High-gravity Spreading of Liquid Coatings on Wetting Flexible Substrates

TL;DR

This study demonstrates that applying high gravitational acceleration via centrifugation significantly accelerates liquid spreading on flexible substrates, reducing spreading time from hours to minutes and extending gravity-driven spreading models to high-g conditions.

Contribution

It introduces a mechanical centrifugation method to manipulate capillary length and spreading rate on flexible substrates under high gravity, a novel approach not previously explored.

Findings

Capillary length reduces by a factor of 24.5 at 600 g

Spreading time decreases from 12 hours to less than 1 minute at 600 g

Gravity-driven spreading models extend to high-g conditions

Abstract

This work describes a mechanical approach for manipulating the capillary length and spreading of liquid coatings on flexible substrates with high gravity. Experimental verification in the literature has focused on cases under standard gravity on earth, and to the authors' knowledge, this work is the first to explore its relevance to spreading puddles under high gravity. By using centrifugation with a high-density liquid base underneath a coated substrate, it is possible to apply acceleration normal to a substrate to decrease the capillary length and increase the rate of spreading. Due to the nature of centrifugation, this method works primarily on flexible substrates, which bend with a curvature that conforms to a contour of uniformly distributed centrifugal acceleration. With high gravity of 600 g applied, the capillary length reduces by a factor of 24.5, and the spreading shifts from "transient spreading" between the surface tension-driven and gravity-driven regimes to the gravity-driven regime. Experimental results show that high gravitational acceleration will enhance the rate of spreading such that a puddle, which would require 12 hours under standard gravity on earth to go from an 8-{\mu}l droplet to a 40-{\mu}m thick puddle, would require less than 1 minute under 600 g. Overall, this work suggests that previously derived expressions for gravity-driven spreading of puddles under earth's standard gravity extend to predicting the behavior of puddles spreading on smooth, wetting substrates exposed to more than 100 g's of acceleration.