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Surface roughness accelerates liquid-liquid transition

Posted March 29, 2017

Researchers at the University of Tokyo have found that transition between two liquid states, characterized by different local structures, in a single-component substance can be accelerated by rubbing the cell’s surfaces, a widely used treatment method to align liquid crystal molecules in liquid crystal displays.

Wedge formed by rubbing filled up with liquid 2. At top, a microscope image captures a wedge formed by rubbing that is completely filled with liquid 2 (bar corresponds to 10μm); at bottom is its schematic representation. Image credit: Hajime Tanaka.

Altering the temperature induces liquid-liquid transition, based on the phenomenon that liquid 1 is more stable than liquid 2 at high temperature, while the reverse is true at low temperature, in which liquid 2 becomes more stable than liquid 1. There are two types of kinetic processes for liquid 1 to transform into liquid 2: It involves either the nucleation-growth type of transformation, which occurs after a long incubation period following the temperature change; or the spinodal-decomposition type, which is initiated immediately after the temperature change, at low temperature. Scientists have studied liquid-liquid transition mainly from the fundamental viewpoint until now, but its applications have remained unexplored.

The research group of Professor Hajime Tanaka and then-Project Researcher Ken-ichiro Murata (currently assistant professor at Hokkaido University) at the University of Tokyo’s Institute of Industrial Science found that confining an organic liquid, triphenyl phosphite, into a cell whose surfaces have been treated by rubbing leads to the disappearance of the incubation time even at a temperature where nucleation growth-type liquid-liquid transition is expected under usual conditions, thus accelerating its transformation significantly. The group also revealed the mechanism: Nanometer-scale surface roughness formed by rubbing leads to the immediate formation of liquid 2 that is more friendly with the surface than liquid 1, even above the spinodal temperature. Furthermore, the researchers found that such rubbing treatments have no effect on spinodal-decomposition-type liquid-liquid transition. The current finding serves as direct experimental evidence that there are two types of transformation and the transition between them takes place rather sharply at the spinodal temperature.

“Liquid-liquid transition has attracted considerable attention from the fundamental viewpoint because of its counterintuitive nature—that there exists more than two liquid states for a single-component substance and a transition occurs between them. However, there have been few studies on its applications,” says Tanaka. He continues, “Our finding that surface treatments by rubbing significantly accelerates liquid-liquid transition may open a new avenue towards applications of liquid-liquid transition phenomena.”

Source: University of Tokyo

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