Scientists explain the behavior of supercooled liquids

Their findings indicate that there is a hidden phase transition between the liquid and solid states.
Loukia Papadopoulos
The liquid and solid states.jpg
The liquid and solid states.

Kranthi Mandadapu 

Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have come closer to explaining why a liquid behaves like a solid in amorphous materials (also known as glassy dynamics) by understanding the molecular behavior in supercooled liquids that represents a hidden phase transition between the two states.

This is according to a press release by the institution published on Tuesday.

A theory that predicts the onset temperature

The scientists combined theory, computer simulations, and previous experiments to understand why molecules in amorphous materials, when cooled, remain disordered like a liquid until transforming suddenly and unexpectedly into a solid-like state at a certain temperature referred to as the onset temperature. It was this transition phase that was previously unknown.

“Our theory predicts the onset temperature measured in model systems and explains why the behavior of supercooled liquids around that temperature is reminiscent of solids even though their structure is the same as that of the liquid,” said Kranthi Mandadapu, a staff scientist in Berkeley Lab’s Chemical Sciences Division and professor of chemical engineering at the University of California, Berkeley, who led the work.

So what exactly did they find?

Alternating between multiple configurations of molecules

All supercooled liquids constantly alternate between multiple configurations of molecules. This sees the creation of localized particle movements known as excitations. The researchers approached these excitations in a 2D supercooled liquid as if they were defects in a crystalline solid. As the liquid’s temperature rose to the onset temperature, they discovered that every instance of a bound pair of defects broke apart into an unbounded pair. This process is what allowed the system to lose its rigidity and begin to behave like a liquid.

“The onset temperature for glassy dynamics is like a melting temperature that ‘melts’ a supercooled liquid into a liquid. This should be relevant for all supercooled liquids or glassy systems,” said Mandadapu.

“The whole quest is to understand microscopically what separates the supercooled liquid and a high temperature liquid.”

Now, the researchers hope to apply their theories to 3D systems and to seek to “explain how localized motions lead to further nearby excitations resulting in the relaxation of the entire liquid.” 

“It’s fascinating from a basic science point of view to examine why these supercooled liquids exhibit remarkably different dynamics than the regular liquids that we know,” added Mandadapu.

In addition, the team's findings could aid in the development of “new amorphous materials for use in medical devices, drug delivery, and additive manufacturing.”

The study was published in the journal PNAS.

Study abstract:

Below the onset temperature To, the equilibrium relaxation time of most glass-forming liquids exhibits glassy dynamics characterized by a super-Arrhenius temperature dependence. In this supercooled regime, the relaxation dynamics also proceeds through localized elastic excitations corresponding to hopping events between inherent states, i.e., potential-energy-minimizing configurations of the liquid. Despite its importance in distinguishing the supercooled regime from the high-temperature regime, the microscopic origin of To is not yet known. Here, we construct a theory for the onset temperature in two dimensions and find that an inherent-state melting transition, described by the binding–unbinding transition of dipolar elastic excitations, delineates the supercooled regime from the high-temperature regime. The corresponding melting transition temperature is in good agreement with the onset temperature found in various two-dimensional (2D) atomistic models of glass formers and an experimental binary colloidal system confined to a water–air interface. Additionally, we find the predictions for the renormalized elastic moduli to agree with the experimentally observed values for the latter 2D colloidal system. We further discuss the predictions of our theory on the displacement and density correlations at supercooled conditions, which are consistent with observations of the Mermin–Wagner fluctuations in experiments and molecular simulations.