Physicists make square droplets and liquid lattices

When two substances come together, they end up settling in a stable state called thermodynamic equilibrium; examples include oil floating above water and milk mixing evenly in coffee. Researchers at Aalto University in Finland wanted to disrupt this type of condition to see what happens and if they can control the outcome.

“Things in balance tend to be quite boring,” says Professor Jaakko Timonen, whose research group has completed new work published in Scientists progress September 15. “It’s fascinating to drive systems out of balance and see if the out-of-balance structures can be controlled or be of use. Biological life itself is a good example of really complex behavior in a group of molecules that are out of thermodynamic equilibrium. “

In their work, the team used combinations of oils with different dielectric constants and conductivities. They then subjected the liquids to an electric field.

“When we activate an electric field on the mixture, an electric charge builds up at the interface between the oils. This charge density shears the interface out of thermodynamic equilibrium and into interesting formations,” says Dr Nikos Kyriakopoulos , one of the authors of the paper. In addition to being disturbed by the electric field, the liquids were confined in a thin, almost two-dimensional sheet. This combination caused the oils to reshape into various completely unexpected droplets and patterns.

The droplets of the experiment could be turned into squares and hexagons with right sides, which is almost impossible in nature, where small bubbles and droplets tend to form spheres. The two liquids could also be made to form interconnected networks: grid patterns that occur regularly in solid materials but are unknown in liquid mixtures. Liquids can even be caused to form a torus, a donut shape, which was stable and held its shape while the field was applied, unlike in nature, as liquids have a strong tendency to collapse and fill the hole in the center. Liquids can also form filaments that roll and spin around an axis.

“All of these strange shapes are caused and maintained by the fact that they are prevented from returning to equilibrium by the movement of electric charges accumulating at the interface,” explains Geet Raju, the first author of the article.

One of the exciting results of this work is the ability to create temporary structures with a controlled, well-defined size that can be turned on and off with voltage, an area that researchers want to explore further to create voltage-controlled optical devices. . . Another potential outcome is the ability to create interactive populations of coiled microfilaments and microdroplets that, on some elementary level, mimic the dynamics and collective behavior of microorganisms like bacteria and microalgae that propel themselves using completely different mechanisms.

The research was carried out at the Department of Applied Physics of the Active Matter research group, headed by Professor Timonen. The article “Diversity of non-equilibrium patterns and merge of activity in confined electrohydrodynamically driven liquids” is published in open access in Scientists progress.