Square droplets and liquid lattices produced by subjecting combinations of oils with different dielectric constants and conductivities to an electric field. Image: Aalto University.When two substances are brought together, they will eventually settle into a steady state called the thermodynamic equilibrium. There are numerous examples of this in everyday life, such as when oil floats on top of water and milk mixes uniformly into coffee. Researchers at Aalto University in Finland wanted to see what would happen if they disrupted this steady state — and whether they could control the outcome.
"Things in equilibrium tend to be quite boring," says Jaakko Timonen, whose research group conducted the study. "It’s fascinating to drive systems out of equilibrium and see if the non-equilibrium structures can be controlled or be useful. 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 researchers took combinations of oils with different dielectric constants and conductivities, and subjected them to an electric field. They report their findings in a paper in Science Advances.
"When we turn on an electric field over the mixture, electrical charge accumulates at the interface between the oils," explains Nikos Kyriakopoulos, one of the authors of the paper. "This charge density shears the interface out of thermodynamic equilibrium and into interesting formations."
As well as being disrupted by the electric field, the oils were confined into a thin, nearly two-dimensional sheet. This combination led to the oils reshaping into various, completely unexpected shapes and patterns.
These shapes included squares and hexagons with straight sides, which are almost impossible to produce in nature, where small bubbles and droplets tend to be spherical. The two oils could also be made to form interconnected lattices: grid patterns that occur regularly in solid materials but are unheard of in liquid mixtures.
The oils could even be coaxed into forming a donut-shaped torus, which was stable and held its shape while the field was applied – unlike in nature, where liquids have a strong tendency to collapse in and fill the hole at the center. In addition, the oils could 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," says Geet Raju, the first author of the paper.
One of the exciting outcomes of this work is the ability to create temporary structures with a controlled and well-defined size that can be turned on and off with voltage, a finding that the researchers 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 behaviour of microorganisms like bacteria and microalgae, which propel themselves using completely different mechanisms.
This story is adapted from material from Aalto University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.