Toroidal Droplets & Liquid Lattices

When two substances are brought together, they will eventually settle into a steady state called the thermodynamic equilibrium. In new research, a team of physicists at Aalto University wanted to disrupt this state to see what happens; they subjected combinations of oils with different dielectric constants and conductivities to an electric field.
An image of a typical non-equilibrium state comprising active filaments, random filament networks and ordered 2D fluidic lattices, and active and self-propulsive electrohydrodynamically sculpted droplets and active emulsions. Image credit: Raju et al., doi: 10.1126/sciadv.abh1642.
“When we turn on an electric field over the mixture, electrical charge accumulates at the interface between the oils,” said senior author Professor Jaakko Timonen, a researcher in the Department of Applied Physics at Aalto University.
“This charge density shears the interface out of thermodynamic equilibrium and into interesting formations,” added co-author Dr. Nikos Kyriakopoulos, a postdoctoral researcher in the Department of Applied Physics at Aalto University.
“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 team’s 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.
“All these strange shapes are caused and sustained by the fact that they are prevented from collapsing back into equilibrium by the motion of the electrical charges building up at the interface,” said first author Geet Raju, a researcher in the Department of Applied Physics at Aalto University.
“One of the exciting results of our work is the ability to create temporary structures with a controlled and well-defined size which can be turned on and off with voltage, an area that we are interested in exploring further for creating voltage-controlled optical devices.”
“Another potential outcome is the ability to create interacting populations of rolling microfilaments and microdroplets that, at some elementary level, mimic the dynamics and collective behavior of microorganisms like bacteria and microalgae that propel themselves using completely different mechanisms.”
The results appear today in the journal Science Advances.
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Geet Raju et al. Diversity of non-equilibrium patterns and emergence of activity in confined electrohydrodynamically driven liquids. Science Advances, published online September 15, 2021; doi: 10.1126/sciadv.abh1642

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