Nanoscale wettability characterization—Interpreting droplet morphological evolution in nanopores

Abstract

Nanoscale wettability, crucial for various disciplines in science and engineering, challenges traditional theory, particularly the Young's equation. This study proposes and validates a modified format of the Young's equation under nano-confinement and, for the first time, the nano-confined droplet morphological evolution and transition are investigated from thermodynamic theories and molecular dynamics simulation. The morphologies of droplets in nano-silts, identified as double-cap, single-cap, and bridge-shaped, underscore the critical roles of line tension and nano-confinement in characterizing wetting behavior. In hydrophobic nano-slits, droplets transition from the double-cap to the single-cap shape at the critical point of $r_0/H$ = 0.31 and to bridge shape at the critical point of $r_0/H$ = 0.40. Moreover, the relative width of the neck region in the bridge-shaped droplets is found to stabilize at $w/r$ ratio of 1.8. Particularly, linear relationships have been established between the droplet contact angle and the parameter $H/r_0$, which identify condensation and breakage of droplets within hydrophobic nano-slits. This model effectively characterizes nanoscale wettability, with precise droplet behavior predictions, which could be beneficial to enhance nano-fluid dynamics understanding and its applications in science and engineering.