Pillared Surfaces Research Articles

Impact dynamics of nanodroplets on pillared surfaces

Impact dynamics of nanodroplets on pillared surfaces

Molecular dynamics simulation study on the wetting characteristics of carbon dioxide droplets on smooth and rough surfaces

A thorough investigation of the wetting behavior of carbon dioxide (CO2) is crucial for enhancing carbon capture efficiency and improving carbon storage and sequestration processes. This work investigates the wetting characteristics of CO2 droplets on smooth and rough Cu-like surfaces using molecular dynamics simulations. The results indicate that an increase in surface energy causes a linear decrease in the contact angle of CO2 droplets on smooth surfaces. An increase in temperature enhances the wetting tendency of the smooth surface toward CO2 droplets. On CO2-phobic surfaces, CO2 droplets tend to detach from the surface, causing the contact angle to increase. On neutral surfaces, CO2 droplets generally maintain wettability, with contact angles remaining almost unchanged. On CO2-philic surfaces, CO2 droplets rapidly wet and spread to form liquid film layers, causing the contact angle to decrease rapidly. In addition, constructing rough surfaces is beneficial for enhancing the surface CO2-phobic property, but the wettability of CO2 droplets varies slightly on different rough surfaces (cylindrical pillars, square pillars, and grooves), the fluctuation of contact angles of CO2 droplets on the grooved surface is relatively small. For square pillared surfaces, in contrast to smooth surfaces, as the surface energy increases, the CO2-phobic property of CO2-phobic surfaces is enhanced, and neutral surfaces can be transformed into CO2-phobic surfaces, with the CO2-philic property of CO2-philic surfaces enhanced. For CO2-phobic square pillared surfaces (α = 0.08), the wetting morphology and wetting modes of CO2 droplets on the square pillared surface are jointly influenced by both the pillar height and the interpillar spacing. When the interpillar spacing is constant, there exists a critical pillar height that can induce the transition of the wetting mode of CO2 droplets from Wenzel to Cassie state.

Effect of liquid splattering on thermal performance of jets and sprays over plain and pillared surfaces

A comparative study of liquid jet and spray is explored over plain and pillared copper surfaces with coolant flow (water) ranging over 3000 < Re < 10500, and heat fluxes up to the critical heat flux limit. Liquid jets and sprays are hydrodynamically characterized using high-speed videography and the phase-Doppler particle analyzer system. The interaction of liquid jets and sprays with the respective metallic substrates is experimentally investigated and strong liquid splattering is observed over the pillared surfaces at higher coolant flow rates. The resulting splattering mass fraction is measured over a range of coolant flow rates and radial positions of the nozzle. Subsequently, the effect of splattering on heat transfer performance is compared for various heat flux conditions by measuring the steady-state surface temperature. Heat transfer coefficient is found to be a strong function of coolant flow rate, operating heat flux conditions, and splattered mass fraction. Significant differences are not seen in these quantities between the liquid jet and spray for a plain surface. However, spray cooling clearly shows superior heat transfer enhancement over a liquid jet for a pillared surface where the critical heat flux limit is significantly delayed. The apparent advantages of heat transfer performance enhancement with patterning of surfaces are not always guaranteed, and the design consideration must address the coolant interaction with it to ensure a net advantage, which may get diminished due to liquid splattering induced by the interaction of the coolant with the patterned surface.

Droplet Size-Assisted Polysiloxane Architecting

(Super)antiwetting shielding around engineering materials and protecting them against harsh environmental conditions have been achieved via growing various geometry polysiloxane (or silicone) patterns around them by using a droplet-assisted growth method, where the polymerization takes place inside of the water droplets acting as reaction vessels. The size and distribution of these reaction vessels are the main factors in making different geometry silicone patterns; however, very little is known about the fate of these droplets throughout the polymerization. Here, we propose keeping the relative humidity (% RH) inside the reactor stable throughout the polymerization as a new coating parameter to force the size of the reaction vessel water droplets to be the same for building simply shaped silicone rods with controlled geometry and distribution. In this manner, we grew simple geometry cylindric microrods on surfaces and could tune their length, diameter, inter-rod spacing, and thus the (super)hydrophobicity. Here, we also demonstrate that with changes in the amplitude and stability of the % RH, it is possible to fabricate different (super)hydrophobic nanograsses, conical silicone microrods, and isotropic silicone nanofilaments. The proposed new way of tuning initial and in situ reaction vessel droplet size can be used as a single factor to formulate different geometry silicone patterns with tunable dimensions, leading to different roughness and hydrophobicity. To a certain extent, the droplet size-assisted silicone shaping in this work provides a new way to control the length, diameter, morphology, inter-rod spacing, and thus the (super)hydrophobicity of the silicone patterns, especially those in the shape of simple cylindrical microrods. This control over silicone architecting will help to prepare new (super)hydrophobic coatings with more controlled morphology and thus wettability; on the contrary, it will support surface scientists modeling the connection between surface geometry and (super)antiwetting of such irregular pillared surfaces that remain elusive.

