CHAMPAIGN, Ill. -- When under stress, liquids -- like people -- may behave in unexpected ways. Squeezed into progressively thinner films, some liquids actually will behave like soft solids, reports Steve Granick, a materials scientist at the University of Illinois. The reduced molecular mobility occurs through a transition similar to that which affects glass.
Granick's findings, reported in the Sept. 9 issue of Physical Review Letters, have important implications for fields such as tribology (where thin films of lubricant separate moving parts), geology (where rocks often scrape past other rocks separated by thin films of water), and biology (where giant protein molecules glide past other molecules and cell membranes via thin films of liquid).
"In all these diverse areas, it is important to understand how thin liquid films behave, and how they affect their surroundings," Granick said. "Now, when we think of modifying, manipulating and tailoring thin films, we can look for analogies with soft solids. It provides an exciting new way to think about things."
To examine the behavior of ultrathin liquid films, Granick and graduate student A. Levent Demirel placed a drop of octamethylcyclotetrasiloxane (OMCTS) -- a fluid composed of simple, ball-shaped molecules -- between two mica plates. They then monitored the viscoelasticity of the film as the plates were slowly pushed together.
"The fluid drained smoothly until the film was about seven molecular layers thick," Granick said. "Then it got stuck. When additional pressure was applied, the fluid drained in discrete steps that corresponded to squeezing out successive molecular layers. Each additional layer required more pressure." Granick's experiments showed that OMCTS did not abruptly "freeze" into a solid as the film grew thinner. Instead, the change in viscoelastic response was more gradual and continuous.
"The nature of the solidification is a smooth transition toward increasing rigidity with decreasing film thickness," Granick said. "This is definitely a glass-like transition."
Although glass, at first glance, may appear to be a hard, brittle substance, it really is better described as a "soft" solid, Granick said. Over time, window panes will "flow" due to gravity's downward pull, making the glass thicker at the bottom than at the top.
Silly Putty is another common example of a soft solid, Granick said. "Silly Putty bounces when dropped on a floor, but flows when it sits on a table." By understanding the "soft solid" characteristics of thin liquid films, scientists and engineers might design more effective lubricants and better understand certain natural phenomena, such as earthquakes.