transportation. [ABSTRACT FROM AUTHOR]

Driving microfluidic flow in micropixels by electrowetting to realize light switches and displays is of both practical and fundamental significance. The electro-optical performance related to microfluidic behavior needs to be clarified to optimize device functions. In this article, the microfluidic performance in electrowetting display devices was categorized according to the oil-water interface shape and response. The oil film movement was divided into vertically "thinning" and transversally "opening," for which the "thinning" process was found the key factor determining the pixel switching speed rather than the "opening" process. Therefore, the breakup point and the oil film thickness were critical, which could be controlled by surface wettability and oil volume. We have also realized a new oil filling method with controllable dosing volume assisted by the microfluidic creation of microdroplets. This study could help quantitatively understand electrowetting display performance in both its theoretical and practical aspects. [ABSTRACT FROM AUTHOR]

We demonstrate real-time quantitativephase imaging as a new opticalapproach for measuring the evaporation dynamics of sessile microdroplets.Quantitative phase images of various droplets were captured duringevaporation. The images enabled us to generate time-resolved three-dimensionaltopographic profiles of droplet shape with nanometer accuracy and,without any assumptions about droplet geometry, to directly measureimportant physical parameters that characterize surface wetting processes.Specifically, the time-dependent variation of the droplet height,volume, contact radius, contact angle distribution along the droplet’sperimeter, and mass flux density for two different surface preparationsare reported. The studies clearly demonstrate three phases of evaporationreported previously: pinned, depinned, and drying modes; the studiesalso reveal instances of partial pinning. Finally, the apparatus isemployed to investigate the cooperative evaporation of the sprayeddroplets. We observe and explain the neighbor-induced reduction inevaporation rate, that is, as compared to predictions for isolateddroplets. In the future, the new experimental methods should stimulatethe exploration of colloidal particle dynamics on the gas–liquid–solidinterface. [ABSTRACT FROM AUTHOR]

Microfluidic systems are often dominated by their surfaces because of the high surface area to volume ratios in microchannel flows or drop-based systems. Here we highlight recent work on engineering and exploiting surface effects to control the formation and motion of microdrops. We highlight work using precisely microstructured wetting surfaces to repel all manner of liquids even when the liquid–air surface tension is low. In a second paper, selective capillary filling and draining is used to pattern liquid and cell-laden gels for 3D culture. A final paper making use of vapor-driven surface tension effects to drive the motion of drop ensembles is also examined, exploring a new mechanism for drop control – including motion and merging. Surface-driven motion and patterning has been a widely successful area in microfluidics (e.g. electrowetting or patterned self-assembled monolayers) and recent work is extending into new directions that, once well-understood, should enable new applications. [ABSTRACT FROM AUTHOR]

The article discusses the study of fabricating the biomimetic polymer nanocone films with condensate microdrop self-removal (CMDSR) function. Topics discussed include the sharpening of the nanonipples for the desired CMDSR function, tailoring biomimetic polymer nanocones through controllable chemical etching and understanding the surface wettability difference among species of bio-samples.

The autonomous and directional transport of microdroplets is critical to the development of self-powered droplet-based microfluidic devices. So far the studies on the self-transport of droplets have employed chemically inhomogeneous surfaces. Remarkably, we observed the self-propulsion of aqueous droplets on a chemically homogeneous superhydrophilic surface. The self-transport phenomenon is attributed to the existence of a precursor layer between droplet and surface that minimizes contact line pinning and droplet is driven by a net force owing to the asymmetry in the precursor layer width across droplet. We provide a theoretical model considering the electrostatic, capillary, and evaporation induced forces to quantitatively predict transport velocity which is in good agreement with experimental observations. The effect of droplet volume, surface roughness, relative humidity, and composition on the droplet velocity is studied. We demonstrated the directional self-transport and coalescence of droplets on a superhydrophilic track in a superhydrophobic background and the self-transport of droplets containing biological cells depicting biological cargos. [Display omitted] [ABSTRACT FROM AUTHOR]

