Three-phase Contact Line Research Articles

Regulating Ru-Ru Distance in RuO2 Catalyst by Lattice Hydroxyl for Efficient Water Oxidation.

Highly active and durable electrocatalysts for the oxygen evolution reaction (OER) are crucial for proton exchange membrane water electrolysis (PEMWE). While doped RuO2 catalysts demonstrate good activity and stability, the presence of dopants limits the number of exposed active sites and complicates Ru recovery. Here, we present a monometallic RuO2 (d-RuO2) with lattice hydroxyl in the periodic structure as a high-performance OER electrocatalyst. The obtained d-RuO2 catalyst exhibits a low overpotential of 150 mV and long-term operational stability of 500 h at 10 mA cm-2, outperforming many Ru/Ir-based oxides ever reported. A PEMWE device using d-RuO2 sustains operation for 348 h at 200 mA cm-2. In-situ characterization reveals that the incorporation of lattice hydroxyl increases the Ru-Ru distance, which facilitates the turnover of the Ru oxidation state and promotes the formation of stable edge-sharing [RuO6] octahedra during the OER, thereby accelerating the formation of O-O bonds and suppressing the overoxidation of Ru sites. Additionally, the small particle size of the catalyst decreases the three-phase contact line and promotes bubble release. This study will provide insights into the design and optimization of catalysts for various electrochemical reactions.

A wetting force-based model for contact line dynamics in droplet impact on curved substrates

Purpose Numerical simulations of moving three-phase contact line on curved substrates are performed in scenarios without any splashing or rebounding after liquid impact. While velocity-based dynamic contact angle models have been used previously, a force-based approach that closely relates dynamic contact angle to underlying flow physics has not been implemented for curved surfaces. The purpose of this study is to develop and implement a force-based model for curved substrates where dynamic contact angle is adjusted by wetting force at contact line. Design/methodology/approach The magnitude and direction of wetting force are calculated for different geometries after computing dynamic contact angle with respect to equilibrium contact angle while considering the effect of curvature of substrate during contact line motion. The resolved components of wetting force are included as source terms in radial and axial momentum equations, for which a sign convention is derived for different configurations. The overall algorithm for wetting force is implemented using user-defined routines within the framework of an existing CFD solver using volume of fluid method. Adaptive mesh refinement is also used near the interface because of intensive nature of the computations. The model is used to simulate droplet impact on convex and concave spherical surfaces, and conical surface along with water entry of a spherical ball. The effect of curvature and impact velocity on contact line motion over convex spherical surface is studied, while the role of contact angle for different surfaces is also examined. Findings The results from the simulations show that the present force-based methodology is able to capture the temporal evolution of dynamic contact angle closely based on the underlying physical mechanisms, without resorting to any empiricism or approximations. The simulations also bring forth the deviations of the dynamic contact angle from the specified equilibrium contact angle values during contact line motion on different curved geometries, the reasons for which are adequately discussed. A validation with existing numerical and experimental results shows the effectiveness of the proposed methodology in accurately capturing contact line motion. Originality/value The results showcase several new and important findings as no prior investigation has been done with regard to implementation of such a force-based approach to study moving contact lines on curved surfaces, to the best of the authors’ knowledge. This study comprehensively outlines and presents all the steps involved in implementing the force-based model while considering the effect of curvature on different geometries under various conditions, and establishes it as an effective and accurate approach to capture contact line dynamics. This study can definitely be helpful to the modelling community towards accurate, physics-based modelling of moving contact lines.

Understanding ultrafast free-rising bubble capturing on nano/micro-structured super-aerophilic surfaces

Rapid bubble capture is essential for collecting targeted gaseous media and eliminating floating impurities across aquatic environments. While the role of nanostructures during the collision of free-rising bubbles with super-aerophilic surfaces is well established, the fundamental contribution of microtextures in promoting initial capture, even before contact, has yet to be fully understood. We report the rising bubble-induced large deformation of the entrapped gas layer, rapidly thinning the liquid film to its rupture threshold and thus achieving an ultrafast bubble capture down to about 1 ms with an array of microcones, decorated with nanoparticles as a convenient example to obtain super-aerophilicity. This rapid capture is also very stable due to the hysteresis movement of three-phase contact lines that inspired a critical pressure criterion for ensuring gas-layer stability and capture efficacy. The present nano/microstructured surface supports prolonged, loss-free gas transport in challenging shear flow as well, providing robust bubble control strategies for diverse systems.

