Newswise – When two substances come together, they eventually settle into a stable state known as the thermodynamic equilibrium. We see this in everyday life when oil floats on top and when milk is uniformly mixed into coffee. Aalto University in Finland sought to disturb this state to determine if they could control its outcome.
‘Things that are in equilibrium can be very boring’, says Professor Jaakko Timothyonen whose research group published new work in Science Advances on 15 September. It’s interesting to drive systems out from equilibrium and then see if non-equilibrium structures are possible to control or be used. Biological life itself is a good example of truly complex behavior in a bunch of molecules that are out of thermodynamic equilibrium.’
In their work, the team used combinations of oils with different dielectric constants and conductivities. The liquids were then exposed to an electric field.
‘ When we apply an electric field to the mixture, the electrical charge builds up at the interface of the oils. This charge density shears the interface out of thermodynamic equilibrium and into interesting formations,’ explains Dr Nikos Kyriakopoulos, one of the authors of the paper. The electric field also disrupted the flow of liquids, resulting in a nearly two-dimensional sheet. The combination of these two factors led to oils changing into completely new droplets and patterns.
The droplets could be transformed into hexagons or squares by using the same technique. This is nearly impossible in nature where droplets and small bubbles tend to form spheres. It is possible to make interconnected lattices from the liquids. These grid patterns are common in solid materials, but uncommon in liquid mixtures. You can coax the liquids into creating a torus. This is a donut-shaped shape made from liquids. Liquids can also form filaments which roll around an axis.
‘ These strange shapes are caused by the fact they are prevented collapsing into equilibrium by motion of the electric charges building up at their interface.
One of the most exciting outcomes of this research is the ability create temporary structures of a well-defined and controlled size that can be turned on/off with voltage. This area of research is something the researchers are keen to explore further in order to create voltage-controlled optical devices. One potential outcome of this work is the possibility of creating interacting populations, or microfilaments, of microdroplets and microfilaments that mimic the dynamic and collective behaviour of microorganisms such as bacteria and microalgae, which propel themselves using entirely different mechanisms.
The research was conducted at the Department of Applied Physics, in the Active Matter research team led by Professor Timonen. The paper Diversity of non-equilibrium patterns and emergence of activity in confined electrohydrodynamically driven liquids is published open-access in Science Advances, freely available here as of 15 September 2021 18: 00 GMT: DOI 10. 1126/sciadv.abh1642
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