Surface tension and microgravity

Abstract

The behaviour of confined liquids on board an orbiting spacecraft is mainly driven by surface tension phenomena, which cause an apparently anomalous response of the liquid when compared with the behaviour that can be observed on an Earth laboratory provided that the amount of liquid is high enough. The reason is that in an orbiting spacecraft the different inertial forces acting on the bulk of the liquid are almost zero, causing thus capillary forces to be the dominant ones. Of course, since gravity forces are proportional to the liquid volume, whereas surface tension forces are proportional to the liquid surface, there are situations on Earth where capillarity can be the dominant effect, as it happens when very small volume liquid samples are considered. However, work with small size samples may require the use of sophisticated optical devices. Leaving aside the neutral buoyancy technique, a way of handling large liquid interfaces is by using drop towers, where the sample falls subjected to the action of Earth's gravity. This approach is suitable when the characteristic time of the problem under consideration is much smaller than the drop time. In this work the transformation of an out-of-use chimney into a drop tower is presented. Because of the miniaturization, hardiness and low cost of current electronic devices, a drop tower can be used as an inexpensive tool for undergraduate students to experimentally analyse a large variety of surface tension driven phenomena.

Drag-shield drop tower residual acceleration optimisation

“…The payload elevation system is a winch equipped with a dc electric motor provided with the necessary control devices to move the release mechanism up and down, which consists of a lock driven by a solenoid, mounted on a horizontal bar anchored to the hoist cable. When the release mechanism is unlocked, the drop package can free fall down to the base of the tower, where it is decelerated by a large spike that penetrates a dry sand reservoir, the setup is described in more detail in Meseguer et al (2014). To validate the theoretical models different experiments can be performed.…”

Liquid Rise in Uniform Screens under Normal Gravity and Microgravity Conditions

“…Capillary flows are of great importance in space technology, because the capillary force is sometimes the only force to drive liquid flow under a microgravity condition [1][2]. Two important applications of capillary transport in spacecraft are heat pipes and Propellant Management Devices (PMD).…”

More investigations in capillary fluidics using a drop tower

“…As briefly reviewed in that article, a drop tower provides a near step reduction in effective gravity level allowing for a wide range of often unearthly observations of large length scale capillary phenomena. Unique balances of inertia and surface tension forces permit observations of enormous systems despite the short duration usually afforded by a drop tower (i.e., typically <10 s) (Steinberg 2008;Chunhui 1993;Dittus 1991;Suñol and González-Cinca 2011;Lekan et al 1993; Wollman and Weislogel 2013;Meseguer et al 2014;Wollman 2012. For example, in the absence of significant body forces (Bond number Bo = ρgR 2 /σ ≪ 1) a balance of inertia and capillarity gives R DT ∼ (σ t 2 DT /ρ) 1/3 , where R DT is the maximum characteristic dimension of the capillary surface, σ is the surface tension, ρ is the density difference across the fluid interface, g is the local acceleration level, and t DT is the effective free fall time of the drop tower.…”