Soap Film Transfer Printing for Ultrathin Electronics.

Flexible and stretchable electronics have attractive applications inaccessible to conventional rigid electronics. However, the mainstream transfer printing techniques have challenges for electronic films in terms of thickness and size and limitations for target substrates in terms of curvature, depth, and interfacial adhesion. Here a facile, damage-free, and contamination-free soap film transfer printing technique is reported that enables the wrinkle-free transfer of ultrathin electronic films, precise alignment in a transparent manner, and conformal and adhesion-independent printing onto various substrates, including those too topographically and adhesively challenging by existing methods. In principle, not only the pattern, resolution, and thickness of transferred films, but also the curvature, depth, and adhesion of target substrates are unlimited, while the size of transferred films can be as high as meter-scale. To demonstrate the capabilities of soap film transfer printing, pre-fabricated ultrathin electronics with multiple patterns, single micron resolution, sub-micron thickness, and centimeter size are conformably integrated onto the ultrathin web, ultra-soft cotton, DVD-R disk with the minimum radius of curvature of 131 nm, interior cavity of Klein bottle and dandelion with ultralow adhesion. The printed ultrathin sensors show superior conformabilities and robust adhesion, leading to engineering opportunities including electrocardiogram (ECG) signal acquisition and temperature measurement in aqueous environments.

Wetting state transitions of individual condensed droplets on pillared textured surfaces.

The ability to realize the self-removal of condensed droplets from a surface is of critical importance for science and applications such as water harvesting and thermal engineering. Despite the enormous interest in micro/nanotextured superhydrophobic materials for high-efficiency condensation, a clear picture of the wetting state transition of condensed droplets is missing, particularly, on a single-droplet level of the order of micrometers. Herein, by varying a substantial parameter space of the contact angle and the geometry of the pillared textures, we have quantified the wetting transition of individual droplets during condensation. We found that a droplet is finally either spontaneously removed from the textures due to a Laplace pressure difference or wets the textures; four different wetting state transition modes have been identified numerically and they are classified in a phase diagram. Simple theories have been constructed to correlate the critical conditions of the wetting state transition to the wettability and geometry of the textures, and they were verified experimentally. We found that the self-removal of condensed droplets benefits from the contact angle and the height of the pillars. These findings not only enhance our fundamental understanding of the wetting state transition of condensed droplets but also allow the rational design of micro/nanotextured water-repellent materials for anti-fogging and anti-wetting.

Impact Forces of Water Drops Falling on Superhydrophobic Surfaces.

A falling liquid drop, after impact on a rigid substrate, deforms and spreads, owing to the normal reaction force. Subsequently, if the substrate is nonwetting, the drop retracts and then jumps off. As we show here, not only is the impact itself associated with a distinct peak in the temporal evolution of the normal force, but also the jump-off, which was hitherto unknown. We characterize both peaks and elucidate how they relate to the different stages of the drop impact process. The time at which the second peak appears coincides with the formation of a Worthington jet, emerging through flow focusing. Even low-velocity impacts can lead to a surprisingly high second peak in the normal force, even larger than the first one, namely when the Worthington jet becomes singular due to the collapse of an air cavity in the drop.

Experimental and theoretical studies on the dynamic landing of water striders on water.

As a species of insects living on water, water striders jump from the water surface to avoid predation and then steadily land without piercing the surface. This spectacular property has attracted extensive interests since it provides bio-inspirations for designing functional microrobots moving on water. In this work, we investigate the landing dynamics of water striders by using artificial striders with different masses and leg lengths. It is found that once a water strider has landed, it oscillates on the water surface and the amplitude decays gradually, triggering a sequence of surface waves. Through scaling analysis, we relate the depth of the dimple that the strider leg displaces to its landing velocity, as well as its leg length and body mass. The subsequent time evolution of the interface where the strider lands is modeled as a damped oscillator, and its energy is exhausted by the surface waves. Moreover, we discuss the maximum depth of the dimple excited by the landing and find that the dynamic process can store more energy than the statically deforming process. Finally, we put forward a criterion of piercing the water surface from the energy point of view. These findings should be of great importance for understanding the locomotion of insects on water and for designing robust water-walking bionic robots.

