Spreading of droplets under various gravitational accelerations

TL;DR

This paper presents a versatile experimental setup to study the spreading dynamics of complex fluid droplets under varying gravitational accelerations, including microgravity, enabling insights into surface tension-driven behaviors.

Contribution

The study introduces the VIP-DROP2 module, capable of depositing multiple droplets and analyzing their flow and stress fields under different gravity conditions, including microgravity.

Findings

Microgravity allows formation of larger droplets dominated by surface tension.

The setup can analyze flow fields and stress distributions inside droplets.

Droplet spreading behavior varies significantly with gravitational acceleration.

Abstract

We describe a setup to perform systematic studies on the spreading of droplets of complex fluids under microgravity conditions. Tweaking the gravitational acceleration under which droplets are deposited provides access to different regimes of the spreading dynamics, quantified through the Bond number. In particular, microgravity allows to form large droplets while remaining in the regime where surface tension effects and internal driving stresses are predominant over hydrostatic forces. The VIP-DROP2 experimental module provides a versatile platform to study a wide range of complex fluids through the deposition of axisymmetric droplets. The module offers the possibility to deposit droplets on a precursor layer, which can be composed of the same or of a different fluid. Besides, it allows to deposit four droplets simultaneously, while conducting shadowgraphy on all of them, and observing either the flow field (through particle image velocimetry), or the stress distribution inside the droplet in the case of stress birefringent fluids. Developed for a drop tower catapult system, it is designed to withstand a vertical acceleration of up to 30 times Earth's gravitational acceleration in the downwards direction, and can operate remotely, under microgravity conditions. We provide a detailed description of the module, and exemplary data analysis for droplets spreading on-ground and in microgravity.