Paradoxical Coffee-Stain Effect Driven by the Marangoni Flow Observed on Oil-Infused Surfaces

TL;DR

This study investigates the coffee-stain effect on oil-infused surfaces, revealing that soluto-capillary Marangoni flow causes ring deposits during droplet evaporation, contrary to typical expectations.

Contribution

It demonstrates the paradoxical coffee-stain formation driven by Marangoni flow on oil-infused surfaces, highlighting the role of contact line depinning and evaporation dynamics.

Findings

Ring-like deposits form across various salt concentrations.

The effect occurs when the droplet contact line depins.

Marangoni flow, not thermo-capillary flow, drives deposit formation.

Abstract

Formation of coffee stain deposits under evaporation of droplets containing aqueous solution of salts placed on silicone-oil impregnated substrates was observed. The formation of ring-like deposits was registered for various molar concentrations of salts for the droplets of 5-300 microlitres in volume. The effect occurred when the contact line was de-pinned, and the evaporation from the edge of a droplet was stopped by the silicone oil. The formation of the coffee stain deposit is related to the soluto-capillary Marangoni flow; the influence of the thermo-capillary flow taking place in parallel is negligible.