More than one century ago, Lippmann found that capillary forces can be effectively controlled by external electrostatic forces. As a simple example, by applying a voltage between a conducting liquid droplet and the surface it is sitting on we are able to adjust the wetting angle of the drop. Since Lippmann's findings, electrocapillary phenomena - or electrowetting - have developed into a series of tools for manipulating microdroplets on solid surfaces, or small amounts of liquids in capillaries for microfluidic applications. In this article, we briefly review some recent progress of fundamental understanding of electrowetting and address some still unsolved issues. Specifically, we focus on static and dynamic electrowetting. In static electrowetting, we discuss some basic phenomena found in DC and AC electrowetting, and some theories about the origin of contact angle saturation. In dynamic electrowetting, we introduce some studies about this rather recent area. At last, we address some other capillary phenomena governed by electrostatics and we give an outlook that might stimulate further investigations on electrowetting. [ABSTRACT FROM AUTHOR]

Highlights: [•] Imbibition of model porous media by sessile droplets of ethanol/water mixtures. [•] Model porous media based on polystyrene microbeads assembly. [•] Imbibition kinetics following a linear time regime over seconds. [•] Hypothesis of a prewetting by condensation of ethanol in the throats. [ABSTRACT FROM AUTHOR]

The dynamic behaviors of microdroplets that impact on textured surfaces with various patterns of microscale pillars are experimentally investigated in this study. A piezoelectric inkjet is used to generate the microdroplets that have a diameter of less than 46 μm and a controlled Weber number. The impact and spreading dynamics of an individual droplet are captured by using a high-speed imaging system. The anisotropic and directional wettability and the wetting states on the textured surfaces with anisotropically arranged pillars are revealed for the first time in this study. The impalement transition from the Cassie-Baxter state to the partially impaled state is evaluated by balancing the wetting pressure P and the capillary pressure P even on the anisotropic textured surfaces. The maximum spreading factor is measured and compared with the theoretical prediction to elucidate the wettability of the textured surfaces. For a given Weber number, the maximum spreading factor decreases as the texture area fraction of the textured surface decreases. In addition, the maximum spreading factors along the direction of longer inter-pillar spacing always have smaller values than those along the direction of shorter inter-pillar spacing when a droplet impacts on the anisotropic arrays of pillars. [ABSTRACT FROM AUTHOR]

Electrowetting-on-dielectric (EWOD), in which microdroplets are manipulated using electrical inputs, has drawn a great deal of attraction with applications of digital lab-on-a-chip and hot-spot cooling. In most EWOD actuations, the commonly used powering method is wired transmission, which may not be suitable for isolating and employing EWOD devices in hard-to-reach areas. In this study, we investigate wireless power transmission for EWOD utilizing inductive coupling. Since EWOD is typically operated by a high-input voltage although the current is minimal, wireless EWOD also requires a similarly high voltage at the receiver, unlike conventional inductive coupling. To meet this condition, the resonant inductive coupling method at a high resonant frequency is introduced and investigated. To optimize the transmission efficiency, we study the effects of many parameters, such as the frequency, inductance, and capacitance at the transmitter and receiver, the gap between the transmitter coil and receiver coil, and the droplet size, by measuring the voltage at the receiver and the contact angle of the droplet placed on a wirelessly operated EWOD chip. In addition, by applying amplitude modulation to the resonant inductive coupling, wireless AC-EWOD, which generates droplet oscillations and is a common mode for EWOD droplet handling, is also achieved. Finally, it is successfully demonstrated that a droplet is transported laterally by using an array of electrodes, which is also powered by an amplitude-modulated wireless signal. [ABSTRACT FROM AUTHOR]

A specific criterion is often used to judge the transitionfromCassie–Baxter wetting state to that of Wenzel. In this work,we examine the applicability of this specific criterion to the caseof micropillars with circular or polygonal cross-sections. For thispurpose, we derive an angle inequality. When this angle inequalityis violated, the specific criterion holds for the corresponding micropillars.Otherwise, the specific criterion may not be applicable to such micropillars.These results arevalidated by pressing tests on six types of micropillars, which havecircular, triangular, square, hexagonal, T-shaped, and star-like cross-sections. [ABSTRACT FROM AUTHOR]