A multiscale volume of fluid method with self-consistent boundary conditions derived from molecular dynamics

Molecular dynamics (MD) and volume of fluid (VOF) are powerful methods for the simulation of dynamic wetting at the nanoscale and macroscale, respectively, but the massive computational cost of MD and the sensitivity and uncertainty of boundary conditions in VOF limit their applications to other scales. In this work, we propose a multiscale simulation strategy by enhancing VOF simulations using self-consistent boundary conditions derived from MD. Specifically, the boundary conditions include a particular slip model based on the molecular kinetic theory for the three-phase contact line to account for the interfacial molecular physics, the classical Navier slip model for the remaining part of the liquid–solid interface, and a new source term supplemented to the momentum equation in VOF to replace the convectional dynamic contact angle model. Each slip model has been calibrated by the MD simulations. The simulation results demonstrate that with these new boundary conditions, the enhanced VOF simulations can provide consistent predictions with full MD simulations for the dynamic wetting of nanodroplets on both smooth and pillared surfaces, and its performance is better than those with other VOF models, especially for the pinning–depinning phenomenon. This multiscale simulation strategy is also proved to be capable of simulating dynamic wetting above the nanoscale, where the pure MD simulations are inaccessible due to the computational cost.

Study of the Classical Cassie Theory and Wenzel Theory Used in Nanoscale

Recently, there are hesitations in the application scope of the classical Cassie theory and Wenzel theory. In this paper, Molecular Dynamics (MD) simulations are used to study these two theories used in the nanoscale and find their limitations. The effect of parameters including solid fractions (or roughness factors), arrangement of pillars (with same solid fractions), pillar height, and droplet size on contact angles was investigated. It shows that the Cassie equation is suitable for droplets on uniform pillared surfaces including different solid fractions, arrangement of pillars and pillar height, when there is no meniscus of droplets. The Wenzel equation is also suitable for droplets on uniform pillared surfaces including different roughness factors, arrangement of pillars and pillar heights. Moreover, whether the droplet size has an influence on the contact angle depends on the pinned place of the contact line. In the Wenzel state, the contact line is pinned although increasing the droplet size, resulting in increasing the contact angle, while the contact angle decreases to the initial value again when the droplet size increases enough to allow the contact line moving to the next pillar. The results provide insights toward the wettability of droplets on surfaces in nanoscale.

Lattice Boltzmann study of bubble dynamics and heat transfer on a hybrid rough surface with a cavity-pillar structure

In this work, a two-dimensional phase-change lattice Boltzmann model is used to investigate the pool boiling from a hybrid rough surface with a single cavity and one or two pillars on the superheated plate for an entire ebullition cycle. The bubble growth and departure dynamics on the hybrid pillared cavity surfaces, namely a single-pillared cavity surface (a surface with one cavity and one neighbouring pillar) and a double-pillared cavity surface (a surface with one cavity and one pillar on its both sides), are also compared with a simple cavity surface (a surface with only one cavity). The results showed that the incorporation of the pillar to the simple cavity surface increases the surface area and distance between the pinning locations of the three-phase contact line (TCL), and thus, enhances the bubble departure diameter and heat flux. It is found that bubble departure diameter and heat flux increase with cavity-pillar spacing up to a certain value, in this case sp∗ = 0.50, beyond which they start to decease and approach the simple cavity surface case at sp∗ = 1.0, while the opposite trend is observed for bubble departure frequency. The bubble departure diameter, frequency and heat flux also increase with pillar width. Compared to uniformly hydrophilic surfaces, the mixed wettability surfaces exhibit large heat flux with relatively small increase in bubble departure diameter.

Topography-Dependent Effective Contact Line in Droplet Depinning.

The mobility of a fakir state droplet on a structured surface is fundamentally determined by the effective length of a microscopic contact line. However, it is largely unknown how the surface topography determines the effective contact line length. Based on the direct measurement of droplet adhesion force and the visualization of contact line, this work shows that effective contact line length is topography dependent as opposed to prior notion. On pored surfaces, contact line is not distorted, and the effective length approaches the droplet apparent perimeter regardless of pore dimensions. On pillared surfaces, the distortion of contact line is significantly dependent on the packing density of the pillar structures so that the effective length is as small as a pillar diameter on densely packed pillars and as large as a pillar perimeter on sparsely-packed pillars, while changing linearly between the two extremes.

