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Droplets rapidly changing into bubbles and enhanced turbulence caused by sandpaper Couple defends PhD theses

A slowly vaporizing droplet, fully surrounded by a gel, will rapidly change into a bubble of the same size. A curved wall that has sand paper attached, shows more turbulent flow and drag than a smooth wall. Two results of fluid dynamics research projects of the University of Twente, but very different ones. They come together because Myrthe Bruning and Pieter Berghout, who are a couple, defend their PhD research on the same day, Friday 15 January. On top of that, their PhD theses are the 99th and the 100th of the Physics of Fluids group.

They know each other’s work so well, that they could almost take over their respective PhD defence, even considering that their work is in very different fields of fluid physics. Myrthe Bruning and Pieter Berghout became a couple when they both already were PhD students. Only on one occasion, they worked together on a small project.

Coffee stain

Bruning’s research is about vaporating droplets containing particles. Those particles can be chemicals or biochemicals you want to analyze. For amplifying their small signals, metal particles can be added in the same droplet. What you expect is that evaporation gives an ordening that is needed for the analysis. But here, the well-known coffee stain effect comes in: the particles will move to the edge of the droplet en form a ring. To have a homogeneous distribution instead, it is possible to add a bit of salt, Bruning describes in her thesis. So, on a flat surface, the desired structure can be realized.

Unexpected effect

Going from a flat surface to a confined bubble, a bubble surrounded by a gel, another phenomenon happens: if the droplet evaporates, the gel will start to fold or crease. The pressure inside the droplet goes down and in very short time, a bubble is formed that has almost the same size of the droplet. This effect is even enhanced when the droplet contains particles. This effect resembles an effect that can be seen in nature, among farn plants. They change shape during dehydration. As a consequence, the ‘droplets’ in which sporangia are confined, change shape as well. The moment cavitation happens, they are catapulted out of the plant.

Unstructured roughness

Berghout did his research on turbulent flow. The group has an advanced Taylor-Couette setup in which turbulence can be reached by letting its cylinders rotate fast. Recent work showed that by adding a structure of ridges to he surface, the turbulence enters a regime that would be scalable to phenomena in nature. Berghout focused on an unstructured form of roughness: no ridges at predefined places, but sand. There are known result results for example in fluid pipes. Still, Berghout concluded that before evaluating the effect of roughness, he needed more knowledge on curved surface that are smooth. This is necessary for unraveling the role of the various effects. The curvature causes turbulence, so does roughness: what effect dominates, so what can we say about the effect of roughness on turbulent flow?

In the end, his work provides more insight into turbulent flows at the boundary layer of a curved wall. Roughness causes many more fluctuation and swirls, even at some distance from the wall. The drag this causes, can now better be predicted.

The PhD defences of Myrthe Bruning and Pieter Berghout, which are the 99th and the 100th of the group Physics of Fluids led by Prof Detlef Lohse, take place 15 January, 14h45 and 16h45. The ceremonies are held fully online due the corona restrictions.

Myrthe Bruning (Arnhem, 1993) defends her PhD thesis ‘Confined and colloidal droplets’, while her partner Pieter Berghout (Gouda, 1992) defends the thesis ‘Wall-bounded turbulence with streamwise curvature’. Bruning started a new job as a policy advisor at the Dutch Ministry of Health, Welfare and Sport, Berghout is now a researcher at chip machine manufacturer ASML

ir. W.R. van der Veen (Wiebe)
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