Horizontal Motion of a Superhydrophobic Substrate Affects the Drop Bouncing Dynamics.

While the drop impact dynamics on stationary surfaces has been widely studied, the way a drop impacts a moving solid is by far less known. Here, we report the physical mechanisms of water drops impacting on superhydrophobic surfaces with horizontal motions. We find that a viscous force is created due to the entrainment of a thin air layer between the liquid and solid interfaces, which competes with the capillary and inertia forces, leading to an asymmetric elongation of the drop and an unexpected contact time reduction. Our experimental and theoretical results uncover consolidated scaling relations: the maximum spreading diameter is controlled by both the Weber and capillary numbers D_{max}/D_{0}∼We^{1/4}Ca^{1/6}, while the dimensionless contact time depends on the capillary number τ/τ_{0}∼Ca^{-1/6}. These findings strengthen our fundamental understandings of interactions between drops and moving solids and open up new opportunities for controlling the preferred water repellency through largely unexplored active approaches.

Directional Droplet Transport Mediated by Circular Groove Arrays. Part II: Theory of Effect.

In the first part of this research, we reported the experimental study of the drop impact on the superhydrophobic circular groove arrays, which resulted in a directional droplet transport. In the second part, we further explored the influence of the Weber number (We), ridge height (H), and the deviation distance (r) between the impacting point and the center of curvature on the lateral offset distance (ΔL) of bouncing drops. The suggested theoretical analysis is in reasonable agreement with the experimental observations. We demonstrate that a Cassie-Wenzel wetting transition occurred within the microstructures of the relief under the threshold Weber number, for example, We ≅ 19-25, which switched the nature of drop bouncing. The dynamic pressure plays a decisive role in the directional droplet transport. The reported investigation may shed light on the solid-liquid interactions occurring on the patterned hierarchical surfaces and open up new opportunities for directional droplet transportation.

Wetting of a liquid annulus in a capillary tube.

In this paper, we systematically investigate the static wetting behavior of a liquid ring in a cylindrical capillary tube. We obtain analytical solutions of the axisymmetric Young-Laplace equation for arbitrary contact angles. We find that, for specific values of the contact angle and the volume of the liquid ring, two solutions of the Young-Laplace equation exist, but only the one with the lower value of the total interfacial energy corresponds to a stable configuration. Based on a numerical scheme determining configurations with a local minimum of the interfacial energy, we also discuss the stability limit between axisymmetric rings and non-axisymmetric configurations. Beyond the stable regime, a liquid plug or a sessile droplet exists instead of a liquid ring, depending on the values of the liquid volume and the contact angle. The stability limit is characterized by specific critical parameters such as the liquid volume, throat diameter, etc. The results are presented in terms of a map showing the different stable liquid morphologies that are obtained from an axisymmetric ring as base state.

Effects of Geometric Confinement on Zero-Gravity Droplets between Two Parallel Planes.

Investigation of geometric effects on confined droplets is important for the fundamental understanding of the ubiquitous wetting phenomena in nature as well as for a variety of practical applications. In this study, the effect of geometric confinement on the wetting behavior of a droplet confined between two parallel rigid planes was investigated. The closed solutions of the Young-Laplace equation were derived through an analytical method. These solutions are applicable for arbitrary values of the contact angles and the ratio of the size of the droplet to the separation of the planes. For completely nonwetting and wetting cases, an asymptotic method was employed, and the sophisticated analytical solutions of the Laplace pressure, droplet volume, surface energy, and capillary force were expressed as functions of the size and the separation of the droplet in a simple way. In order to check the applicability of the results, experiments were designed to counteract the gravity of the droplets. The asymptotic results not only quantitatively agree well with the theoretical and experimental results over a large range of the parameter space, but also provide a straightforward view for reflecting the effect of the geometric confinement on the wetting behaviors of droplets.

Directional Droplet Transport Mediated by Circular Groove Arrays. Part I: Experimental Findings.