CO2 wetting on pillar-nanostructured substrates

CO2 capture by dropwise CO2 condensation on cold solid surfaces is a promising technology. Understanding the role of the nanoscale surface and topographical features of CO2 droplet wetting characteristics is of importance for CO2 capture by this technology, but this remains unexplored as of yet. Here, using large-scale molecular dynamics (MD) simulations, the contact angle and wetting behaviors of CO2 droplets on pillar-structured Cu-like surfaces are investigated for the first time. Dynamic wetting simulations show that, by changing the strength of the solid–liquid attraction a smooth Cu-like surface offers a transition from CO2-philic to CO2-phobic. By periodically pillared roughening of the Cu-like surfaces, however, a higher contact angle and a smaller spreading exponent of a liquid CO2 droplet are realized. Particularly, a critical crossover of CO2-philic to CO2-phobic can appear. The wetting of the pillared surfaces by a liquid CO2 droplet proceeds non-uniformly. A liquid CO2 droplet is capable of exhibiting a transition from the Cassie state to the Wenzel state with increasing increasing inter-pillar distance, and increasing pillar width. The wetting morphologies of the metastable Wenzel state of a CO2 droplet are very different from each other. The findings will inform the ongoing design of CO2-phobic solid surfaces for practical dropwise condensation-based CO2 capture applications.

Electrowetting-Dominated Instability of Cassie Droplets on Superlyophobic Pillared Surfaces.

The liquid-air interface of Cassie droplets on superhydrophobic/superlyophobic surfaces has been directly captured with a high-precision laser displacement meter. The measured profile of the interface shape and the critical voltage with which the Cassie-to-Wenzel transition occurs are compared against those from numerical simulations of the electric field coupled with the interface shape. Under the applied voltage, the collapsing behavior of water, glycerol, and hexadecane droplets on SU-8, CYTOP, and overhanging Si/SiO2 pillars has been uniquely identified depending on the liquid properties, the pillar geometry, and the pillar material. It is shown that, with increasing voltage, the contact angle at the three-phase contact line approaches the maximum advancing angle along the pillar sidewalls, above which the depinning from the pillar edge leads to a slide-down motion. The slide-down instability is dominant over the pull-in instability both on dielectric pillars and conductive overhanging pillars examined in the present study. It is indicated that the collapsing behavior on the present overhanging pillars is closely related to contact angle saturation in electrowetting and stick-slip motion of the contact line.

Modeling and simulating the growth of ellipsoidal droplets during dropwise condensation on pillared surfaces

A numerical simulation model is proposed to describe ellipsoidal droplets growth on pillared substrates during dropwise condensation. Comparison between gray scale images taken from droplets growing on pillared surfaces with those growing on flat surfaces shows considerable differences that must be taken into account in the simulation model. Firstly, since the droplets can be easily canalized between the pillars, the shape of droplets change from spherical to ellipsoidal specially in coalescence step. Secondly, on pillared surfaces due to droplets three dimensional configuration, coalescence can occur in all three dimensions. And thirdly, if a droplet touches a pillar, they will stay in touch until the end of the process. The model proposed here calculates the growth rate of ellipsoidal droplets during adsorption and coalescence steps, and checks the coalescence by verifying the existence of real intersection between each pair of droplets. The results of this model is compared with real data from 6 different configurations of pillars. The mean relative error of the model for droplets density is 10.00±0.68%, while it proposes more accurate results for predicting droplets radius with the mean error of 2.28±0.42%.

Spontaneous Spreading of a Droplet: The Role of Solid Continuity and Advancing Contact Angle

Spontaneous spreading of a droplet on a solid surface is poorly understood from a macroscopic level down to a molecular level. Here, we investigate the effect of surface topography and wettability on spontaneous spreading of a water droplet. Spreading force is measured for a suspended droplet that minimizes interference of kinetic energy in the spontaneous spreading during its contact with solid surfaces of discontinuous (pillar) and continuous (pore) patterns with various shapes and dimensions. Results show that a droplet cannot spread spontaneously on pillared surfaces regardless of their shapes or dimensions because of the solid discontinuity. On the contrary, a droplet on pored surfaces can undergo spontaneous spreading whose force increases with a decrease in the advancing contact angle. Theoretical models based on both the system free energy and capillary force along the contact line validate the direct and universal dependency of the spontaneous spreading force on the advancing contact angle.

