Microscopic Origin of Capillary Force Balance at Contact Line

Soft wetting: Substrate softness- and time-dependent droplet/bubble adhesion

HYPOTHESIS

The droplet/bubble adhesion characteristics depend on the length of the droplet/bubble three-phase contact line. Since the deformation caused by the liquid-gas interfacial tension on the soft substrate, referred as to the wetting ridge, retards contact line spreading and retraction, we conjecture that the droplet/bubble adhesion characteristics depend also on the substrate softness.

EXPERIMENTS

Soft substrates with various shear moduli are prepared and characterized by the spreading and receding dynamics of water droplets and underwater bubbles. Snap-in and normal adhesion forces of droplets/bubbles on such soft substrates are directly measured along with the visualized droplet/bubble shape profiles.

FINDINGS

The droplet/bubble snap-in force, which corresponds to the short-time spreading dynamics, decreases with a decrease in the substrate shear modulus because of the retarded contact line spreading. The droplet maximal adhesion force on a soft substrate can be counterintuitively either smaller or larger than its counterpart on the rigid substrate depending on different dwelling times, i.e., the droplet/bubble-substrate contact time before droplet/bubble-substrate separation. The former is attributed to the retarded contact line spreading, whereas the latter is attributed to the retarded contact line retraction. The substrate softness- and dwelling time-dependent droplet/bubble adhesion reported in this study will benefit various applications related to soft substrates.

Soft wetting: an analytical model for pillar topography- and softness-dependent droplet depinning force

The extent to which a droplet pins on a textured substrate is determined by the dynamics of the contact line and the liquid-vapor interface. However, the synergistic contribution of contact line sliding and interface distortion to the droplet depinning force remains unknown. More strikingly, current models fail to predict the depinning force per unit length of droplets on soft pillar arrays. Therefore, we fabricate soft pillar arrays with varying geometrical dimensions and mechanical properties and measure the depinning forces per unit length by allowing droplets to evaporate on such substrates. We then analyze the decrease in excess Gibbs free energy of the apparent droplet caused by the detachment of the droplet boundary from the previously pinned pillars. In contrast to prior notions, based on the measured decreases in excess Gibbs free energy, we find that the coefficient, that governs the ratio of interface distortion's contribution to the depinning force to that of the sliding contact line, increases with a decrease in pillar packing density. By considering the combined contribution from contact line sliding, liquid-vapor interface distortion, and pillar deflection, we introduce an analytical model to predict the droplet depinning force per unit length and corroborate the model using experimental data reported in this and prior studies.

Discretely-supported transfer nanoimprint anti-reflection nanostructures on complex uneven surface of Fresnel lenses

Nanopatterning complex uneven surface of numerous functional devices to improve their performance is significantly appealing; however, it is extremely challenging. This study proposes a discretely-supported transfer nanoimprint technique to fabricate nanostructures on complex device surfaces containing multi-spatial frequencies. First, a discretely-supported nanoimprint template was designed based on the built energy criterion. A contact fidelity of over 99% was achieved between the designed template and the targeted complex uneven substrate surface. Next, the prefilled nanostructures on the template were transferred to the target surface after contact. By precisely controlling the amount of micro-droplet jetting on the template on-demand, the accumulation of the polymer in the micro-valley sites on the complex substrate was avoided, thus maintaining the morphology and generating function of the devices. Finally, high-quality Fresnel lenses with broadband wide-directional antireflection and excellent imaging performance were developed by imprinting subwavelength-tapered nanostructures on the relief surface.

Soft Wetting: Droplet Receding Contact Angles on Soft Superhydrophobic Surfaces

Despite intensive investigations on the droplet receding contact angle on superhydrophobic surfaces, i.e., a key parameter characterizing surface wettability and adhesion, the quantitative correlation between the surface structure mechanical properties (softness) and the droplet receding contact angles remains vague. By systematically varying the geometric dimensions and mechanical properties of soft pillar arrays, we find that the droplet receding contact angles decrease with the decrease in the pillar spring constant. Most surprisingly, the densely packed pillar arrays may result in larger receding contact angles than those on sparsely packed pillars, opposing the understanding of rigid pillar arrays, where the receding contact angles increase with a decrease in the packing density of pillars. This is attributed to the collective effects of capillarity and elasticity, where the energy consumed by the sliding contact line, the energy stored in the distorted liquid-vapor interface, and the energy stored in the deflected pillar contribute to the droplet depinning characteristics. We develop an analytical model to predict the droplet receding contact angles on soft superhydrophobic pillar arrays with knowledge of the material intrinsic receding contact angle, the pillar geometry, and the pillar mechanical properties. The predictions are corroborated by the experimental data measured in this and prior studies.

