Driving Droplets by Curvi-Propulsion

TL;DR

This paper introduces a curvature-gradient mechanism called curvi-propulsion that significantly enhances the spontaneous and directional movement of small liquid droplets on solid surfaces, outperforming traditional wettability gradient methods.

Contribution

The study demonstrates that curvature gradients can effectively accelerate small droplets, with experimental and simulation evidence showing speeds much higher than previous methods, applicable to both hydrophilic and hydrophobic surfaces.

Findings

Maximum droplet speed of 0.28 m/s on glass cones

Force on droplets scales with curvature gradient and diverges near cone tips

Nanometer-scale droplets can move at over 100 m/s on nano-cones

Abstract

How to make small liquid droplets move spontaneously and directionally on solid surfaces is a challenge in lab-on-chip technologies, DNA analysis, and heat exchangers. The best-known mechanism, a wettability gradient, does not move droplets rapidly enough for most purposes and cannot move droplets smaller than a critical size defined by the contact angle hysteresis. Here we report on a mechanism using curvature gradients, which we show is particularly effective at accelerating small droplets, and works for both hydrophilic and hydrophobic surfaces. Experiments for water droplets on glass cones in the sub-millimeter range show a maximum speed of 0.28 m/s, two orders of magnitude higher than obtained by wettability gradient. From simple considerations of droplet surface area change, we show that the force exerted on a droplet on a conical surface scales as the curvature gradient. This force therefore diverges for small droplets near the tip of a cone. We illustrate this using molecular dynamics simulations, and describe nanometer-scale droplets moving spontaneously at over 100 m/s on nano-cones.