Water Droplet and Its Contact Line Characteristics on Hydrophobic and Hydrophilic Surfaces: A Molecular Dynamics Simulation Approach.

Abstract

In literature, for a water droplet on a solid surface, the set of points at the intersection of the three phases, solid_ liquid_ gas, is referred to as the triple phase contact line (TPCL). However, recent studies indicate that the intersections of these phases form a region, which we refer to as the triple phase contact vicinity (TPCV). In the present work, the dimensions of the TPCV, including its width and cross-sectional area, have been calculated for a water droplet on a wide range of hydrophilic and hydrophobic surfaces, using molecular dynamics simulations. Additionally, the behavior of molecules, including their presence frequency, velocity, and displacement, has been studied. The results indicate that, as the surface becomes more hydrophobic, the width of the TPCV increases and its cross-sectional area decreases. The presence frequency of molecules located at the TPCV in equilibrium shows that the molecules are arranged in a Gaussian distribution and exhibit oscillatory movements around their average positions. It has also been shown that in equilibrium TPCV of hydrophobic surfaces, there are more molecules moving toward the center of the droplet relative to hydrophilic surfaces. Conversely, in equilibrium TPCV of hydrophilic surfaces, there are more molecules that move toward wetting the substrate relative to hydrophobic surfaces. Furthermore, for hydrophobic surfaces, the velocity of molecules moving toward the center of the droplet is greater than the velocity of molecules moving toward wetting the substrate. On hydrophilic surfaces, these two velocities are almost identical. Water molecules on hydrophobic surfaces move faster in all directions; however, on hydrophilic surfaces, they move slower in one direction. The density profile of the droplet on the surface shows that for hydrophobic surfaces, the highest density is reported at the center of the droplet. In contrast, for hydrophilic surfaces, due to the formation of layers of water molecules parallel to the substrate, the highest density is reported in the closest layer to the surface. At the end, considering the importance of graphene and its wettability behavior, which has recently attracted significant attention, the investigations related to the TPCV on the graphene surface are also reported.