Spontaneous Capillarity-Driven Droplet Ejection

TL;DR

This paper explores how capillary forces can spontaneously eject droplets at large scales, demonstrating the effects of geometry and fluid properties in microgravity and terrestrial environments, with implications for fluid dynamics research.

Contribution

It introduces a novel understanding of capillarity-driven droplet ejection at large length scales, supported by experiments and scaling analysis.

Findings

Droplets in microgravity are a million times larger than terrestrial ones.

Capillary flow can be manipulated to produce jets and drops.

Dimensionless groups identify different droplet behavior regimes.

Abstract

The first large length-scale capillary rise experiments were conducted by R. Siegel using a drop tower at NASA LeRC shortly after the 1957 launch of Sputnik I. Siegel was curious if the wetting fluid would expel from the end of short capillary tubes in a low-gravity environment. He observed that although the fluid partially left the tubes, it was always pulled back by surface tension, which caused the fluid to remain pinned to the tubes' end. By exploiting tube geometry and fluid properties, we demonstrate that such capillary flows can in fact eject a variety of jets and drops. This fluid dynamics video provides a historical overview of such spontaneous capillarity-driven droplet ejection. Footage of terrestrial and low earth orbit experiments are also shown. Droplets generated in a microgravity environment are $10^6$ times larger than those ejected in a terrestrial environment. The accompanying article provides a summary of the critical parameters and experimental procedures. Scaling the governing equations reveals the dimensionless groups that identify topological regimes of droplet behavior which provides a novel perspective from which to further investigate jets, droplets, and other capillary phenomena over large length scales.