Coffee-stain growth dynamics on dry and wet surfaces

TL;DR

This study combines theoretical and experimental approaches to analyze how evaporation conditions influence particle deposit growth in drying colloidal drops, revealing that evaporation dynamics change but final deposits remain similar.

Contribution

It introduces a comprehensive analysis of particle deposit growth under dry and wet evaporation conditions, highlighting the influence of surrounding vapor concentration on drying dynamics.

Findings

Evaporation on wet surfaces suppresses flux singularity and leads to homogeneous evaporation.

Dry and wet conditions alter the drying dynamics but not the final particle accumulation.

Theoretical predictions match experimental results for deposit growth and drying timescales.

Abstract

The drying of a drop containing particles often results in the accumulation of the particles at the contact line. In this work, we investigate the drying of an aqueous colloidal drop surrounded by a hydrogel that is also evaporating. We combine theoretical and experimental studies to understand how the surrounding vapor concentration affects the particle deposit during the constant radius evaporation mode. In addition to the common case of evaporation on an otherwise dry surface, we show that in a configuration where liquid is evaporating from a flat surface around the drop, the singularity of the evaporative flux at the contact line is suppressed and the drop evaporation is homogeneous. For both conditions, we derive the velocity field and we establish the temporal evolution of the number of particles accumulated at the contact line. We predict the growth dynamics of the stain and the drying timescales. Thus, dry and wet conditions are compared with experimental results and we highlight that only the dynamics is modified by the evaporation conditions, not the final accumulation at the contact line.