Two-component Marangoni-contracted droplets: friction and shape

TL;DR

This study investigates the motion, deformation, and dynamics of self-contracting, Marangoni-stabilized droplets on inclined surfaces, revealing their unique frictionless behavior and how they deform at high speeds.

Contribution

It provides a detailed analysis of the motion and deformation of Marangoni-contracted droplets, highlighting their negligible pinning and the relationship between capillary and Bond numbers.

Findings

Droplets exhibit negligible pinning forces.

Capillary number is proportional to Bond number at small velocities.

Droplets deform and deposit smaller droplets at high velocities.

Abstract

When a mixture of propylene glycol and water is deposited on a clean glass slide, it forms a droplet of a given apparent contact angle rather than spreading as one would expect on such a high-energy surface. The droplet is stabilized by a Marangoni flow due to the non-uniformity of the components' concentrations between the border and the center of the droplet, itself a result of evaporation. These self-contracting droplets have unusual properties such as absence of pinning and the ability to move under an external humidity gradient. The droplets' apparent contact angle is a function of their concentration and the external humidity. Here we study the motion of such droplets sliding down slopes, how they deform when moving at large speeds, and compare the results to normal non-volatile droplets. We precisely control the external humidity and explore the influence of the volume, viscosity, surface tension, and contact angle. We find that the droplets suffer a negligible pinning force so that for small velocities the capillary number ($\mathrm{Ca}$) is directly proportional to the Bond number ($\mathrm{Bo}$): $\mathrm{Ca}=\mathrm{Bo} \sin\alpha$ with $\alpha$ the angle of the slope. When the droplets move at larger velocities they deform when Ca exceeds a threshold, and deposit smaller droplets when $\mathrm{Ca}$ reaches twice this threshold.