Physicists make square drops and liquid latticework

When two substances come together, they will end up settling into a steady state called thermodynamic equilibrium; Examples are oil floating on water and milk mixing evenly with coffee. Researchers at Aalto University in Finland wanted to interrupt this type of state to see what happens and whether they can control the outcome.

“Things in balance tend to be pretty boring,” says Professor Jaakko Timonen, whose research group did new work published in Scientific advances on September 15th. “It’s fascinating to expel systems from equilibrium and see if non-equilibrium structures can be controlled or useful. Biological life itself is a good example of really complex behavior in a lot of molecules that are out of balance. ‘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 ignite an electric field on the mixture, the electric charge accumulates at the interface between the oils. This charge density depletes the interface out of thermodynamic equilibrium and becomes interesting formations,” explains Dr. Nikos Kyriakopoulos, one of the authors of the paper. In addition to being interrupted by the electric field, the liquids were confined in a thin, almost two-dimensional sheet. This combination caused the oils to transform into several completely unexpected drops and patterns.

The drops in the experiment could turn into squares and hexagons with straight sides, which is almost impossible by nature, where small bubbles and drops tend to form spheres. The two liquids could also become interconnected lattices: grid patterns that regularly occur in solid materials, but are not known to be liquid mixtures. Liquids can even be persuaded to form a bull, a donut shape, which was stable and maintained its shape while the field was applied, unlike nature, as liquids have a strong tendency to col. lapse and fill the center hole. Liquids can also form filaments that roll and rotate around an axis.

“All of these strange shapes are caused and sustained by preventing them from collapsing back into equilibrium by the movement of electrical charges that accumulate at the interface,” says Geet Raju, the first author of the paper.

One of the exciting results of this work is the ability to create temporal structures with a controlled, well-defined size that can be turned on and off with voltage, an area researchers are interested in exploring further to create voltage-controlled optical devices. . Another potential outcome is the ability to create interactive populations of rolling microfilaments and microdroplets that, at some elemental level, mimic the collective dynamics and behavior of microorganisms such as bacteria and microalgae that are propelled by completely different mechanisms.

The research was conducted in the Department of Applied Physics of the Active Matter research group, led by Professor Timonen. The document “Diversity of non-equilibrium patterns and emergence of activity in electrohydrodynamically driven confined liquids” is published with open access to Scientific advances.