Neon lights and the perfect blur
Researchers are preparing for a new round of International Space Station experiments in a facility designed to demystify the role of surface tension and instabilities (Marangoni effects) on heat transfer to, and within an evaporating or condensing fluid. The tools: lasers, purple light and an out-of-focus camera to get the sharpest result.
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The waist-high facility Heat Transfer Host-2 fits within the European Drawer Rack-2 and is one of many upgrades planned for the International Space Station’s European Columbus laboratory in the coming years. The design allows experiment inserts to be slotted in and run autonomously, making use of the weightlessness in Earth orbit.
The ‘Marangoni in Films’ and ‘Condensation on Fins’ experiments are part of a larger campaign to assess how heat is transferred through gases and liquids during phase change. Investigating the process in space allows researchers to look at the underlying mechanics of strong, large-scale movements, without gravity getting in the way. A better understanding should improve future satellite cooling systems as well as confirm or fine-tune computer models that can be applied on Earth – improving cooling for electronics such as smartphones and computers, leading to optimised industrial processes, such as coatings and deposits.
Atypical observing
Before any experiment can take place, the scientific tools need to be perfected. For these experiments, the first inserts are set to launch in 2023. The researchers are looking to observe changes on the micron level – smaller than bacteria and viruses.
In December last year, researchers tested a system at Lambda-X in Nivelles, Belgium, shining a laser on a metal fin and using a high-precision interferometer to record changes. During the experiment, the fin is cooled and subsequently covered with a condensing liquid film. The interferometer records the temperature changes and vapour concentration variations around the fin, while the interferometer’s optical mode tracks the liquid film’s thickness with high precision.
“Interestingly we need the optical camera to be slightly out of focus to get the best result,” says ESA Payload System Engineer Ana Frutos Pastor, “by focussing just behind the fins, we can distinguish the contours with microscopic accuracy.”
Marangoni in space
The Marangoni effect describes how particles can be moved along liquids as they interact with changing temperatures. To better understand and control the instability, a second set of experiments will focus on small 20 mm square plaques with minute peaks and valleys, just a few hundred microns high. Flooded with liquid and heated, a technique based on blue and red light that shows as purple will be used to measure down to the micron how the temperature differences at the liquid’s surface lead to the formation of peaks and valleys.
“These experiments mainly serve to confirm or refine mathematical models, this is fundamental physics,” explains Balazs Toth of ESA’s Fluid Science Payloads Team, “but the effects they are studying play on many things around us, from how a coffee stain evaporates to how computers are cooled as you read this sentence, and how life support systems of spacecraft could be improved.”
The first hardware models were built by QinetiQ Space in Kruibeke, Belgium, and have been tested by the science teams. These models contain all of the fluid and thermal management systems as well as the shiny new diagnostic methods, of course.
