Ultrafast Spontaneous Motion of Nanodroplets

TL;DR

This paper demonstrates that curvature gradients can induce ultrafast spontaneous motion of nanodroplets, surpassing traditional wettability gradient methods, with experimental and simulation evidence showing speeds up to 100 m/s.

Contribution

It introduces a novel mechanism using curvature gradients to accelerate small droplets rapidly, applicable to both hydrophilic and hydrophobic surfaces.

Findings

Maximum droplet speed of 0.28 m/s on tapered surfaces

Nanoscale droplets reach over 100 m/s in simulations

Force scales with surface curvature gradient

Abstract

Making liquid droplets move spontaneously on solid surfaces is a key challenge in lab-on-chip and heat exchanger technologies. The best-known mechanism, a wettability gradient, does not generally move droplets rapidly enough and cannot drive droplets smaller than a critical size. Here we report how a curvature gradient is particularly effective at accelerating small droplets, and works for both hydrophilic and hydrophobic surfaces. Experiments for water droplets on tapered surfaces with curvature radii in the sub-millimeter range show a maximum speed of 0.28 m/s, two orders of magnitude higher than obtained by wettability gradient. We show that the force exerted on a droplet scales as the surface curvature gradient. Using molecular dynamics simulations, we observe nanoscale droplets moving spontaneously at over 100 m/s on tapered surfaces.