Wettability of Reentrant Surfaces: A Global Energy Approach.

In this work, we consider two possible wetting states for a droplet when placed on a substrate: the Fakir configuration of a Cassie-Baxter (CB) state with a droplet residing on top of roughness grooves and the one characterized by the homogeneous wetting of the surface, referred as the Wenzel (W) state. We extend a theoretical model based on the global interfacial energies for both states CB and W to study the wetting behavior of simple and double reentrant surfaces. Due to the minimization of the energies associated with each wetting state, we predict the thermodynamic wetting state of the droplet for a given surface texture and obtain its contact angle θC. We first use this model to find the geometries for pillared, simple and double reentrant surfaces that most enhances θC and conclude that the repellent behavior of these surfaces is governed by the relation between the height and width of the reentrances. We compare our results with recent experiments and discuss the limitations of this thermodynamic approach. To address one of these limitations, we implement Monte Carlo simulations of the cellular Potts Model in three dimensions, which allow us to investigate the dependency of the wetting state on the initial state of the droplet. We find that when the droplet is initialized in a CB state, it gets trapped in a local minimum and stays in the repellent behavior irrespective of the theoretical prediction. When the initial state is W, simulations show a good agreement with theory for pillared surfaces for all geometries, but for reentrant surfaces the agreement only happens in few cases: for most simulated geometries the contact angle reached by the droplet in simulations is higher than θC predicted by the model. Moreover, we find that the contact angle of the simulated droplet is higher when placed on the reentrant surfaces than for a pillared surfaces with the same height, width and pillar distance.

Wetting transitions on patterned surfaces with diffuse interaction potentials embedded in a Young-Laplace formulation.

The Minimum Energy Paths (MEPs) of wetting transitions on pillared surfaces are computed with the Young-Laplace equation, augmented with a pressure term that accounts for liquid-solid interactions. The interactions are smoothed over a short range from the solid phase, therefore facilitating the numerical solution of problems concerning wetting on complex surface patterns. The patterns may include abrupt geometric features, e.g., arrays of rectangular pillars, where the application of the unmodified Young-Laplace is not practical. The MEPs are obtained by coupling the augmented Young-Laplace with the modified string method from which the energy barriers of wetting transitions are eventually extracted. We demonstrate the method on a wetting transition that is associated with the breakdown of superhydrophobic behavior, i.e., the transition from the Cassie-Baxter state to the Wenzel state, taking place on a superhydrophobic pillared surface. The computed energy barriers quantify the resistance of the system to these transitions and therefore, they can be used to evaluate superhydrophobic performance or provide guidelines for optimal pattern design.

Electrostatic instability of liquid droplets on MEMS-based pillared surfaces

The electrostatic instability of the Cassie-state droplets for different pitches of pillared surfaces and various ion concentration of liquids has been investigated. MEMS-based pillar structures with well-defined geometrical parameters have been fabricated and wetting behaviors were characterized through systematic experiments. A mathematical model for estimating the critical voltage with which the Cassie-state liquid droplet collapses has been developed incorporating the pinning force and the surface tension under the electric field. The present model is found to be in good agreement with the experimental results, reproducing the electrostatic instability due to the pull-in of the liquid–air interface.

Nanopatterned polymer surfaces with bactericidal properties.

Bacteria that adhere to the surfaces of implanted medical devices can cause catastrophic infection. Since chemical modifications of materials' surfaces have poor long-term performance in preventing bacterial buildup, approaches using bactericidal physical surface topography have been investigated. The authors used Nanoimprint Lithography was used to fabricate a library of biomimetic nanopillars on the surfaces of poly(methyl methacrylate) (PMMA) films. After incubation of Escherichia coli (E. coli) on the structured PMMA surfaces, pillared surfaces were found to have lower densities of adherent cells compared to flat films (67%-91% of densities on flat films). Moreover, of the E. coli that did adhere a greater fraction of them were dead on pillared surfaces (16%-141% higher dead fraction than on flat films). Smaller more closely spaced nanopillars had better performance. The smallest most closely spaced nanopillars were found to reduce the bacterial load in contaminated aqueous suspensions by 50% over a 24-h period compared to flat controls. Through quantitative analysis of cell orientation data, it was determined that the minimum threshold for optimal nanopillar spacing is between 130 and 380 nm. Measurements of bacterial cell length indicate that nanopillars adversely affect E. coli morphology, eliciting a filamentous response. Taken together, this work shows that imprinted polymer nanostructures with precisely defined geometries can kill bacteria without any chemical modifications. These results effectively translate bactericidal nanopillar topographies to PMMA, an important polymer used for medical devices.

