By disrupting the thermodynamic equilibrium of liquids, physicists have made them behave very differently from how they do in nature, succeeding in convincing liquids in squares and hexagons of straight sides and lattice patterns.
This is not only fascinating in itself, but it could help us better understand how liquids behave under different conditions, which has implications for a variety of fields, from physics to medical research.
“Things in balance tend to be pretty boring,” said physicist Jaakko Timonen of Aalto University in Finland.
“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’.
You are likely to regularly see the thermodynamic equilibrium without even realizing it. It is the phenomenon that allows your cold milk to mix evenly throughout the hot coffee, as the temperatures (and therefore the kinetic energy of the molecules) of the two liquids become uniform.
But when the thermodynamic equilibrium is wrong, interesting things can happen, such as the spontaneous appearance of ordered states. This is of interest to scientists and engineers. It can help us not only to understand the thermodynamic equilibrium itself, but to various materials.
The research team, led by Aalto physicist Geet Raju, designed an experiment to explore it. They placed two liquids, oils, with different relative conductivities and permittivities under confinement between two flat, non-wetting surfaces to induce an almost two-dimensional plane. Then they applied an electric field.
“When we ignite an electric field over the mixture, the electric charge builds up at the interface between the oils,” said physicist Nikos Kyriakopoulos of Aalto University. “This load density depletes the interface out of thermodynamic equilibrium and into interesting formations.”
In nature, liquids are curved. In the absence of a container, they form small round and plump drops, bound by their surface tension that contains them on the smallest possible surface. In team experiments, they were induced to organize themselves into patterns that never occur in liquids in nature.
These include the straight-faced geometric shapes mentioned, as well as interconnected latticework. The team also created bulls (donut shapes) that do not usually occur in nature, as the liquid tends to fill the center hole and also the filament nets. They even saw filaments revolving around an axis.
“All of these strange shapes are caused and sustained by preventing them from collapsing back into balance by the movement of electrical charges that build up at the interface,” Raju said.
The researchers said the ability to control the shapes generated by the application of a tuned electric field has a wide range of very exciting applications.
For example, it could be used to assemble objects in specific locations in larger structures and for automatic assembly of liquids. Rotary filaments have implications for particle physics. Last, but not least, is the potential of optics.
“The two-phase system studied here offers exciting possibilities as optical devices due to the exceptional control over the liquid-liquid interface and fluid structures with electric field,” the researchers wrote in their paper.
“This will immediately lead to technologically relevant voltage-controlled unbalanced optical diffusers and structural colors based on crystals and photonic glasses by controlling the formation, interactions and self-assembly of the various fluid structures demonstrated here.”
The research has been published in Scientific advances.