Rugotaxis: Droplet motion without external energy supply

TL;DR

This paper investigates droplet movement on nano-patterned, wrinkled substrates with gradient wavelengths, using theoretical and molecular dynamics methods to understand how surface features influence spontaneous droplet motion without external energy.

Contribution

It introduces a comprehensive analysis of rugotaxis on wrinkled surfaces, identifying key parameters that enable droplet motion driven by interfacial energy changes.

Findings

Shallow wrinkles with small wavelength gradients promote rugotaxis.

Moderate droplet adhesion enhances movement, especially for larger droplets.

Interfacial energy gain drives droplet motion toward denser wrinkle regions.

Abstract

Nano-patterned substrates offer possibilities for controlling the motion of fluids without external energy supply in novel technologies in microfluidics, coatings, etc. Here, we report on the rugotaxial motion of droplets on wrinkled substrates with gradient in the wavelength of the wrinkles by exploring a broad range of parameters, such as amplitude of the wrinkles, substrate wettability, droplet size and wavelength gradient. Adopting a theoretical and molecular dynamics approach, we determine the Cassie-Baxter and Wenzel states of the droplets, investigate the efficiency of rugotaxis as a function of different parameters, and discuss additional effects, such as pinning. We find that shallow wrinkles characterised by small wavelength gradients, and moderate adhesion of the droplet to the substrate favour the rugotaxis motion with growing droplet size, when pinning is avoided. We also find that the driving force in rugotaxis is the gain in interfacial energy between the droplet and the substrate as the droplet enters regions of denser wrinkles (smaller wavelengths of the wrinkles).