Preparation of Long-Range Ordered 1D Nanowire Arrays on PVP-Modified Hydrophobic Highly Adhesive Templates Using Conical Fiber Arrays.

Suspended nanoscale one-dimensional (1D) arrays have attracted substantial interest due to their promising applications in nanodevice fabrication. In this study, we propose a novel strategy for fabricating precisely positioned, long-range ordered nanowire arrays by controlling the directional liquid transport of conical fiber arrays (CFAs) on asymmetrically modified silicon templates patterned with periodic spindle-shaped micropillars. The intrinsic properties of CFAs and the tailored wettability of silicon templates play critical roles in nanowire fabrication. CFAs generates quasi-unidirectional surface tension (Fγ), facilitating precise control over the retraction of liquid films and ensuring strict nanowire alignment in the dewetting direction. Meanwhile, high-adhesion hydrophobic surfaces effectively enhance the pinning behavior of the three-phase contact line during the retraction process, thereby improving the liquid bridge stability. It is noteworthy that the method developed for preparing high-yield arrays of ultralong nanowires exhibits remarkable universality. This approach can be widely applied to the synthesis of suspended nanowires using diverse polymers such as polystyrene sulfonic acid, poly(vinyl alcohol), polyvinylpyrrolidone, polyethylene glycol, and sodium alginate as solutes, achieving a robust formation rate exceeding 80% for nanowires that surpass 16 μm in length. These findings contribute valuable knowledge for the scalable production of suspended 1D nanostructures, furthering advancements in nanoscale device development.

Mechanism and strategy of self-assembly of quaternary ammonium surfactant molecules to regulate pesticide droplet impact and wetting of hydrophobic surfaces.

Surfactants regulate the interaction between pesticide droplets and the surfaces of plants on which they are sprayed. The influence of the key structural functional groups of surfactants on the interaction between pesticide droplets and hydrophobic pear leaves has not been explored. The behavior of Imidacloprid (Imid) droplets regulated by cationic quaternary ammonium surfactants with different structures on hydrophobic pear leaves and their bouncing dynamics were studied. The properties of pesticide droplets regulated by rosin-based bicationic quaternary ammonium salt and ethylene (dodecyl polyoxyethylene/tetradecyl polyoxyethylene) chloride/ammonium bromide were well matched with those of pear leaves with a waxy layer. This structure was closely related to the double-chain structure corresponding to that of double N-head groups in quaternary ammonium surfactants. Quaternary ammonium surfactants regulate the wetting of droplets by forming semi-micellar structures near the three-phase contact line, which drives the droplets to wet and spread on the leaf. The quaternary ammonium surfactant containing the double N-head structure enabled strong wetting and adhesion of pesticide droplets on the hydrophobic surface. The key structural functional groups of different quaternary ammonium surfactants directionally modified the impact kinetics of Imid droplets on the leaf surfaces and their changing trend. The double N-head structure played a key role in the molecular structure of quaternary ammonium surfactants, and the hyperbranched ethylene oxide (EO)chain played a small role in the molecular structure. These results clearly indicate how the structure of key functional groups of quaternary ammonium surfactants regulated the interface adhesion of pesticide droplets on the leaf surfaces and explain the microscopic mechanism of their interaction. © 2024 Society of Chemical Industry.

Evolution Characteristics and Mechanism of Three-Phase Contact Line of a DNA Microdroplet Impacting and Spreading on a Hydrophilic Horizontal Solid Surface.

The phenomena and characteristics of droplets impacting and spreading on a horizontal solid surface are key scientific issues in microfluidic technology and surface physics. The movement regularity and mechanism of the three-phase contact line (TCL) of DNA microdroplets spreading on a hydrophilic PMMA smooth surface without splashing or rebounding are investigated by high-speed camera technology. Some significant results are achieved, for example: based on the changes in droplet morphology, the changes of DNA droplet TCL are divided into eight stages, i.e., expanding, pinning, retracting, repinning, microexpanding, micro-oscillating, stabilizing, and retraction drying, which are helpful to understand the deposition of DNA molecules on the substrate surface. Among them, the TCL moves intensely in the stages of expanding and retracting, respectively, following the cubic function and quadratic function; then, TCL remains virtually unchanged since entering the repinning stage until the stabilizing stage, and DNA molecules are deposited on the surface of PMMA, accumulating to form crystals. Furthermore, the relationship between three correlated damped-like oscillations, i.e., droplet top height (htop), TCL position, and dynamic contact angle (DCA) size, is systematically investigated; the htop oscillates first, then the TCL and DCA oscillate after a delay of 5 ms, and the oscillation period increases sequentially, which is 7.55, 7.90, and 8.40 ms, respectively. Finally, the effects of dynamic characteristics and crystallization of DNA molecules on TCL movement and DCA size are expounded. This research provides detailed experimental data and proposes a theoretical model for understanding the dynamic mechanism of DNA droplet spreading on solid surfaces and has been helpful for designing DNA chips and developing micro-/nano-fluidic sensors.

