Sandip Khan

Wetting transition of nanodroplets of water on textured surfaces: a molecular dynamics study

The crossover behaviour of water droplets state from the Wenzel state to the Cassie state with varying pillar height and surface fraction is examined critically using molecular dynamics. We report the effect of the system size on the wetting behaviour of water droplets by examining the contact angle for both regimes. We observe that when the droplet size is comparable to the pillar dimension, the contact angle of droplets fluctuates with increasing droplet size because of the contact line pinning, which is more pronounced in the Wenzel regime. We further demonstrate the phantom-wall method to evaluate free energy of intermediate wetting states.

Vapour–liquid phase equilibria of simple fluids confined in patterned slit pores

We present the influence of surface heterogeneity on the vapour–liquid phase behaviour of square-well fluids in slit pores using grand-canonical transition-matrix Monte Carlo simulations along with the histogram-reweighting method. Properties such as phase coexistence envelopes, critical properties and local density profiles of the confined SW fluid are reported for chemically and physically patterned slit surfaces. It is observed that in the chemically patterned pores, fluid–fluid and surface attraction parameters along with the width of attractive and inert stripes play fundamentally different roles in the phase coexistence and critical properties. On the other hand, pillar gap and height significantly affect the vapour–liquid equilibria in the physically patterned slit pores. We also present the effect of chemically and physically patterned slit surfaces on the spreading pressure.

Prewetting transitions of one site associating fluids

Prewetting transitions are studied for Lennard-Jones (LJ) based dimer forming associating fluids, on a structureless surface represented by LJ 9-3 type potential, for various association strengths using grand-canonical transition matrix Monte Carlo (GC-TMMC) and histogram reweighting techniques. Occurrences of prewetting transition are observed for association strengths: epsilon(af)=2.0, 4.0, 6.0, 8.0, and 10.0. Structural properties, monomer fraction, and orientation order profile of thin-thick film of one-site associating fluids are presented. Wetting temperature, T(w), and prewetting critical temperature, T(pwc), increases with increasing association strength, which is in agreement with the results of the density functional theory (DFT). Length of prewetting line, on the other hand, is found to decrease first with increasing association energy until epsilon(af)=8.0 and subsequently found to increase substantially for epsilon(af)=10. This behavior is contrary to the prediction from the DFT. We observe that the boundary tension of thin-thick film via GC-TMMC and finite size scaling exhibits a maximum with respect to association strength.

Surface phase transitions of multiple-site associating fluids

Surface phase transitions of Lennard–Jones (LJ) based two- and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor–liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor–liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.