Wetting transition of a nanodrop on switchable hydrophilic-hydrophobic surfaces

• Published in: Surfaces and interfaces 2020-12, Vol.21, p.100628, Article 100628
• Main Authors: Roy Choudhuri, J., Nath, P.
• Format: Article
• Language: English
• Subjects: Contact angle; Friction; Insulator; Molecular simulation; Relaxation
• ISSN: 2468-0230; 2468-0230
• DOI: 10.1016/j.surfin.2020.100628

The dynamic modulation of contact angle on switchable hydrophilic-hydrophobic surfaces is one of the important factors in designing miniaturized optical devices. A spreading and retention of a nano water droplet placed on an insulator surface covering two planar electrode layers and perpendicularly in contact with a conductive tip is studied using atomistic molecular simulation. The change in the droplet shape is initiated by modifying interaction energy between the insulator atoms and the water molecules. The contact angle analysis shows a unique dependence on the nature of the underlying substrate. Time correlation function analysis based on the calculation of variation of the height of the drop’s center of mass with time estimates the rate of relaxations for wetting-dewetting behavior subject to the condition of surface hydrophilic and hydrophobic nature. For the hydrophilic case, the relaxation rate of the drop is found to be twice slower than the hydrophilic one. The difference in the rate of relaxations is analyzed by considering the effects of the frictional force on the contact line motion of the drop. Friction coefficients are calculated on the basis of continuum hydrodynamic (HD) and molecular kinetic theory (MKT) theories. The dependence of the friction coefficients on the surface properties depicts the strong dominance of the liquid-substrate friction on the drop’s perimeter motion based on molecular kinetic theory.