Surface Wettability Effects on Droplet Dynamics and Heat Transfer on Heated Stainless-Steel Foils

Abstract We report the dynamics and heat transfer characteristics of water droplets impacting a thin stainless-steel foil maintained at different temperatures. The hydrophobic characteristics are imparted to the surface through polysiloxane coating, and water droplets impact the uncoated and coated heated surfaces at different velocities. High-speed videography is utilized to capture the dynamics of the droplet upon impact, while the temperature field of the substrate, during the phenomenon, is simultaneously recorded using high-speed infrared thermography. Heat transfer to the droplet over different surfaces is determined through energy balance on the foil using the captured thermographs. The results reveal that the spreading phase duration is independent of droplet impact velocity, irrespective of surface wettability, whereas surface wettability primarily affects the receding phase. The coated hydrophobic surfaces exhibited lower resistance to motion at the three-phase contact line, resulting in reduced spread ratios during the receding phase. It is noted that the majority of heat transfer occurred during the initial spreading and receding phases, driven primarily by forced convection. The maximum heat fluxes were observed along the three-phase contact line, particularly at the onset of the receding phase. The coated surface demonstrated lower overall heat transfer rates compared to non-coated surfaces, with the difference increasing at higher surface temperatures. Additionally, an increase in surface temperature to 75 °C enhanced the hydrophobicity of the coated surface, leading to prolonged receding phases and extended time to reach the sessile state.

Multistep Phase Transition and Molecular Reaction of Plasmonic Nanoparticles at the Three-Phase Contact Line of an Evaporating Sessile Droplet

Multistep Phase Transition and Molecular Reaction of Plasmonic Nanoparticles at the Three-Phase Contact Line of an Evaporating Sessile Droplet

Coalescence-Induced Spontaneous Shedding of Microdroplets on Superhydrophobic Surfaces Featuring Enclosed Micropillars with Hierarchical Roughness.

This study investigated water vapor condensation on superhydrophobic surfaces (SHSs) featuring micropillars enclosed by wall lattices and having three-tier hierarchical roughness. A total of five samples were created with three (NW-J, W200-J and W400-J samples) having large micropillar depth (∼6 μm) and two (NW-S and W200-S samples) having small micropillar depth (∼1 μm). Two distinct condensate removal modes were observed during condensation: coalescence-induced jumping on samples with large micropillar depth and coalescence-induced shedding on samples with small micropillar depth. The results showed that the diameter of the shedding droplet on the W200-S sample having small micropillar depth could be as small as 107 μm, as compared to the theoretical critical diameter of 267 μm for gravitational shedding on the same sample. The enhanced functionality of the three-tier nanotextures on the W200-S sample could effectively suppress localized pinning of the three-phase contact line and Wenzel neck formation during the growth of condensate droplets. Consequently, during multidroplet coalescence, the released surface energy easily overcomes the solid-liquid adhesion, leading to spontaneous shedding of merged droplets. The inclusion of the wall lattice aids condensate growth by the droplet self-alignment along the walls and promoting coalescence. As a result, the W200-S sample exhibited the highest condensate collection as well. The proposed surface design has great potential for scaling up and implementation in heating, ventilation, and air-conditioning equipment due to the simplicity of the surface morphology and the facile spray-coating method used to achieve hierarchical roughness.