Dynamic hydrophobicity of heterogeneous pillared surfaces at the nano-scale

In this study, the static and dynamic behaviors of nano-scale water droplets on heterogeneous surfaces were investigated using molecular dynamics simulations. The surface consisted of a flat plate and pillar structures. The surface was designed with four pillar heights and three pillar characteristic energies. Simulations were first run so that the water droplet reached the static equilibrium state. Once the static water droplets were in Cassie-Baxter state, increasing the pillar height had very little effect on the contact angle. Droplets on the surface with the strongest pillar characteristic energy never reached the Cassie-Baxter state and contact angles tended to decrease with increasing pillar height. Then five forces were applied to the water droplets parallel to the surface to observe the dynamic behavior of the droplets. Then, the effect of the pillar characteristic energy on the behavior of the dynamic water droplet was discussed using the contact angle hysteresis (cosθ Re − cosθ Ad) as the pillar height and the magnitude of the applied force varied. When compared to the homogeneous cases, it was found that except at the lowest pillar height all of the lower pillar characteristic energy cases were hydrophobic and did not depend much on pillar height or magnitude of force. Whereas the higher pillar characteristic energy cases were generally hydrophilic and the hydrophobicity depended greatly on the magnitude of the force.

Dynamic hydrophobicity on flat and pillared graphite surfaces with different pillar surface fractions

The motion of a droplet on a surface is of importance to many fields. While many things are known at the macro-scale still a complete understanding of fluid flow at the nano-scale is far from being known. This study focuses on the dynamic hydrophobicity of a pillar surface with different pillar surface fractions at the nano-scale using molecular dynamics simulations. Five pillar heights and four pillar surface fractions were modeled using a graphite surface which has anisotropic characteristics due to its spaced layers. A nano-sized water droplet with 5124 molecules was run to equilibrium on each surface. Then a body force was applied and the dynamic contact angles were calculated for 5 ns. These contact angles were used to calculate the surface’s effective hydrophobicity. The droplets were categorized into one of three groups as different phenomena were identified depending on the pillar surface fraction, applied force, and pillar height. It was found that at the nano-scale smooth, flat surfaces are dynamically more hydrophobic than any of the cases with pillars. Larger pillar surface fractions tended to be more hydrophobic and the pillar surface fraction of 36% was least affected by pillar height and applied body force.

Automatic Actin Filament Quantification of Osteoblasts and Their Morphometric Analysis on Microtextured Silicon-Titanium Arrays

Microtexturing of implant surfaces is of major relevance in the endeavor to improve biorelevant implant designs. In order to elucidate the role of biomaterial’s topography on cell physiology, obtaining quantitative correlations between cellular behavior and distinct microarchitectural properties is in great demand. Until now, the microscopically observed reorganization of the cytoskeleton on structured biomaterials has been difficult to convert into data. We used geometrically microtextured silicon-titanium arrays as a model system. Samples were prepared by deep reactive-ion etching of silicon wafers, resulting in rectangular grooves (width and height: 2 µm) and cubic pillars (pillar dimensions: 2 × 2 × 5 and 5 × 5 × 5 µm); finally sputter-coated with 100 nm titanium. We focused on the morphometric analysis of MG-63 osteoblasts, including a quantification of the actin cytoskeleton. By means of our novel software FilaQuant, especially developed for automatic actin filament recognition, we were first able to quantify the alterations of the actin network dependent on the microtexture of a material surface. The cells’ actin fibers were significantly reduced in length on the pillared surfaces versus the grooved array (4–5 fold) and completely reorganized on the micropillars, but without altering the orientation of cells. Our morpho-functional approach opens new possibilities for the data correlation of cell-material interactions.

Cassie–Wenzel and Wenzel–Cassie transitions on immersed superhydrophobic surfaces under hydrostatic pressure

For incorporating superhydrophobic surfaces in microfluidic systems, it is important to understand the ability of the superhydrophobic state to withstand hydraulic pressure. In this paper we describe experiments to probe the collapse transition on superhydrophobic surfaces completely covered by water, where the air film formed on the surface is closed. Polyethylene foils nanoimprinted with micrometre sized pillars in different geometries and densities are used as the model superhydrophobic surfaces. The pressure required for the transition from Cassie to Wenzel state is measured for all surfaces and also compared to analytical and numerical models. We find that the closed film of trapped air helps stabilise the Cassie state at low pillar densities and that the effect of a small change in pillar sidewall angle can drastically change the collapse behaviour. Finally, the reverse transition, from Wenzel to Cassie state, is observed on densely pillared surfaces at low water pressure.