Directional transport of liquid droplets is crucial for various applications including water harvesting, anti-icing, and condensation heat transfer. Here, bouncing of water droplets with patterned superhydrophobic surfaces composed of circular equidistant grooves was studied. The directional transport of droplets toward the pole of the grooves was observed. The impact of the Weber number, initial polar distance r, and geometrical parameters of the surface on the directional droplet bouncing was experimentally explored. The nature of bouncing was switched when the Weber numbers exceeded We ≅ 20-25. The rebouncing height was slightly dependent on the initial polar coordinate of the impact point for a fixed We, whereas it grew for We > 20. The weak dependence of the droplet spreading time on the Weber number was close to the dependence predicted by the Hertz bouncing, thus evidencing the negligible influence of viscosity in the process. Change in the scaling exponent describing the dependence of the normalized spreading time on the Weber number was registered for We ≅ 25. The universal dependence of the offset distance ΔL of the droplets on the Weber number ΔL/D0 ∼ We1.5 was established. The normalized offset distance decreased with the normalized initial polar distance as ΔL/D0 ∼ (r/S)-1, where D0 and S are the droplet diameter and groove width, respectively. This research may yield more insights into droplet bouncing on patterned surfaces and offer more options in directed droplet transportation.

Temperature-regulated adhesion of impacting drops on nano/microtextured monostable superrepellent surfaces.

The monostable Cassie state is a favorable wetting state for superhydrophobic materials, in which water drops can automatically transfer from the Wenzel wetting state to the Cassie wetting state, such that as a consequence the water repellency can be maintained. Drop impact phenomena are ubiquitous in nature and of critical importance in industry, and previous works show that the efficiency of self-cleaning and dropwise condensation could benefit from drop impact on monostable surfaces. However, whether such a feature is sufficiently robust remains unclear when the temperature of the surface is taken into consideration. Here, we report that there exists a lower bound of the temperature of the surface, under which a transition from the Cassie wetting state to the Wenzel wetting state arises. By varying the temperature of the surface, it is found that the solid-liquid wetting region could be regulated. Based on thermodynamics, we propose a model to predict the controllable wetting region, and we show that the gradual transition of the wetting state is a result of the accumulation of droplets on the nanoscale. Connections between the dynamics occurring at the solid-liquid interfaces on the microscale and the condensation occurring in the nanotextures are constructed. These results deepen our understanding of the breakdown of superhydrophobicity under dynamic impinging in high humidity. Moreover, this study will shed new light on the applications for controllable liquid deposition and surface decoration, such as catalysts on the superhydrophobic surfaces.

Monostable superrepellent materials.

Superrepellency is an extreme situation where liquids stay at the tops of rough surfaces, in the so-called Cassie state. Owing to the dramatic reduction of solid/liquid contact, such states lead to many applications, such as antifouling, droplet manipulation, hydrodynamic slip, and self-cleaning. However, superrepellency is often destroyed by impalement transitions triggered by environmental disturbances whereas inverse transitions are not observed without energy input. Here we show through controlled experiments the existence of a "monostable" region in the phase space of surface chemistry and roughness, where transitions from Cassie to (impaled) Wenzel states become spontaneously reversible. We establish the condition for observing monostability, which might guide further design and engineering of robust superrepellent materials.

Controlling the Trajectories of Nano/Micro Particles Using Light-Actuated Marangoni Flow.

The ability to manipulate small objects and to produce patterns on the nano- and microscale is of great importance, both with respect to fundamentals and technological applications. The manipulation of particles with diameters of the order of 100 nm or below is a challenge because of their Brownian motion but also because of the scaling behavior of methods such as optical trapping. The unification of optical and hydrodynamic forces is a potential route toward the manipulation of tiny objects. Herein we demonstrate the trapping and manipulation of nano- and microparticles based on interfacial flows controlled by visible light, a method we denote as "Light-Actuated Marangoni Tweezer (LAMT)". We experimentally study the manipulation of particles having diameters ranging from 20 nm to 10 µm, including quantum dots and polystyrene nano/microparticles. The particles can be manipulated by scanning a light beam along a liquid surface. In this way, we are able to define almost arbitrary particle trajectories, for example, in the form of letters. In addition, we are able to handle a number of particles in parallel by creating an optical "landscape" consisting of a multitude of laser spots. The inherent advantages of LAMTs are the linear scaling of the trapping force with the particle diameter and the fact that the force is less dependent on particle properties than in the case of conventional methods.