On the influence of optically controlled thermocapillary flow during vertical convective assembly: Origin of remora disk-like patterns

Vertical convective assembly, a cost-effective and efficient colloidal assembly strategy, has garnered interest from a wide range of disciplines, including photonics and sensors. In this work, we reveal the role of nonuniform temperature distribution at the three-phase contact line (TPCL) during the vertical lifting of the substrate from the colloidal suspension. Conventionally, vertical assembly is performed under isothermal conditions, and the possible outcomes are uniform particle deposits and discrete lines based on stick-slip behavior. We demonstrate that exposing the TPCL with a nearly Gaussian-type temperature profile under an optimal lifting speed of 0.8–5 μm/s results in a new kind of particle pattern, which we call remora disk-like assembly, with periodic central thick regions and lamella-kind structures on either side. We generate the required temperature gradient by irradiating the TPCL with a laser beam, whose emission wavelength matches the plasmonic excitation of the nanoparticles used (λ = 532 nm). The nonuniform temperature distribution at the TPCL (ΔT = 13 °C) generates a corresponding thermocapillary flow, which drives the particles toward the TPCL in a gradient manner. We develop a physical model to explain the particle deposition mechanism, the nature of the remora disk assembly, and the asymmetric depinning behavior of the meniscus. Furthermore, by varying the lifting speed, we could tune the morphology and spacing of the patterns. We believe the new insights on the particle dynamics under optically controlled thermocapillary flow could significantly contribute to the fundamental understanding as well as enriching the applied aspects of the vertical lifting-based colloidal lithography.

Dynamic characteristics and energy variation of droplet impacting on a quartz surface

Understanding the impact process of droplets on surfaces is a crucial prerequisite for enhancing the efficiency of wetting dust removal. The collision dynamics between water droplets and quartz surfaces were investigated using a high-speed camera, revealing four distinct stages in this process. During the first three stages, there was an increase in droplet width and three-phase contact line over time, while the droplet height and contact angle decreased. These observations can be attributed to the combined effects of impulsive force and surface tension. With increasing droplet velocity, the duration of the first three stages prolonged, accompanied by an increase in both the diameter of the three-phase contact line and droplet width during the final stage, whereas there was a decrease in droplet height and contact angle. This behavior primarily arises from energy transfer involving kinetic energy converted into contact surface energy, droplet surface energy, and dissipated energy during collision events. Contact surface energy and droplet surface energy exhibited an upward trend with rising droplet velocity. Simultaneously, collision-induced dissipated energy increased proportionally with respect to droplet velocity. Notably, both the rate and ratio of dissipated energy demonstrated positive correlations with the Weber number; specifically following a linear relationship characterized by a slope value of 2.81. These findings offer valuable insights for advancing technology development related to equipment used for wetting dust removal.

Insight into the behavior and dynamics of extended thin films in shear-thinning liquids

Extended liquid thin films are essential and ubiquitous in the field of microfluidics. Mass and energy transfer in microfluidic systems, such as micro-scale heat pipes, falling film reactors, etc., depend on the forces acting near the three-phase contact line. Within the extended thin film region, the solid–liquid intermolecular force becomes significant along with the surface force. Several experiments have been conducted to understand and optimize the forces involved in mass and energy transport for Newtonian liquids. However, in real-world situations, these extended thin films are usually made of non-Newtonian liquids. The impact of high viscous forces and the complex rheology of non-Newtonian liquids on the extended thin film remains largely unexplored. This work pioneers a detailed experimental investigation into the extended thin film behavior of a shear-thinning polymeric liquid solution, offering new insights into this understudied phenomenon. The polymeric solution is supplemented with a surfactant to adjust the surface tension. The interplay between surfactant and the intrinsic nature of polymer solutions is studied by measuring their rheological properties. The extended thin film thickness is measured using image-analysis interferometry for polymer solutions with varying concentrations. The Hamaker constant is calculated from the slope and curvature profiles. A theoretical model is developed using the augmented Young–Laplace equation. The model can predict the extended film thickness profile near the three-phase contact line region. The model's predictions are favorably compared with experimental results. This work advances the understanding of extended thin film dynamics in non-Newtonian fluids, with broad implications for industrial and scientific applications.

Recirculatory Solvotaxis of a Nematic Droplet on Water Surface Enabling Miniaturization.

