Impact of droplets on spherical surfaces with different wettability at the nanoscale: A molecular dynamics study

Abstract

Spherical surfaces that can manipulate impacting droplets are of great importance to a wide range of practical applications, such as lab on a chip and digital microfluidics. In this work, we utilize spherical surfaces with varying wettability to achieve the objectives of directional movement, deflected bounce, and deflected dripping. These outcomes may be a good candidate for the use of classification or screening to impact water droplets. A total of five outcomes with respect to We and λ (λ = D0/DJ) have been reported by drawing a phase diagram. The mechanisms behind each outcome are discussed systematically by observing snapshots, extracting spreading factor, and analyzing instantaneous velocity distributions. The asymmetric behavior of the droplet's recoil is observed, which is a consequence of the unbalanced line tension. Moreover, the difference in Young's forces promotes the behavior of directional movement and deflected bounce. Among these outcomes, the deflected dripping is the unique outcome that occurs at a high We range but with a small λ value. The specific dynamic behavior stems from the energy difference within covering droplets, which generates a special interflow toward the moderately hydrophobic side. Finally, in the outlook part, several other types of wettability-difference particles are proposed, and their potential effect on altering impacting behavior has been discussed briefly. This work paves the way to understand the effect of anisotropy of solid spheres on nanoscale impingement and provides new insights into how to control impacting droplets using well-performed wettability-difference particles.