Theoretical and Three-Dimensional Molecular Dynamics Study of Droplet Wettability and Mobility on Lubricant-Infused Porous Surfaces

Profiting from their slippery nature, lubricant-infused porous surfaces endow with droplets excellent mobility and consequently promise remarkable heat transfer improvement for dropwise condensation. To be a four-phase wetting system, the droplet wettability configurations and the corresponding dynamic characteristics on lubricant-infused porous surfaces are closely related to many factors, such as multiple interfacial interactions, surface features, and lubricant thickness, which keeps a long-standing challenge to promulgate the underlying physics. In this work, thermodynamically theoretical analysis and three-dimensional molecular dynamics simulations with the coarse-grained water and hexane models are carried out to explore droplet wettability and mobility on lubricant-infused porous surfaces. Combined with accessible theoretical criteria, phase diagrams of droplet configurations are constructed with a comprehensive consideration of interfacial interactions, surface structures, and lubricant thickness. Subsequently, droplet sliding and coalescence dynamics are quantitatively defined under different configurations. Finally, in terms of the promotion of dropwise condensation, a non-cloaking configuration with the encapsulated state underneath the droplet is recommended to achieve high droplet mobility owing to the low viscous drag of the lubricant and the eliminated pinning effect of the contact line. On the basis of the low oil-water and water-solid interactions, a stable lubricant layer with a relatively low thickness is suggested to construct slippery surfaces.

Molecular insights into fluid-solid interfacial tensions in water + gas + solid systems at various temperatures and pressures

The fluid-solid interfacial tension is of great importance to many applications including the geological storage of greenhouse gases and enhancing the recovery of geo-resources, but it is rarely studied. Extensive molecular dynamics simulations are conducted to calculate fluid-solid interfacial properties in H2O + gas (H2, N2, CH4, and CO2) + rigid solid three-phase systems at various temperatures (298-403 K), pressures (0-100 MPa), and wettabilities (hydrophilic, neutral, and hydrophobic). Our results on the H2O + solid system show that vapor-solid interfacial tension should not be ignored in cases where the fluid-solid interaction energy is strong or the contact angle is close to 90°. As the temperature rises, the magnitude of H2O's liquid-solid interfacial tension declines because the oscillation of the interfacial density/pressure profile weakens at high temperatures. However, the magnitude of H2O vapor-solid interfacial tension is enhanced with temperature due to the stronger adsorption of H2O. Moreover, the H2O-solid interfacial tension in H2O + gas (H2 or N2) + solid systems is weakly dependent on pressure, while the pressure effects on H2O-solid interfacial tensions in systems with CH4 or CO2 are significant. We show that the assumption of pressure independent H2O-solid interfacial tensions should be cautiously applied to Neumann's method for systems containing non-hydrophilic surfaces with strong gas-solid interaction. Meanwhile, the magnitude of gas-solid interfacial tension increases with pressure and gas-solid interaction. High temperatures generally decrease the magnitude of gas-solid interfacial tensions. Further, we found that the increment of contact angle due to the presence of gases follows this order: H2 < N2 < CH4 < CO2.

Direct observation of spreading precursor liquids in a corner

Precursor liquid is a nanoscale liquid creeping ahead of the macroscopic edge of spreading liquids, whose behaviors tightly correlate with the three-phase reaction efficiency and patterning accuracy. However, the important spatial-temporal characteristic of the precursor liquid still remains obscure because its real-time spreading process has not been directly observed. Here, we report that the spreading ionic liquid precursors in a silicon corner can be directly captured on video using in situ scanning electron microscopy. In situ spreading videos show that the precursor liquid spreads linearly over time ([Formula: see text]) rather than obeying the classic Lucas-Washburn law ([Formula: see text]) and possesses a characteristic width of ∼250-310 nm. Theoretical analyses and molecular dynamics simulations demonstrate that the unique behaviors of precursor liquids originate from the competing effect of van der Waals force and surface energy. These findings provide avenues for directly observing liquid/solid interfacial phenomena on a microscopic level.