Small degree of anisotropic wetting on self-similar hierarchical wrinkled surfaces.

We studied the wetting behavior of multiscale self-similar hierarchical wrinkled surfaces. The hierarchical surface was fabricated on poly(dimethylsiloxane) (PDMS) substrates by manipulating the sequential strain release and combined plasma/ultraviolet ozone (UVO) treatment. The generated structured surface shows an independently controlled dual-scale roughness with level-1 small-wavelength wrinkles (wavelength of 700-1500 nm and amplitude of 50-500 nm) resting on level-2 large-wavelength wrinkles (wavelength of 15-35 µm and amplitude of 3.5-5 µm), as well as accompanying orthogonal cracks. By tuning the aspect ratio of hierarchical wrinkles, the degree of wetting anisotropy in hierarchical wrinkled surfaces, defined as the contact angle difference between the parallel and perpendicular directions to the wrinkle grooves, is found to change between 3° and 9°. Through both experimental characterization (confocal fluorescence imaging) and theoretical analyses, we showed that the wetting state in the hierarchical wrinkled surface is in the Wenzel wetting state. We found that the measured apparent contact angle is larger than the theoretically predicted Wenzel contact angle, which is found to be attributed to the three-phase contact line pinning effect of both wrinkles and cracks that generates energetic barriers during the contact line motion. This is evidenced by the observed sudden drop of over 20° in the static contact angles along both perpendicular and parallel directions after slight vibration perturbation. Finally, we concluded that the observed small degree of wetting anisotropy in the hierarchical wrinkled surfaces mainly arises from the competition between orthogonal wrinkles and cracks in the contact line pinning.

Sunlight-Driven Water Transport via a Reconfigurable Pump.

Fast and controllable water transport in microchannels has implications for many applications. A combination of stimuli-responsive asymmetrical changes in the geometry and gradient in the surface wettability offers the possibility to accelerate the transport and realize controllability. Herein, we introduce a meters-long sunlight-powered reconfigurable water pump constructed by tubular poly(dimethylsiloxane) (PDMS) premixed with chemically reduced graphene oxide (rGO), in which the inner wall is modified with thermal-sensitive poly(N-isopropylacrylamide) hydrogel (PNIPAm). This sunlight-powered water pump delivers a record-high advance speed of 1.5 mm s-1 and 13.6 kg h-1 m-2 under 1.5 sun. Theoretical and experimental results reveal that the remarkable performance results from the synergistic effect of the contact-angle gradient arising from the reversible hydrophilic/hydrophobic switch of PNIPAm and the capillary force arising from the geometric deformation of the microchannel.

Departure of condensation droplets on superhydrophobic surfaces.

This article focuses on the departure of multidroplet coalescence on a superhydrophobic surface with nanoscale roughness. Out-of-plane jumping events triggered by multidroplet coalescence and a single fallen droplet are observed. Experimental data show that the departure of droplets due to the multidroplets coalescence and the jumping modes is dominant for the removal of condensed droplets from the substrate. The energy barrier is easier to overcome and the critical size of the self-propelled droplets could be further decreased in multidroplet coalescence jumping mode. A general theoretical model is developed which accounts quantitatively for determining the jumping velocity and the critical size of the multidroplet coalescence.

From coffee rings to coffee eyes.