Self-organized contact line instabilities (CLI) of a macroscopic liquid crystal (LC) droplet can be an ingenious pathway to generate a large collection of miniaturized LC drops. For example, when a larger drop of volatile solvent (e.g., hexane) is dispensed near a smaller LC drop resting on a soft and slippery surface of a nonsolvent (e.g., water), unique self-organized locomotion in the form of a twin vortex has been observed within the droplets. This phenomenon is driven by the rapid counter diffusion of hexane and LC between the two droplets, resulting in the formation of a pair of vortices within the droplets before instigating a CLI at the three-phase contact line (TPCL) of the LC droplet. Initially, the higher Laplace pressure inside the LC droplet (PL,5CB) due to a net pressure gradient, PL,5CB > PL,Hex, drives the LC toward hexane. However, as the volatile solvent droplet shrinks due to rapid evaporation, a flow reversal happens owing to PL,5CB < PL,Hex. Subsequently, the diffusion of hexane into the LC droplet and its subsequent evaporation manifest a periodic oscillatory CLI expansion and retraction at the TPCL, which in turn form periodic finger-like structures. Following this, the fingers with a higher aspect ratio break into an array of miniaturized satellite LC droplets undergoing Rayleigh-Plateau instability (RPI). The observed deviation in the normalized satellite droplet spacing compared to theoretical value affirm the stabilizing influence of LC elasticity in such fingers, where λ and R5CB are experimentally calculated droplet spacing and 5CB droplet radius. Control experiments elucidate the specific contributions of capillary, drag, solutal Marangoni, and osmotic forces to the 5CB droplet locomotion phenomena. The experimentally and analytically consistent demonstration also supports and predicts pressure drop-induced droplet velocities as v ∼ t1.16.

The detachment of an air bubble from glass plates featured with pore structures filled with gas or water

A pore-featured surface can entrap water or gas phase inside pores and cause phase heterogeneity on the surface. This phenomenon has a significant impact on air bubble adhesion. This paper investigated the air bubble detaching processing on the glass plates featured with single and array pores filled by water or gas. Under a quasi-static and compulsive displacement, an air bubble is attached and then detached from a pore-featured surface in water. The three-phase contact line (TPL) on the pore-featured surface and associated capillary force were measured and calculated. The results showed that continuity of solid glass was intercepted by the water/gas phase on the pore opening. The TPL jumped over and cut off at the edge of pores filled with water and caused the capillary force action length largely shortened. When the pore was filled with air, the air bubble formed a capillary gas bridge that caused a retention of TPL and extended the action length of the capillary force. In the cases of arrayed pores, TPL jumped over small water-filled pore openings and caused a step-decline of capillary force. The TPL was found moving continuously over the small air-filled arrayed pores, which showed a TPL retention effect. Several applied surface porosity Φ = 0.031, 0.126, and 0.283 of the arrayed pore surfaces were found to have similar effects to the single pore surface. The revealed mechanism of the air bubble detaching process on the pore-induced phase heterogeneous surface is important in understanding and controlling the adhesion process of air bubbles.

Influence of surface-active agents on the dynamic wetting film rupture: Gas migration and surface nanobubbles formation

Influence of surface-active agents on the dynamic wetting film rupture: Gas migration and surface nanobubbles formation

Size-Dependent Wetting Contact Angles at the Nanoscale Defined by Equimolar Surfaces and Surfaces of Tension

The wetting characteristics of fluids play a crucial role in various fields of interface and surface science. Contact angle serves as a fundamental indicator of wetting behavior. However, accurate quantification of wetting phenomena even at the macroscale often poses challenges, particularly due to the hysteresis between receding and advancing contact angles. The complexity increases further at the nanoscale, where the significant volume of the interphase region causes ambiguity in defining the “dividing surface.” In this study, we use molecular dynamics simulations to investigate the wetting dynamics of a “cylindrical nanodroplet” and an argon nanofilm. Through analysis of microscopic density distribution maps and tension tensor distributions within the Gibbs framework, we identified equimolar and tension surfaces at both liquid-gas and liquid-solid interfaces. Our results show over 10% discrepancies between equilibrium contact angles calculated for equimolar surfaces and those based on tension surfaces in the case of the cylindrical nanodroplet. We observed a clear dependence of wetting contact angles on the cross-sectional radius of cylindrical droplets with a straight three-phase contact line. As the radius decreases, the differences between contact angles at equimolar and tension surfaces increase, while for larger droplets, these differences diminish and become negligible.

Effect of Mixed Surfactant on Evaporation Driven Salt Crystallization Morphology in Sessile Droplets.

