Ultrafast Drop Movements Arising from Curvature Gradient

TL;DR

This paper reports the discovery of ultrafast spontaneous water drop movements on curved surfaces, driven by curvature gradients, with potential applications in droplet transport technologies.

Contribution

It reveals a universal mechanism where mean curvature gradients induce rapid droplet movements, surpassing known effects like Marangoni.

Findings

Maximum observed speed of 0.22 m/s on experimental surfaces.

Simulated speeds up to 125 m/s on nanoscale cones.

Movement driven by mean curvature gradient regardless of surface wettability.

Abstract

We report experimental observation of a kind of fast spontaneous movements of water drops on surfaces of cones with diameters from 0.1 to 1.5 mm. The observed maximum speed (0.22 m/s) under ambient conditions were at least two orders of magnitude higher than that resulting from any known single spontaneous movement mechanism, for example, Marangoni effect due to gradient of surface tension. We trapped even higher spontaneous movement speeds (up to 125 m/s) in virtual experiments for drops on nanoscale cones by using molecular dynamics simulations. The underlying mechanism is found to be universally effective - drops on any surface either hydrophilic or hydrophobic with varying mean curvature are subject to driving forces toward the gradient direction of the mean curvature. The larger the mean curvature of the surface and the lower the contact angle of the liquid are, the stronger the driving force will be. This discovery can lead to more effective techniques for transporting droplets.