Molecular Dynamics Simulation of Water Near Nanostructured Hydrophobic Surfaces: Interfacial Energies (original) (raw)
Related papers
Langmuir, 2005
The local structure of water near hydrophobic surfaces of different surface topographies has been analyzed by molecular dynamics simulation. An alkane crystal has been taken as the parent model for a hydrophobic surface. Surface structures were created by placing pits into it, which were half a nanometer deep and several nanometers wide. Around all structures, the water has a lower density, less orientational ordering, fewer water-water hydrogen bonds, and fewer surface contacts than for a flat unstructured surface. This indicates that the structured surfaces are more hydrophobic than the flat surface. Of the structures investigated, pits with a diameter of approximately 2.5 nm were effective in increasing the hydrophobic character of the surface.
A study on the dynamic behaviors of water droplets impacting nanostructured surfaces
AIP Advances, 2011
We have investigated the influence of impact velocity and intrinsic surface wettability of nanostructures on the impact dynamic behaviors of water droplets on nanostructure surfaces. Nanowires array surfaces with tunable wettabilities ranging from superhydrophilic to superhydrophobic were fabricated by the deposition of surface modifiers differing in alkyl chain length. The transition criteria of rebound/wetting state and rebound/splashing state based on the relationship between the Webber (We) number and the surface free energy were determined. We have confirmed that the critical We number that determines the transition of the rebound/wetting increased as surface energy decreased. Additionally, the We number at which fragmentation occurred on our superhydrophobic surface was relatively low compared to previously reported values.
Journal of Low Temperature Physics, 2009
We derive coarse-grained potentials to describe the interaction of a physically adsorbed, fluid-phase atom with a solid surface that is patterned with an array of rectangular or cylindrical pillars. The coarse-grained potentials are used in molecular dynamics simulations to probe the wetting of a Lennard-Jones liquid droplet on various patterned solid surfaces. Our results, which indicate that surface patterning can significantly influence wetting, are in agreement with previous studies.
We used molecular dynamics (MD) simulations to study the wetting of Lennard-Jones cylindrical droplets on surfaces patterned with grooves. By scaling the surface topography parameters with the droplet size, we find that the preferred wetting modes and contact angles become independent of the droplet size. This result is in agreement with a mathematical model for the droplet free energy at small Bond numbers for which the effects of gravity are negligible. The MD contact angles for various wetting modes are in good agreement with those predicted by the mathematical model. We construct phase diagrams of the dependence of the wetting modes observed in the MD simulations on the topography of the surface. Depending on the topographical parameters characterizing the surface, multiple wetting modes can be observed, as is also seen experimentally. Thus, our studies indicate that MD simulations can yield insight into the large-length-scale behavior of droplets on patterned surfaces.
Influence of surface morphology and nano-structure on hydrophobicity: A molecular dynamics approach
Applied Surface Science, 2019
In this study, water droplet behavior on hydrophobic and superhydrophobic surfaces with different morphologies and geometries has been investigated using molecular dynamics method. The contact angle and potential energy of water droplet on nanostructured surfaces has been evaluated as criteria of hydrophobicity. Results show that surface morphology, distance between columns, void fraction and the growth angle are the main parameters that significantly influence surface hydrophobicity. Wetting transition from Cassie-Baxter state to Wenzel state of simulated structures show that contact the angle decreases with increasing the distance between the columns. Superhydrophobic thin films with different shapes and dimensions, namely tetragonal helical sculptured, gecko feet and lotus leaves were designed and simulated. A contact angle of 155.7° for tetragonal helical sculptured structure is achieved. Interestingly, gecko feet and lotus leaves structures show oscillatory behavior of potential energy curves. It implies that these surfaces are more hydrophobic than other simulated films.
A Theory for the Morphological Dependence of Wetting on a Physically Patterned Solid Surface
We present a theoretical model for predicting equilibrium wetting configurations of two-dimensional droplets on periodically grooved hydrophobic surfaces. The main advantage of our model is that it accounts for pinning/ depinning of the contact line at step edges, a feature that is not captured by the Cassie and Wenzel models. We also account for the effects of gravity (via the Bond number) on various wetting configurations that can occur. Using freeenergy minimization, we construct phase diagrams depicting the dependence of the wetting modes (including the number of surface grooves involved in the wetting configuration) and their corresponding contact angles on the geometrical parameters characterizing the patterned surface. In the limit of vanishing Bond number, the predicted wetting modes and contact angles become independent of drop size if the geometrical parameters are scaled with drop radius. Contact angles predicted by our continuum-level theoretical model are in good agreement with corresponding results from nanometer-scale molecular dynamics simulations. Our theoretical predictions are also in good agreement with experimentally measured contact angles of small drops, for which gravitational effects on interface deformation are negligible. We show that contact-line pinning is important for superhydrophobicity and that the contact angle is maximized when the droplet size is comparable to the length scale of the surface pattern.
Evaporation-Triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures
Physical Review Letters, 2010
When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.
Wetting of chemically heterogeneous striped surfaces - Molecular dynamics simulations.pdf
Using molecular dynamics simulations, we thoroughly investigated the wetting behaviors of a chemically heterogeneous striped substrate patterned with two different wetting materials, face-centered cubic gold and face-centered cubic silver. We analyzed the density distributions, normal stress distributions, surface tensions, and contact angles of a water droplet placed on the substrates at different heterogeneities. We found that the density and stress profile of the water droplet near the substratewater interface were noticeably affected by altering the gold and silver contents in the substrate. Specifically, a greater portion of gold (more wetting) or smaller portion of silver (less wetting) in the substrate composition induced higher densities and higher normal stresses in the vicinity of the substrate surface. Also, it was observed that the surface tensions at liquid-vapor interface and solid-vapor interface were not largely impacted by the change of the substrate composition while the solid-liquid surface tension decreased exponentially with increasing fraction of gold. Most importantly, we found that contact angle of a nanometer-sized water droplet resting on the chemically heterogeneous striped substrate does not show linear dependence on corresponding surface fractions like that predicted by Cassie-Baxter model at the macro-scale. Consequently, we proposed a method for successfully predicting the contact angle by including the critical effects of the substrate heterogeneity on both surface tensions and line tension at the three-phase contact line of the water droplet and the chemically striped substrate. © 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license