Extensive studies have been conducted to manipulate the morphology of sodium chloride salt crystals to tailor their physical and chemical properties. Among the myriad factors considered, the effects of the substrate and additives have profound impacts on the types of salt depositions. Surface charge effects and various ionic surfactants influence ion movement, resulting in diverse crystal morphologies. This manuscript aims to provide a consolidated summary by concurrently studying multiple effects to uncover the salt crystal morphology under the influence of two oppositely charged ionic surfactants on charged and neutral surfaces. The cationic surfactant cetyltrimethylammonium bromide induces skeletal crystal growth by retarding salt precipitation until supersaturation is reached. Conversely, the anionic surfactant sodium dodecyl sulfate hinders ion diffusion at the three-phase contact line. Each surfactant effect is dominant at higher molar concentrations. Surface charge affects the amount of surface adsorption and free-moving ions within the saline surfactant droplets, greatly influencing the number of salt crystals formed on the neutral substrate. However, charge neutralization at the highest concentrations of both surfactants nullifies the surface charge effect, resulting in practically indistinguishable salt crystals with similar sizes and numbers, leading to only a small area difference of 1461 μm2. This study provides insights into the kinetics of crystallization under the combined influence of anionic, cationic, and surface charge interactions. The findings can serve as a future reference for predicting and controlling ionic interactions and crystal morphology.

Periodic Burst Freezing in a Water-Filled Capillary Tube.

Water-filled porous structures are ubiquitous components in life sciences and engineering. At sufficiently low temperatures, the water inside the porous structures can freeze, triggering the frost heave effect and causing problems such as permafrost, frost damage, and frostbite. In this study, we report a unique pattern of frost heat release through periodic bulging and bursting at the end of a water column inside a capillary tube. When water freezes, it expands in volume and attempts to drain out. However, surface tension can hinder the flow of water, resulting in the formation of a bulge that periodically bursts when the spherical molecule reaches a critical threshold. We determined that the critical contact angle of the bulge is approximately 135°, which is balanced by the surface tension at the three-phase line of contact. The size of the bulge increases nonlinearly with the diameter of the capillary tube, while the number of periodic repetitions is inversely proportional to the tube diameter and directly proportional to the length of the water column. These findings provide insights into the interaction between surface tension and frost heave, which has important implications for the design and optimization of microfluidic devices with improved resistance to freezing.

ON CONTACT WETTING ANGLES IN THE LATTICE BOLTZMANN METHOD AND THEIR MEASUREMENT

Lattice Boltzmann method is applied for the computer simulation of liquid droplets that are in contact with a solid surface. In such problems, the value of contact angle at the three-phase contact line needs to be set. To achieve this, interaction forces between fluid and solid are introduced. The simulation of the equilibrium shapes of droplets is performed. We show that the use of commonly accepted forms of the interaction between fluid and solid in LBM leads to a non-physical change of the density near the solid surface. This hinders the correct calculation of heat fluxes. A new method is proposed to control the contact angles where only the tangential components of forces acting on fluid from solid nodes are taken into account (tangential force method, TFM). Use of this method does not change the fluid density in layers adjacent to the boundary. This is important in the simulation of flows with heat transfer and electrohydrodynamic flows. The dependence of the contact angle on the parameter of fluid-solid interaction is obtained.

Dynamics of drop impact and contact line motion on micro-pillared surfaces

Physical texturing creates patterned and pillared surfaces that display superhydrophobicity in drop spreading and drop impact studies. Often, such surfaces are accompanied by large hysteresis since the three-phase contact line may get trapped over and within the pillars. In this context, wetting characteristics of a water drop spreading over a micro-pillared surface of copper are investigated. Apart from drop spreading on a bare pillared surface, two companion studies where the pillars are fully and partially coated using superhydrophobic and hydrophobic coatings have been carried out. The Weber number and Reynolds number based on the drop diameter and impact speed are varied over the range 1–41 and 440–2870, respectively, while the Bond number remains constant, ∼1.03. Imaging sequences show spreading behavior that is distinctive of the surface chosen. For a fully coated surface, the drop is seen to jump-off upon impact while a residual drop remains over uncoated and partially coated pillars during the receding phase. These observations are compared against three-dimensional numerical simulations that resolve the pillar shapes. Simulations are seen to be qualitatively in good agreement with experiments. Simulations additionally probe the contact line movement over coated and uncoated pillars for comparison with experiments. The filling of the interpillar space with liquid and the resulting interface deformation are examined. Jointly, the emptying of the interpillar gap is also discussed. Experiments and simulation show a jump in drop footprint when the contact line leaves the pillar, and the associated velocity becomes large.