Researchers observe rubber-like elasticity in liquid glycerol for the first time

Simple molecular liquids such as water or glycerol are of great importance for technological applications, in biology or even for understanding properties in the liquid state. Researchers at the Max Planck Institut für Struktur und Dynamik der Materie (MPSD) have now succeeded in observing liquid glycerol in a completely unexpected rubbery state.

In their article published in Proceedings of the National Academy of Sciences, researchers report how they created rapidly expanding bubbles on the surface of a liquid in vacuum using a pulsed laser. However, the bubble’s thin, micrometers-thick fluid envelope does not act like the expected viscous fluid dissipating deformation energy, but like the elastic envelope of a rubber toy balloon, which stores and releases elastic energy.

This is the first time that elasticity dominates the flow behavior in a Newtonian fluid such as glycerol. Its presence is difficult to reconcile with the general concepts of liquid glycerol interactions and prompts the search for more detailed explanations. Surprisingly, elasticity persists over long time scales of several microseconds, which can be important for very fast engineering applications such as micrometer-controlled flows under high pressure. Nevertheless, the question of whether this behavior is a specific property of liquid glycerol, or a phenomenon that occurs under similar conditions in many molecular liquids, but has not been observed so far, remains unresolved.

The team proposes that the high filtration rate and limited thickness of the shell cause individual molecules to form groups that migrate in an interactive and collective manner. This change would stabilize the elastic state for a longer period of time than is possible in the equilibrium state of glycerol, where single molecules undergo rapid diffusion. „We want to better understand this unusual state,” says lead author and doctoral student Meghnath Kayanathle, „because it can tell us a lot about collective excitations in disordered systems.”

The rubbery state of liquid glycerol raises the question: Are similar effects possible in other liquid materials? Creating elastic bubbles in water in particular would be a major achievement, as it is a very important and well-studied liquid with implications in many scientific fields. However, glycerol bubbles only form in a vacuum environment as shown by the MPSD group. This poses some challenges for similar experiments involving water, as it begins to boil below a vapor pressure of 32 mbar – above the pressure at which the experiments must take place.

The research was carried out by members of the company’s science support unit Ultrafast Beams and guest scientist Jipeng Huang from the University of Duisburg-Essen. An innovative scientific approach and proper selection of parameters led to the discovery of this novel elastic behavior. „Our experiment invites us to rethink the interactions and differences between liquids and solids,” said principal investigator Sasha F.

„As a next step, we aim to investigate the molecular interaction and structure of the transient bubble shell, and whether this effect can be reproduced in a range of other liquids different from glycerol.”