Photo: (Left) Above an onset temperature, a 2D material exhibits normal liquid behavior with all particles similarly mobile (yellow). (Right) Below that temperature, it becomes supercooled, with the onset of rigidity leading to just some mobile particles (yellow) amongst solid-like ‘frozen’ regions (blue). Kranthi Mandadapu.
Their improved understanding applies to ordinary materials like plastics and glass, and could help scientists develop new amorphous materials for use in medical devices, drug delivery, and additive manufacturing.
Anything made out of plastic or glass is known as an amorphous material. Unlike many materials that freeze into crystalline solids, the atoms and molecules in amorphous materials never stack together to form crystals when cooled. In fact, although we commonly think of plastic and glass as “solids,” they instead remain in a state that is more accurately described as a supercooled liquid that flows extremely slowly. And although these “glassy dynamic” materials are ubiquitous in our daily lives, how they become rigid at the microscopic scale has long eluded scientists.
Now, researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered molecular behavior in supercooled liquids that represents a hidden phase transition between a liquid and a solid.
Specifically, using theory, computer simulations, and previous experiments, the scientists explained why the molecules in these materials, when cooled, remain disordered like a liquid until taking a sharp turn toward a solid-like state at a certain temperature called the onset temperature – effectively becoming so viscous that they barely move. This onset of rigidity – a previously unknown phase transition – is what separates supercooled from normal liquids.