Control the hydrophilic layer thickness of Janus membranes by manipulating membrane wetting in membrane distillation

Janus membranes with asymmetric wettability have attracted wide attentions for their robust anti-oil-wetting/fouling abilities in membrane distillation (MD). Compared to traditional surface modification approaches, in this study, we provided a new approach which manipulated surfactant-induced wetting to fabricate Janus membrane with a controllable thickness of the hydrophilic layer. The membranes with 10, 20, and 40 μm of wetted layers were obtained by stopping the wetting induced by 40 mg L^-1 Triton X-100 (J = 25 L m^-2 h^-1) at about 15, 40, and 120 s, respectively. Then, the wetted layers were coated using polydopamine (PDA) to fabricate the Janus membranes. The resulting Janus membranes showed no significant change in porosities or pore size distributions compared with the virgin PVDF membrane. These Janus membranes exhibited low in-air water contact angles (< 50°), high underwater oil contact angles (> 145°), and low adhesion with oil droplets. Therefore, they all showed excellent oil-water separation performance with ∼100% rejection and stable flux. The Janus membranes showed no significant decline in flux, but a trade-off existed between the hydrophilic layer thicknesses and the vapor flux. Utilizing membranes with tunable hydrophilic layer thickness, we elucidated the underlying mechanism of such trade-off in mass transfer. Furthermore, the successful modification of membranes with different coatings and in-situ immobilization of silver nanoparticles indicated that this facile modification method is universal and can be further expanded for multifunctional membrane fabrication.

Investigations into wetting and spreading behaviors of impacting metal droplet under ultrasonic vibration control

Ultrasonic-assisted metal droplet deposition (UAMDD) is currently considered a promising technology in droplet-based 3D printing due to its capability to change the wetting and spreading behaviors at the droplet-substrate interface. However, the involved contact dynamics during impacting droplet deposition, particularly the complex physical interaction and metallurgical reaction of induced wetting-spreading-solidification by the external energy, remain unclear to date, which hinders the quantitative prediction and regulation of the microstructures and bonding property of the UAMDD bumps. Here, the wettability of the impacting metal droplet ejected by a piezoelectric micro-jet device (PMJD) on non-wetting and wetting ultrasonic vibration substrates is studied, and the corresponding spreading diameter, contact angle, and bonding strength are also discussed. For the non-wetting substrate, the wettability of the droplet can be significantly increased due to the extrusion of the vibration substrate and the momentum transfer layer at the droplet-substrate interface. And the wettability of the droplet on a wetting substrate is increased at a lower vibration amplitude, which is driven by the momentum transfer layer and the capillary waves at the liquid-vapor interface. Moreover, the effects of the ultrasonic amplitude on the droplet spreading are studied under the resonant frequency of 18.2-18.4 kHz. Compared to deposit droplets on a static substrate, such UAMDD has 31% and 2.1% increments in the spreading diameters for the non-wetting and wetting systems, and the corresponding adhesion tangential forces are increased by 3.85 and 5.59 times.

Contact Angle Measurement on Curved Wetting Surfaces in Multiphase Lattice Boltzmann Method

Contact angle is an essential physical quantity that characterizes the wettability of a substrate. Although it is widely used in the studies of surface wetting, capillary phenomena, and moving contact lines, the contact angle measurements in simulations and experiments are still complicated and time-consuming. In this paper, we present an efficient scheme for the measurement of contact angle on curved wetting surfaces in lattice Boltzmann simulations. The measuring results are in excellent agreement with the theoretical predictions without considering the gravity effect. A series of simulations with various drop sizes and surface curvatures confirm that the present scheme is grid-independent. Then, the scheme is verified in gravitational environments by simulating the deformations of sessile and pendent droplets on the curved wetting surface. The numerical results are highly consistent with experimental observations and support the theoretical analysis that the microscopic contact angle is independent of gravity. Furthermore, the method utilizes only the microscopic geometry of the contact angle and does not depend on the droplet profile; therefore, it can be applied to nonaxisymmetric shapes or moving contact lines. The scheme is applied to capture the dynamic contact angle hysteresis on homogeneous or chemically heterogeneous curved surfaces. Importantly, the accurate contact angle measurement enables the dynamic mechanical analysis of moving contact lines. The present measurement is simple and efficient and can be extended to implementations in various multiphase lattice Boltzmann models.

