Effect of Capillary and Marangoni Forces on Transport Phenomena in Microgravity

Abstract

The Constrained Vapor Bubble (CVB) experiment concerns a transparent, simple, "wickless" heat pipe operated in the microgravity environment of the International Space Station (ISS). In a microgravity environment, the relative effect of Marangoni flow is amplified because of highly reduced buoyancy driven flows as demonstrated herein. In this work, experimental results obtained using a transparent 30 mm long CVB module, 3 mm × 3 mm in square cross-section, with power inputs of up to 3.125 W are presented and discussed. Due to the extremely low Bond number and the dielectric materials of construction, the CVB system was ideally suited to determining if dry-out as a result of Marangoni forces might contribute to limiting heat pipe performance and exactly how that limitation occurs. Using a combination of visual observations and thermal measurements, we find a more complicated phenomenon in which opposing Marangoni and capillary forces lead to flooding of the device. A simple one-dimensional, thermal-fluid flow model describes the essence of the relative importance of the two stresses. Moreover, even though the heater end of the device is flooded and the liquid is highly superheated, boiling does not occur due to high evaporation rates.

Condensation on Highly Superheated Surfaces: Unstable Thin Films in a Wickless Heat Pipe

“…The interfacial region is located between the heater end and the "central drop" and first appears at a power input between 0.6 -0.7 W. At larger power inputs, the temperature gradient was high enough to generate a significant Marangoni flow that drove the liquid away from the heater end. Instead of the heater end becoming dry, liquid accumulated there and reduced the performance of the heat pipe [26,34]. In the sharp corners of the device, the competition between the capillary return flow and the Marangoni flow formed a junction vortex leading to a thick liquid "drop" that formed on the flat surface of the cuvette.…”

“…The Hamaker constant was estimated for pentane-on-silica using Lifshitz theory. The value of the heat transfer coefficient was taken from [34] and was calculated based on the models used to generate As shown in Figure 5b and in Figure S3, the phenomenon also appears in 1-g, if one knows to look for it. However, since the return flow of liquid is meager in 1-g, there is little vapor to condense at the heater end and the phenomenon appears as if pentane were a partially wetting fluid.…”

“…The experimental details have been described in previous publications [26,[33][34][35][36][37] and a synopsis is given in the Supplemental Information section [38]. Figure 1 shows the temperature profiles in a 40 mm, microgravity CVB experiment carried out using a cold finger temperature of 292 K and electrical power inputs to the heater of 1.6 to 3W.…”

Wetting of a liquid annulus in a capillary tube

“…In the following we would like to shine more light on the special case θ = 0°, because of its importance for realistic wetting phenomena in capillary tubes. Examples are the transport of confined foam lamellae (13), the influence of capillary and Marangoni forces on transport phenomena in microgravity (11,12), the pinch-off of bubbles (32), or the instability of confined fluid threads (27). In many of these cases, a completely wetting liquid, i.e.…”

“…Very recently, Bostwick et al (3) reviewed the hydrodynamic stability of capillary surface subject to various constraints, such as volume conservation, contact-line boundary conditions and the geometry of the supporting surface. Owing to its omnipresence and practical relevance in such fields as capillary condensation (8,9), fluid transport (10,13,14), and thermocapillarity (11,12), wetting under confinement has received enormous attention. Especially liquid surfaces that take the shape of a (deformed) cylinder have been in the focus of intense research efforts.…”