Avalanche of particles in evaporating coffee drops

TL;DR

This study uses experimental measurements and hydrodynamic modeling to analyze the particle avalanche phenomenon in evaporating coffee drops, revealing how particles are transported and packed during the final stages of evaporation.

Contribution

It provides detailed experimental evidence and a hydrodynamic explanation for the particle avalanche and packing structures in evaporating droplets, extending prior work on coffee stain formation.

Findings

Particles are dragged in an avalanche during the last evaporation moments.

Outer particle layers form a crystalline array, inner layers resemble jammed granular fluid.

Radial velocity diverges as droplet evaporates completely.

Abstract

The pioneering work of Deegan et al. [Nature 389, (1997)] showed how a drying sessile droplet suspension of particles presents a maximum evaporating flux at its contact line which drags liquid and particles creating the well known coffee stain ring. In this Fluid Dynamics Video, measurements using micro Particle Image Velocimetry and Particle Tracking clearly show an avalanche of particles being dragged in the last moments, for vanishing contact angles and droplet height. This explains the different characteristic packing of the particles in the layers of the ring: the outer one resembles a crystalline array, while the inner one looks more like a jammed granular fluid. Using the basic hydrodynamic model used by Deegan et al. [Phys. Rev. E 62, (2000)] it will be shown how the liquid radial velocity diverges as the droplet life comes to an end, yielding a good comparison with the experimental data.