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July, 2017

It's not often one can stop the flow of time, let alone reverse it. But in the case of emulsions, scientists from the Active and Intelligent Materials (AIM) Lab in Materials Science and Metallurgy at the University of Cambridge and University of Sofia have done just that. Emulsions are a curious state of matter with their own rules. When you break a glass, and it falls apart into thousands of pieces, you could never hope to put it back exactly as it was. This is so ingrained in our experience we can tell the flow of time by seeing the initial and final states of the glass. In emulsions the situation is similar, but time flows in the other direction: the tiny little pieces are the unstable state, and the whole glass (or in this case, single droplet) is the stable one. The broken-up liquid droplets have a self-healing tendency, and as emulsions age, many droplets coalesce and become few, or eventually one (backward arrow in the picture above). Small droplets are often highly desirable, so usually by doing work, one can break up large droplets again into little pieces. Established industrial processes use mechanical shear and ~1000 times more energy than the extra surface energy stored in the newly created tiny droplet interfaces, significantly heating the emulsion in the process. Breaking up droplets by cooling them is attractive not only from energy efficiency, but also because they allow making emulsions compatible with temperature-sensitive ingredients for the first time. Some of the largest companies in Europe have expressed interest, and the team has just won an EU grant to scale-up the technology.

The AIM lab is also investigating the fundamental ability of the system of  just 3 chemicals (water, soap (surfactant), and oil), to harness energy from a few degrees fluctuations around room temperature, which naturally occur in nature. Life is known for harvesting and storing energy. But this peculiar behavior for a simple non-living system could  ring insights into how non-living systems could be made to harvest thermal energy from their environment as well, and ratchet themselves up to higher-and-higher energy states. 

Figure: Before reading, guess which way the arrow of time should point.  Alkane oil droplets, Scale bars: 20 microns.

S. Tcholakova, Z.H. Valkova, D. Cholakova, Z. Vinarov, I. Lesov, N. Denkov, S.K. Smoukov, "Efficient self-emulsification via cooling-heating cycles", Nature Comm. 8, art# 15012 (2017)