The influence of molecular-scale roughness on the surface spreading of an aqueous nanodrop

Abstract

We examine the effect of nanoscale roughness on spreading and surface mobility of water nanodroplets. Using molecular dynamics, we consider model surfaces with sub-nanoscale asperities at varied surface coverage and with different distribution patterns. We test materials that are hydrophobic, and those that are hydrophilic in the absence of surface corrugations. Interestingly, on both types of surfaces, the introduction of surface asperities gives rise to a sharp increase in the apparent contact angle. The Cassie-Baxter equation is obeyed approximately on hydrophobic substrates, however, the increase in the contact angle on a hydrophilic surface differs qualitatively from the behavior on macroscopically rough surfaces described by the Wenzel equation. On the hydrophobic substrate, the superhydrophobic state with the maximal contact angle of 180 degrees is reached when the asperity coverage falls below 25%, suggesting that superhydrophobicity can also be achieved by the nanoscale roughness of a macroscopically smooth material. We further examine the effect of surface roughness on droplet mobility on the substrate. The apparent diffusion constant shows a dramatic slow down of the nanodroplet translation even for asperity coverage in the range of 1% for a hydrophilic surface, while droplets on corrugated hydrophobic surfaces retain the ability to flow around the asperities. In contrast, for smooth surfaces we find that the drop mobility on the hydrophilic surface exceeds that on the hydrophobic one.

Probing surface tension additivity on chemically heterogeneous surfaces by a molecular approach

“…1 reproduces the nonlinear dependence of contact angles on f CH 3 with comparable accuracy to that achieved in the linear, f ðSASÞ-based representation in Fig. 6 B and C. Recent simulation studies (47,48) show Wenzel correction (6) may not always apply at subnanoscale roughness. The assumption we use implicitly is that Wenzel factors r α can account adequately for any changes in accessible areas of moieties due to mixing.…”

“…When adjacent surface areas have very different polarities, fluctuations of nanodroplet base (17,48) favor inclusion of polar patches. Simulated distribution of water atop a heterogeneous surface composed of melittin fragments, (Fig.…”

Nanodrop on a nanorough hydrophilic solid surface: Contact angle dependence on the size, arrangement, and composition of the pillars

“…For nanodrops, there is theoretical evidence, obtained by the density functional theory [8] and Monte Carlo simulations [9], which even on a hydrophilic surface the contact angle increases with increasing roughness. In order to answer to the question of how specific modifications of a hydrophilic surface affect its wetting properties, in the present paper, we examine nanodrops on such a surface in more details than in the previous papers [8,9]. In particular, by changing the distance between pillars, the most effective configuration is identified which provides the largest increase in the contact angle.…”

Hydrophobicity of proteins and nanostructured solutes is governed by topographical and chemical context

“…Theory of inhomogeneous liquids connects water density fluctuations to other quantities, such as water compressibility, transverse water density correlations, and the free energy of cavity formation, all of which can also serve as molecular measures of context-dependent hydrophobicity (24,(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). Such measures have been used previously to study aspects of context-dependent hydrophobicity of nanoscale surfaces (35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48).…”