Microdrop impact and spreading phenomena are explored as an interface formation process using a recently developed computational framework. The accuracy of the results obtained from this framework for the simulation of high deformation free-surface flows is confirmed by a comparison with previous numerical studies for the large amplitude oscillations of free liquid drops. Our code's ability to produce high resolution benchmark calculations for dynamic wetting flows is then demonstrated by simulating microdrop impact and spreading on surfaces of greatly differing wettability. The simulations allow one to see features of the process which go beyond the resolution available to experimental analysis. Strong interfacial effects which are observed at the microfluidic scale are then harnessed by designing surfaces of varying wettability that allow new methods of flow control to be developed. [ABSTRACT FROM AUTHOR]

An in situ observation method to understand thermal Marangoni convection in sessile droplets is reported in this paper. Infrared (IR) thermal imaging is applied in the experiments to visualize the transient temperature distribution on the surface of the droplet. Additionally, micro particle image velocimetry (micro-PIV) sys-tem illustrates the fluid field inside the droplet. Two kinds of droplets are considered in this paper: sessile droplets heated with a hotplate and droplets heated by an electrode at the apex. Convection in both droplets is observed in situ, and the experimental results are in accord with the theory of fluid mechanics. [ABSTRACT FROM AUTHOR]

In this paper, the free energy based Lattice Boltzmann Method (LBM) which has been recently extended by the authors for modeling and simulation of Electrowetting (EW) phenomenon is applied to another application of EW, i.e., droplets merging. The results were compared with experimental data and the results show good accuracy of the numerical simulation. Also, the stability of a droplet in a finite system was studied analytically and numerically (by LBM) and it was shown that the droplet stability is a function of system size and droplet contact angle. The simulation results were in good agreement with analytical solution and it has been concluded that in the finite systems an unstable droplet can be stabilized by applying a voltage (i.e., using EW). Based on this observation, a new method for droplets merging in finite systems has been proposed. [ABSTRACT FROM AUTHOR]

This paper describes the effect of the contact angle hysteresis caused by the adhesion between the liquid droplet and the solid surface on the threshold voltage for droplet transport in three different ground-type electrowetting configurations. It was found that the threshold voltage equals the hysteresis voltages in both single-plate and twin-plate configurations and the square root of 2 times the hysteresis voltage in the two-plate configuration by analyzing the contact angles at electrowetting threshold theoretically. The fabricated devices using 2.5 pm thick polyimide (er = 3.3) as the dielectric layer and thin Teflon AF as the hydrophobic layer (Oq = 116° and a = 4-6°) showed the threshold voltages for droplet transport initiation of 28, 40 and 28 V in the single-plate, two-plate and twin-plate devices, respectively, and the threshold voltages for stable droplet transport of 35,50 and 35 V, respectively. All the results are in very good agreement with theoretical values. [ABSTRACT FROM AUTHOR]

In this paper, the dynamic ac electrowetting (EW) behavior of liquid crystal (LC) droplets was investigated on inhomogeneous substrates by inducing LC molecules to orient themselves on a pre-rubbed poiyimide substrate. For the first time, we studied the influence of inhomogeneous molecular orientation on the oscilla-tion response of dielectric droplets during ac EW. The experimental results show that the ac EW oscillation amplitude of LC droplet is also inhomogeneous depending on the polyimide-induced molecular orienta-tions on the substrate. The oscillation amplitude that is parallel to the LC molecular orientation is about ∼40% larger than the oscillation amplitude that is vertical to the LC molecular orientation. The oscillation amplitude on the homogeneous un-rubbed poiyimide substrate falls in between. Meanwhile, the droplet oscillation amplitude decreases as the ac EW driving frequency increases. An analysis of the EW driving force and various resistance forces for dielectric droplet revealed that intermolecular resistance dominates in the inhomogeneous response of LC droplet, i.e., the lower intermolecular friction between parallel-arranged molecules and the higher intermolecular friction between the cross-arranged molecules cause the inhomo-geneous hydrodynamic damping effect on the LC droplet. The findings provide a potential mechanism for optimizing the driving approach for the ac electrowetting of LC droplets by tailoring the substrate and the LC molecular orientation. [ABSTRACT FROM AUTHOR]

