Capillary droplets on Leidenfrost micro-ratchets

TL;DR

This paper investigates the movement of micron-sized capillary droplets on heated micro-ratchets, extending the viscous mechanism theory to explain the forces involved, supported by new experimental measurements.

Contribution

It introduces a simple measurement method for droplet thrust on micro-ratchets and extends the viscous mechanism theory to capillary-sized droplets.

Findings

Good agreement between the extended viscous mechanism and experimental data.

Micro-ratchets effectively transport capillary droplets at terminal velocities around 10 cm/s.

New laser-ablated micro-ratchets enable detailed study of droplet propulsion mechanisms.

Abstract

Leidenfrost ratchets are structures with the ability of transporting liquid droplets when heated over the critical Leidenfrost temperature. Once this temperature is reached, the droplet levitates over the surface and moves in the direction marked by the slope of the ratchet at terminal velocities around 10 cm/s. Here we provide new experiments with micron-sized ratchets, which have been produced with picosecond pulse laser ablation. In the following work, we use a simple method to measure the thrust driving droplets of capillary size over the micro-ratchets. The mechanism responsible for the force acting on the drop on superheated ratchets has been recently under debate. We extend the recently proposed 'viscous mechanism' proposed by Dupeaux et al. [Europhys. Lett., 96, 58001 (2011)] to capillary droplets and find good agreement with our measurements.