When two substances are brought together, they will eventually settle into a steady state called thermodynamic equilibrium; examples include oil floating on top of water and milk mixing uniformly into coffee. Researchers at Aalto University in Finland wanted to disrupt this sort of state to see what happens—and whether they can control the outcome.
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 turn on an electric field over the mixture, electrical charge accumulates at the interface between 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. As well as being disrupted by the electric field, the liquids were confined into a thin, nearly two-dimensional sheet. This combination led to the oils reshaping into various completely unexpected droplets and patterns.
The droplets in the experiment could be made into squares and hexagons with straight sides, which is almost impossible in nature, where small bubbles and droplets tend to form spheres. The two liquids could be also made to form into interconnected lattices: grid patterns that occur regularly in solid materials but are unheard of in liquid mixtures. The liquids can even be coaxed into forming 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 in and fill the hole at the center. The liquids can also form filaments that roll and rotate around an axis.
The research was carried out at the Department of Applied Physics in the Active Matter research group, 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.
More information: Diversity of non-equilibrium patterns and emergence of activity in confined electrohydrodynamically driven liquids, Science Advances (2021). DOI: 10.1126/sciadv.abh1642