We discuss how the stain left after evaporation of a suspension evolves with heating of the glass or plastic on which the liquid has been deposited. Upon increasing the substrate temperature, it is found that the stain gradually changes from the usually observed ring to an "eye" shape, that is, a combination of the thick central stain and the thin outer ring. Both the size and the relative volume of the central stain increase with the substrate temperature. The main mechanism for this phenomenon is proposed to be an enhanced Marangoni recirculation flow on hot substrates. These findings can be exploited to continuously tune the morphology of coffee stains, with potential applications in self-assembly and ink-jet printing.

Substrate curvature gradient drives rapid droplet motion.

Making small liquid droplets move spontaneously on solid surfaces is a key challenge in lab-on-chip and heat exchanger technologies. Here, we report that a substrate curvature gradient can accelerate micro- and nanodroplets to high speeds on both hydrophilic and hydrophobic substrates. Experiments for microscale water droplets on tapered surfaces show a maximum speed of 0.42 m/s, 2 orders of magnitude higher than with a wettability gradient. We show that the total free energy and driving force exerted on a droplet are determined by the substrate curvature and substrate curvature gradient, respectively. Using molecular dynamics simulations, we predict nanoscale droplets moving spontaneously at over 100 m/s on tapered surfaces.

Wetting of graphene oxide: a molecular dynamics study.

We characterize the wetting properties of graphene oxide by performing classical molecular dynamics simulations. With oxygen-containing functional groups on the basal plane, graphene becomes hydrophilic and the water contact angle decreases with their concentration, c. The concentration dependence displays a transition at c ≈ 11% as defined by the interacting range of hydrogen bonds with oxidized groups and water. Patterns of the oxidized region and the morphological corrugation of the sheet strongly influence the spreading of water droplets with their lateral spans defined by corresponding geometrical parameters and thus can be used to control their behavior on the surface. These results are discussed with respect to relevant applications in graphene oxide-derived functional materials and offer a fundamental understanding of their wetting and flow phenomena.

Ice Adhesion Properties on Micropillared Superhydrophobic Surfaces.

In this work, we investigate the freezing behavior and ice adhesion properties of sessile drops on micropillared superhydrophobic surfaces (SHSs) with various sizes, which are of practical importance for anti/deicing. First of all, it is demonstrated that the recalescence is related only to the supercooling degree of drops but not to the geometrical parameters of micropillars. The freezing time of sessile drops first increases and then decreases with the area fraction of the SHSs, which demonstrates the nonmonotonic dependence of the icing time on the area fraction. Moreover, the influence of the geometrical parameters of the micropillars on the ice adhesion is discussed. With the decrease of the substrate temperature, the wetting state of the adhesive ice can be transformed from the Cassie ice to the Wenzel ice. For the Cassie ice, the adhesive force is proportional to the area fraction of the SHSs. Interestingly, experimental results show that there exist two interfacial debonding modes of the Wenzel ice: translational debonding and rotational debonding. Furthermore, it is found that the rotational debonding mode contributes to a much lower adhesive force between the ice and the micropillared surface compared to that of the translational debonding mode. By analyzing the critical interfacial energy release rate of the two modes, we deduce the threshold between the two modes, which is quantified as the geometrical parameters of the micropillars. In addition, quantitative relations between the geometrical parameters and the adhesion strengths of the two modes are also obtained. We envision that this work would shed new light on the design optimization of anti/deicing materials.

Droplet detachment by air flow for microstructured superhydrophobic surfaces.

Quantitative correlation between critical air velocity and roughness of microstructured surface has still not been established systematically until the present; the dynamics of water droplet detachment by air flow from micropillar-like superhydrophobic surfaces is investigated by combining experiments and simulation comparisons. Experimental evidence demonstrates that the onset of water droplet detachment from horizontal micropillar-like superhydrophobic surfaces under air flow always starts with detachment of the rear contact lines of the droplets from the pillar tops, which exhibits a similar dynamic mechanism for water droplet motion under a gravity field. On the basis of theoretical analysis and numerical simulation, an explicit analytical model is proposed for investigating the detaching mechanism, in which the critical air velocity can be fully determined by several intrinsic parameters: water-solid interface area fraction, droplet volume, and Young's contact angle. This model gives predictions of the critical detachment velocity of air flow that agree well with the experimental measurements.