Numerical estimation of droplet motion on linear wettability gradient surface in microgravity environment

Abstract

Self-propelled motion of liquid droplets on wettability gradient surface (WGS) is an emerging field due to its various applications. Simulations are performed to investigate droplet motion on WGS under gravity and microgravity conditions. Herein, transient, laminar, 3-dimensional Continuity and Navier-Stokes equation are solved for the computational domain consisting of liquid water droplet on WGS in presence of air, applying no-slip, constant contact angle boundary conditions at surface using ANSYS FLUENT® for the computational domain. Simulation results are validated with the available experimental and simulation results and are found to be in good agreement. Further, a parametric study is performed to exemplify the effect of gravity, initial contact angle, drop volume and surface gradient on droplet velocity, spreading, position and surface shear stress. Present study demonstrates, shape evolution of droplet moving on WGS is attributed to caterpillar alike inching motion in both gravity and microgravity conditions. Droplet is almost stagnant on surface with initial superhydrophobic angle (θhigh > 150°) with low gradient (Δθe = 0.2° and 1°). However, despite of higher spreading droplet accelerates on hydrophobic surfaces (θhigh = 100°) with higher gradient (Δθe = 10°). The effect of gravity is negligible for small droplets. However, substantially affects large droplet due to increased value of Bond number. Present results are helpful for designing and optimizing wettability graded surfaces which endures droplet splitting and coalescence in various microgravity engineering systems. In addition, the WGS are also advantageous for heat transfer augmentation.