A mesoscopic model is developed to investigate the oscillations of a sub-micrometer droplet in AC elec-trowetting based on dissipative particle dynamics. To simulate the effects of the applied AC voltage, we vary the interaction between the solid and liquid particles aiming to recover the contact angles obeying the Lippmann-Young equation. The low frequency flow obtained in the present study is consistent qualitatively with previous experimental measurement. For the intermediate frequency voltage, generally no significant movement is found inside the droplet except fluid oscillatory motion near the contact line. The contact line dynamics is investigated as well, in which the results show the hysteresis phenomenon of contact line movement, and the phase difference between the applied voltage variation and the contact line oscillation generally increases with the AC frequency except for some resonance frequencies. Furthermore, the ampli-tude of the contact line is found to decrease linearly on a logarithmic scale with the applied frequency. [ABSTRACT FROM AUTHOR]

A kind of superhydrophobic magnetic nano/micropillar array (MNA) with optimized intrinsic dynamic wetting property has characteristics of magnetically induced dynamic tilt‐angle changes, which achieves the controlling of the directional droplet transport effectively. It is revealed that MNA has a low adhesive gradient along the direction of magnetically induced tilt‐angles. A tilt‐angles of nano/micropillars in the range of 0°–59° is realized by controlling the magnetic field intensity (0–490 mT), so that a droplet (10 µL) can be transported on MNA from one pillar to the next along the tilt‐angle direction. It is proposed that the continuous changes of magnetically induced dynamic tilt‐angles on MNA induce a gradient driving force to act on the droplet, in addition to cooperation with the low adhesive direction that results from unidirectional gradient discontinuous solid–liquid–gas three phase contact lines. The finding offers insight into designing of surface materials that can be extended into microfluidics for controlling of droplet motion and others. Magnetically induced dynamic tilt‐angle nano/micropillar arrays with low adhesive direction achieve the directional transport of microdroplets. [ABSTRACT FROM AUTHOR]

Fundamental understanding of the wettability of curved substrates is crucial for the applications of microdroplets in colloidal science, microfluidics, and heat exchanger technologies. Here we report via lattice Boltzmann simulations and energetic analysis that microdroplets show an ability of transporting selectively to appropriate substrates solely according to substrate shape (curvature), which is called the substrate-curvature-dependent droplet targeting because of its similarity to protein targeting by which proteins are transported to the appropriate destinations in the cell. Two dynamic pathways of droplet targeting are identified: one is the Ostwald ripening-like liquid transport between separated droplets via evaporating droplets on more curved convex (or less curved concave) surfaces and growing droplets on less curved convex (or more curved concave) surfaces, and the other is the directional motion of a droplet through contacting simultaneously substrates of different curvatures. Then we demonstrate analytically that droplet targeting is a thermodynamically driven process. The driving force for directional motion of droplets is the surface-curvature-induced modulation of the work of adhesion, while the Ostwald ripening-like transport is ascribed to the substrate-curvature-induced change of droplet curvature radius. Our findings of droplet targeting are potentially useful for a tremendous range of applications, such as microfluidics, thermal control, and microfabrication. [ABSTRACT FROM AUTHOR]

While there are intensive studies on the coalescence of sessile macroscale droplets, there is little study on the coalescence of sessile microdroplets. In this paper, the coalescence process of two sessile microdroplets is studied by using a many-body dissipative particle dynamics numerical method. A comprehensive parametric study is conducted to investigate the effects on the coalescence process from the wettability gradient, hydrophilicity of the solid surface, and symmetric or asymmetric configurations. A water bridge is formed after two microdroplets contact. The temporal evolution of the coalescence process is characterized by the water bridge's radii parallel to the solid surface (Wm) and perpendicular to the solid surface (Hm). It is found that the changes of both Hm and Wm with time follow a power law; i.e., Hm=β1τβ and Wm=α1τα. The growth of Hm and Wm depends on the hydrophilicity of the substrate. Wm grows faster than Hm on a hydrophilic surface, and Hm grows faster than Wm on a hydrophobic surface. This is due to the strong competition between capillary forces induced by the water-bridge curvature and the solid substrate hydrophobicity. Also, flow structure analysis shows that regardless of the coalescence type once the liquid bridge is formed the liquid flow direction inside the capillary bridge is to expand the bridge radius. Finally, we do not observe oscillation of the merged droplet during the coalescence process, possibly due to the significant effects of the viscous forces. [ABSTRACT FROM AUTHOR]