Putting the squeeze on liquid films.

Putting the squeeze on liquid films

Squeezing a liquid into a tight space, such as the narrow gap between two plates, strongly affects its physical properties. Indeed, recent experiments show that in some instances, a thin, confined film behaves more like a solid than a liquid. "We don't know what structure we're forming here," says Steve Granick of the University of Illinois in Urbana-Champaign. "But the liquid appears to be capable of taking on long-range order." The question of what surfaces do to the structure of liquids is an important issue in studies of how liquids behave in porous materials and how lubrication works.

To study the resistance of liquid films to sliding, Granick and colleague John Van Alsten use a custom-built apparatus (SN: 4/30/88, p.283) to squeeze a liquid between two extremely smooth, curved mica surfaces. Under sufficient pressure, the cylindrical surfaces flatten over a small area yet retain enough liquid between them to create a film only a few molecules thick. The top mica surface is mounted on a "boat" that slowly oscillates back and forth in response to an electrical signal. By comparing the boat's actual displacement with the input signal, the researchers can calculate how much the liquid film resists the motion, providing a measure of the film's apparent dynamic viscosity and its rate of energy dissipation.

For liquids such as hexadecane, which consists of long, flexible chains of carbon and hydrogen atoms, Granick and Van Alsten find that a confined liquid's dynamic viscosity is at least 100 times greater than that of the bulk liquid. The viscosity also depends very strongly on the pressure between the plates. For instance, doubling the pressure causes a fivefold increase in the viscosity, whereas the film's thickness changes by less than an angstrom. Details in the pattern of the film's response to pressure suggest that, rather than moving about at random, molecules in a confined liquid somehow coordinate their movements to accommodate the applied stress.

In liquids such as octamethylcyclo-tetrasiloxane, made up of compact, globular molecules, the researchers sometimes see an abrupt transition from a liquid-like to a solid-like response. In other words, at a sufficiently high pressure between the plates, the film somehow locks together. The transition is also reversible: Lowering the pressure brings back the liquid-like response. "This was the kind of [observation] that we didn't know whether to believe when we first saw it," Granick says.

The results suggest the liquid's molecules may be settling into an arrangement resembling a glass. "As we squeeze on these liquids harder and harder, we're freezing out more and more modes of motion until the molecules just sit there," Granick says. However, it's still difficult to account for the rapidity and reversibility of the transition. Adds Granick, "The results raise many questions concerning the packing and crowding of fluid molecules in constricted geometries."