Molecular Dynamics Simulation of Nanodroplet Impacting on Wettability Gradient Surface

The work aims to study the effects of different impact velocity and wettability gradient on droplet movement during the process of droplet impacting on solid surface. The specific data for spreading and bouncing droplets were obtained by molecular dynamics (MD) simulation, which was difficult to be measured quantitatively in the experiment and had reference value for guiding engineering practice. The MD method was used to simulate the nanodroplet impacting on solid surface with wettability gradient at different initial velocities v0. The behavior of droplet impacting on solid surface was affected by droplet impact velocity, presence of impurities in the droplet and solid surface characteristics. The solid surface characteristics mainly included wettability, surface roughness and temperature. The wettability of solid surface could be reflected by the contact angle of droplet surface. The wettability gradient was constructed by changing the gap of nano square columns on solid surface. The smaller the square column gap, the smaller the contact angle, and the better the wettability. When v0=0.3-1.1 nm/ps, the droplet after hitting the surface moved along the direction of good wettability on the solid surface. When v0=0.3-0.5 nm/ps, the droplet was hindered by the pinning effect and hardly spread. When v0=0.7-1.1 nm/ps, the droplet underwent secondary spreading. Owing to the effect of initial kinetic energy and pinning effect of the droplet, two inflection points were observed in the curve of moving velocity vt when the droplet centroid left the gradient surface. The x-direction velocity vt of the droplet leaving the gradient surface was mainly related to the initial velocity v0 and the total spreading time T of the droplet. The x-direction velocity vt increased with the increase of the initial kinetic energy of the droplet. vt decreased with the increase of T, because the kinetic energy of droplets was consumed more in the spreading process. When v0=1.2-1.5 nm/ps, the droplet bounced after hitting solid surface. At this time, the velocity component of the droplet in the vertical direction of v0 increased with the increase of v0, and the relationship between them was linear, while the velocity component in the horizontal direction of v0 remained unchanged, which corresponded to the fixed value of 0.017 nm/ps. The bouncing velocity v and angle α of droplet increased with the increase of v0. In the process when a droplet impacts on a solid surface with wettability gradient, the relationship between maximum spreading factor βmax and v0 is proposed to be approximately linear. After the impact, the low-velocity droplets are captured, which then move along the direction of good wettability. The relationship between vt and v0 in different impact velocity ranges is proposed herein. The high-velocity droplets bounce along the side with good wettability. Furthermore, the relationship among droplet bounce velocity, angle, and v0 is proposed, respectively. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.