Challenges, Prospects, and Emerging Applications of Inkjet-Printed Electronics: A Chemist's Point of View

Driven by the development of new functional inks, inkjet-printed electronics has achieved several milestones upon moving from the integration of simple electronic elements (e.g., temperature and pressure sensors, RFID antennas, etc.) to high-tech applications (e.g. in optoelectronics, energy storage and harvesting, medical diagnosis). Currently, inkjet printing techniques are limited by spatial resolution higher than several micrometers, which sets a redhibitorythreshold for miniaturization and for many applications that require the controlled organization of constituents at the nanometer scale. In this Review, we present the physico-chemical concepts and the equipment constraints underpinning the resolution limit of inkjet printing and describe the contributions from molecular, supramolecular, and nanomaterials-based approaches for their circumvention. Based on these considerations, we propose future trajectories for improving inkjet-printing resolution that will be driven and supported by breakthroughs coming from chemistry. Please check all text carefully as extensive language polishing was necessary. Title ok? Yes.

A Biocompatible Vibration-Actuated Omni-Droplets Rectifier with Large Volume Range Fabricated by Femtosecond Laser

High-performance droplet transport is crucial for diverse applications including biomedical detection, chemical micro-reaction, and droplet microfluidics. Despite extensive progress, traditional passive and active strategies are restricted to limited liquid types, small droplet volume ranges, and poor biocompatibilities. Moreover, more challenges occur for biological fluids due to large viscosity and low surface tension. Here, a vibration-actuated omni-droplets rectifier (VAODR) consisting of slippery ratchet arrays fabricated by femtosecond laser and vibration platforms is reported. Through the relative competition between the asymmetric adhesive resistance originating from the lubricant meniscus on the VAODR and the periodic inertial driving force originating from isotropic vibration, the fast (up to ≈60 mm s^-1 ), programmable, and robust transport of droplets is achieved for a large volume range (0.05-2000 µL, V_max /V_min  ≈ 40 000) and in various transport modes including transport of liquid slugs in tubes, programmable and sequential transport, and bidirectional transport. This VAODR is general to a high diversity of biological and medical fluids, and thus can be used for biomedical detection including ABO blood-group tests and anticancer drugs screening. These strategies provide a complementary and promising platform for maneuvering omni-droplets that are fundamental to biomedical applications and other high-throughput omni-droplet operation fields.

Molecular origin of fast evaporation at the solid-water-vapor line in a sessile droplet

By analyzing the behaviors of water molecules at the solid-water-vapor contact line, we explore the molecular origin of large evaporation rates at the contact line and find new ways to increase the evaporation of the droplet. In contrast to previous models considering macroscopic surroundings and the geometry of the droplet, here we study the behaviors of water molecules by introducing cohesive energy which includes interactions of water molecules with both other water molecules in the droplet and atoms in the substrate. Molecules at the contact line bear the smallest evaporating energy barrier and therefore, possess the largest possibility to evaporate. Further analyses show that the evaporation rate of the droplet is enhanced through the large length of the contact line. These analyses are corroborated by experiments, where the evaporation rate of the droplet is enhanced up to 30% by incorporating hollow glass spheres in the droplet. Our theoretical and experimental efforts illustrate the underlying molecular mechanisms of large evaporation rates of a droplet, providing new avenues to accelerate droplet evaporation.

Superhydrophobic modification of nanocellulose based on an octadecylamine/dopamine system

We prepared super-hydrophobic nanocellulose films using a non-toxic octadecylamine/polydopamine system. Octadecylamine, a low surface energy material, was used to provide hydrophobic alkyl long chains. Polydopamine was produced by dopamine under alkaline conditions, creating an adhesive substance, which reinforced the hydrophobic long chains and increased the surface roughness of nanocellulose. The effects of reagent concentration, reaction temperature, and reaction time on hydrophobicity were then investigated. The results showed that with a 1:1 mass ratio of nanocellulose to octadecylamine, and reacting at 60 °C for 4 h, the contact angle of the obtained composite membrane reached 168.2°. Scanning electron microscope images revealed that the modified nanocellulose had a smaller particle size and more uniform distribution, which effectively improved the hydrophobicity of the nanocellulose. Thus, the green preparation of superhydrophobic films with high-temperature resistance and wear resistance was realized, which contributed to the high-value utilization of nanocellulose.

