Surfactant-driven flow transitions in evaporating droplets

TL;DR

This study investigates how surfactants influence flow patterns in evaporating droplets, revealing that surfactants can alter surface properties and flow directions, affecting the coffee-stain effect through detailed experimental analysis.

Contribution

The paper provides new insights into how different surfactants modify droplet surface properties and flow dynamics, with detailed 3D particle tracking and surface tension measurements.

Findings

Surfactants can make the droplet surface rigid or elastic.

Flow can be enhanced or inverted depending on surfactant type.

Detailed surface tension differences are measured with high resolution.

Abstract

An evaporating droplet is a dynamic system in which flow is spontaneously generated to minimize the surface energy, dragging particles to the borders and ultimately resulting in the so-called "coffee-stain effect". The situation becomes more complex at the droplet's surface, where surface tension gradients of different nature can compete with each other yielding different scenarios. With careful experiments and with the aid of 3D particle tracking techniques, we are able to show that different types of surfactants turn the droplet's surface either rigid or elastic, which alters the evaporating fluid flow, either enhancing the classical coffee-stain effect or leading to a total flow inversion. Our measurements lead to unprecedented and detailed measurements of the surface tension difference along an evaporating droplet's surface with good temporal and spatial resolution.