How can the geometry of a rough surface affect its wettability? - A coarse-grained simulation analysis

Abstract

Governing the physical and chemical characteristics of contact area among solid substrate and liquid droplets is a widely used strategy to fabricate superhydrophobic and superhydrophilic surfaces.While both surface morphology and surface free energy of a solid substrate conclude its wettability, designing the surface with right topology has reserved immense focus by researchers in last few decades. In the pursuit to achieve such goals of immaculate surfaces, some intriguing question need to be answered, for instance, what will happen if one takes same material for the solid substrate but having different morphological features? Irrespective of chemical nature, how the physical appearance of a rough surface controls the wettability? To unravel these questions, we examine the wettability of different surrogate models using coarse-grain (CG) simulations. Interesting results were obtained for apparent contact angles on varying the morphology of a particular surface with different geometrical shapes. Different surface geometries such as square, nail, solid sphere, hollow sphere, rod, and hollow sphere of hydrophobic (poly(dimethylsiloxane)) (PDMS) and hydrophilic (poly (vinyl alcohol)) (PVA) polymers were created. In hydrophobic case, the square followed by nail shape corrugated surface demonstrated least wettability by restricting the penetration of water beads inside the grooves. Whereas, for hydrophilic case, the shell shaped surface showed excellent wetting condition due to maximum availability of unoccupied volume. It was concluded from the present study that hollow cylinder and square shaped corrugated surface could be respectively used to obtain the maximum and the least surface wettability possible. This investigation, through the evolved understanding of the role of surface geometry at the nano level could guide researchers and materials scientists to develop effective materials with desired wetting conditions.