Origin of Rebound Suppression for Dilute Polymer Solution Droplets on Superhydrophobic Substrate

Controlling droplet deposition with a minute amount of polymer additives is of profound practical importance in a wild range of applications. Previous work ascribed the relevant mechanisms to extensional viscosity, normal stress, wetting properties, etc., but the mechanism remains controversial. In this paper, we employ molecular dynamics simulations systematically for the first time to investigate the origin of rebound suppression for dilute polymer solution droplets on a flat superhydrophobic substrate. The results demonstrate that polymer-substrate interactions and impact velocities dominate the antirebound phenomenon. For low impact velocities, the dynamic characteristics of droplets are insensitive to polymer additives. However, large impact velocities will enhance the stretch behavior of polymer chains and make the chains closer to the substrate, increasing the probability of polymer molecules contacting the bottom substrate. With the cooperation of strong polymer-substrate interactions, polymer molecules can be absorbed easily by the bottom substrate, resisting the retraction process and leading to the onset of the antirebound behavior.

Exploring Contact Angle Hysteresis Behavior of Droplets on the Surface Microstructure

It is confirmed that surfaces with specific microstructures could exhibit good superhydrophobic properties, and there are also a lot of conclusions about droplet hysteresis behavior. However, most of the research methods are based on two-dimensional ideal model and experimental observation at the macroscale. Further research needs to be conducted about the hysteresis behavior of droplets on the microstructure surface under three-dimensional conditions. In this paper, the influence of curvature variation of the liquid surface between pillars on the contact angle hysteresis (CAH) has been investigated. The simulation results were in good agreement with the experimental measurements. Analyses were conducted on the morphology change and force of the liquid surface between pillars, and an index was proposed to describe the degree of difficulty of liquid surface movement. It was revealed that a change in the direction of the surface tension at the three-phase interface caused by curvature variation of the liquid surface between pillars played an important role in the movement of the liquid surface. The greater the surface tension component in the normal direction of the liquid surface, the more likely it was for the liquid surface to advance or recede. The local curvature of the liquid surface increased or the angles between the pillars increased, and the effect of the CAH would be weakened.

Open Problems in Wetting Phenomena: Pinning Retention Forces

We review existing explanations for drop pinning and the origin of the force required to initiate the sliding of a drop on a solid surface (depinning). Theories that describe these phenomena include de Gennes', Marmur's, Furmidge's, the related Furmidge-Extrand's, and Tadmor's theory. These theories are all well cited but generally do not address each other, and usually papers that cite one of them ignore the others. Here, we discuss the advantages and disadvantages of these theories and their applicability to different experimental systems. Thus, we link different experimental systems to the theories that describe them best. We describe the force laws that can be deduced should these theories be united and the major open problems that remain. We describe a physical meaning that can be extracted from retention force measurements, specifically, the interfacial modulus that describes the tendency of a solid to conform to the liquid. This has implications for various wetting phenomena such as adhesion robustness, drug penetration into biological tissues, and solid robustness/resilience versus solid degradation over time as a result of its contact with a liquid.

A generalized examination of capillary force balance at contact line: On rough surfaces or in two-liquid systems

We investigate the capillary force balance at the contact line on rough solid surfaces and in two-liquid systems. Our results confirm that solid-liquid interactions perpendicular to the interface have a significant influence on the lateral component of the capillary force exerted on the contact line. Surface roughness of the solid substrate reduces the mobility of liquid and alters how the perpendicular solid-liquid interactions transfer into a force acting parallel to the interface. A quantitative relation between surface roughness and the transfer strategy is proposed. Moreover, when a liquid is in coexistence with another immiscible liquid on a solid, the capillary forces exerted on liquids of both sides are involved in our theoretical model. The contact angle can be predicted by calculating three interfacial tensions. These arguments are then verified by molecular dynamics simulations. Our findings set up the generalized theoretical framework for the capillary force balance at the contact line and broaden its application in more realistic scenarios.