Molecular dynamics simulation of water condensation on surfaces with tunable wettability (original) (raw)
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Molecular Dynamics Simulations of Water Condensation on Surfaces with Tunable Wettability
Langmuir, 2020
Water condensation plays a major role in a wide range of industrial applications. Over the past few years, many studies have shown interest in designing surfaces with enhanced water condensation and removal properties. It is well known that heterogeneous nucleation outperforms homogeneous nucleation in the condensation process. Because heterogeneous nucleation initiates on a surface at a small scale, it is highly desirable to characterize water-surface interactions at the molecular level. Molecular dynamics (MD) simulations can provide direct insight into heterogeneous nucleation and advance surface designs. Existing MD simulations of water condensation on surfaces were conducted by tuning the solid-water van der Waals interaction energy as a substitute for modeling surfaces with different wettabilities. However, this approach cannot reflect the real intermolecular interactions between the surface and water molecules. Here, we report MD simulations of water condensation on realistic surfaces of alkanethiol self-assembled monolayers with different head group chemistries. We show that decreasing surface hydrophobicity significantly increases the electrostatic forces between water molecules and the surface, thus increasing the water condensation rate. We observe a strong correlation between our rate of condensation results and the results from other surface characterization metrics, such as the interfacial thermal conductance, contact angle, and the molecular-scale wettability metric of Garde and co-workers. This work provides insight into the water condensation process at the molecular scale on surfaces with tunable wettability.
Chemphyschem, 2005
We present results from molecular dynamics simulations of water near structured hydrophobic surfaces. The surface structures reported herein are a planar alkane crystal as a reference and crystals with a hole and a protrusion of approximately 2.5 nm diameter and 0.5 nm depth or height. All indicators show that surface structuring increases the hydrophobicity: The water density is reduced near the structure elements, and the number of residual contacts between water and the surface decreases by about 40 % with respect to the planar surface. Thermodynamic integration shows that the interfacial energy of the structured surfaces is about 7 mJ m−2higher for structured surfaces than for the planar surface. The hydrophobicity increases by a similar amount for the hole and the protrusion geometries compared to the planar surface.
The Journal of Physical Chemistry C, 2014
First principles molecular dynamics simulations are used to gain an atomistic-level insight into how the molecular behavior of interfacial water is influenced by specific surface adsorbates. Although the overall hydrophobic versus hydrophilic character of a given surface is widely recognized to be important in determining the behavior of interfacial water molecules, we show that subtle molecular details may also play a role in determining the dynamical behavior of water. By comparing water diffusivity at three different non-polar surfaces, we find that specific surface features can lead to a suppression of hydrogen bond network ring structures by enhancing hexagonal spatial distributions of water molecules near the surface. Such a distinct molecular dependent behavior of the interfacial water was found to persist well into the liquid, while the most structural properties are noticeably influenced in only the first water layer. Fig 8. Enumeration of rings in (a) region I and (b) region II at H-, CH3and CF3-Si surfaces
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.
Nanoscale condensation of water on self-assembled monolayers
Soft Matter, 2011
We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage (∼2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures (∼10-8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 °C under atmospheric conditions did lead to evaporation from the surface. We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry. 2011 The Royal Society of Chemistry.
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
Soft Matter, 2010
We carried out molecular dynamics simulations of water droplets on self-assembled monolayers of perfluorocarbon molecules. The interactions between the water droplet and the hydrophobic fluorocarbon surface were studied by systematically changing the molecular surface coverage and the mobility of the tethered head groups of the surface chain molecules. The microscopic contact angles were determined for different fluorocarbon surface densities. The contact angle at a nanometer length scale does not show a large change with the surface density. The structure of the droplets was studied by looking at the water density profiles and water penetration near the hydrophobic surface. At surface densities near close packed coverage of fluorocarbons, the water density shows an oscillating pattern near the boundary with a robust layered structure. As the surface density decreased and more water molecules penetrated into the fluorocarbon surface, the ordering of the water molecules at the boundary became less pronounced and the layered density structure became diffuse. The water droplet is found to induce the interfacial surface molecules to rearrange and form unique topological structures that minimize the unfavorable water-surface contacts. The local density of the fluorocarbon molecules right below the water droplet is measured to be higher than the density outside the droplet. The density difference increases as the overall surface density decreases. Two different surface morphologies emerge from the water-induced surface reorganization over the range of surface coverage explored in the study. For surface densities near closed packed monolayer coverage, the height of the fluorocarbons is maximum at the center of the droplet and minimum at the watervapor-surface triple junction, generating a convex surface morphology under the droplet. For lower surface densities, on the other hand, the height of the fluorocarbon surface becomes maximal at and right outside the water-vapor-surface contact line and decreases quickly towards the center of the droplet, forming a concave shape of the surface. The interplay between the fluorocarbon packing and the water molecules is found to have profound consequences in many aspects of surface-water interactions, including water depletion and penetration, hydrogen bonding, and surface morphologies.
Concentration-driven surface transition in the wetting of mixed alkanethiol monolayers on gold
Journal of The American Chemical Society, 1991
The construction of mixed monolayers containing hydrophobic and hydrophilic components for which the contact angles for three different liquids vary as a highly nonlinear function of the monolayer composition is reported. It is suggested that a prewetting, crystalline-like layer of water, possibly formed from bulk vapor, is present near the hydrophilic surface, because of an enhanced surface chemical potential ("surface field"). As the concentration of the hydrophilic component is lowered, increasing "quenched randomness" in the distribution of surface fields destroys the surface condensed water phase, thus triggering the observed nonlinearity in the contact angles. The microscopic structure of the water molecules adsorbed on an OH surface is revealed by continuum Monte Carlo simulations, with realistic force fields, and the scenario is supported by mean-field calculations on a simplified lattice-gas model. The observed wetting behavior at 30% relative humidity was altered for a relative humidity 52%, as well as when the surface of the monolayer was molecularly roughened by the addition of two CH2 groups to the hydrophobic (CH,-terminated) component of the mixed monolayers. It is suggested that this transitional phenomenon is due to a possible (true or rounded) surface phase transition, due to the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields.
Dynamics of water condensation on a switchable surface originated from molecular orientations
Physical review, 2021
In heat transfer systems, how water condenses on the surface is critical to the energy efficiency of the system. With fixed surface wettability, hydrophilic surfaces enhance the nucleation rate but result in filmwise condensation due to pinning effect, which impedes the heat transfer between water vapor and surface during droplet growth. A hydrophilic surface with high drop mobility is realized with static tailored wettability surfaces, while tunable surfaces have potential in more comprehensive manipulation in condensation with different scale in time and scale. However, the mechanism has rarely been investigated and elucidated. In this paper, we investigate water condensation on a tunable surface originated from surface tension distribution control. The surface tension distribution under applied electric field is modeled and tested. We demonstrate that the surface tension manipulated by liquid crystal orientation alters the nucleation site density. Also, the periodic surface tension distribution aligns condensed water drops and decelerates the radius growth of droplets. The mechanism of active water condensation manipulation can be further applied to other tunable surfaces for various applications such as atmospheric water generator, heat transfer systems, and desalination systems.