Focus: Toothpaste Is a Glass
Your company logo can look sharp on a T-shirt, thanks in part to ink technology. Rheological additives allow the ink to flow through the printer and then stay precisely in place on the fabric. But the properties of these pastes, used for other industrial purposes as well, have been difficult to understand in terms of basic physics, partly because they don’t behave like conventional viscous liquids. In the 27 November PRL a French team shows that some of the puzzling phenomena in pastes are actually commonly observed by physicists studying glassy materials–microscopically disordered systems, such as noncrystalline solids, and including window glass. The results demonstrate that glass physics principles are very general, and the team’s protocol may also help companies develop additives in a more rational way.
For many years researchers have had trouble nailing down the mechanical properties of pastes and similar materials because they seem to change with time and vary from one protocol to the next. Pastes are unusual because under small stresses they spring back elastically, but under a tight squeeze they flow like liquids–as anyone who has squeezed toothpaste from a tube knows.
Motivated by recent theoretical work connecting “soft material” characteristics with glass dynamics [Phys. Rev. Lett. 78, 2020 (1997)], Michel Cloitre and his colleagues at the joint laboratory of the National Center for Scientific Research (CNRS) and the company ATOFINA in Levallois-Perret, France, wanted to establish the glasslike properties of pastes. Using a standard rheological apparatus, they placed a thin sample between a pair of plates and applied a large twisting force (stress)–enough to make the sample flow–to the upper plate. But rather than observing the motion (strain) of the plate resulting from the stress, they followed the strain after the stress was turned off. The paste immediately began to “spring back” slowly, but the recovery continued for several hours and showed no sign of ending. The strain recovery was logarithmic in time, so there was no particular time scale to the motion–no time at which one could say the motion had essentially ceased.
Such a slow “relaxation” of a property following a “resetting” operation is typical of glasses, as was the team’s observation of an “aging” phenomenon: The stiffness of the material increased with time during the relaxation, which they showed by applying small stresses at later times. And the strains induced by those stresses depended on the aging time in a simple way, to give so-called universal behavior.
In other glasses–such as PVC plastic, which is best known as a pipe material–high temperature, rather than high stress, allows the material to flow and erase its memory of any previous relaxation. Quickly cooling the molten glass to a noncrystalline solid allows relaxations to begin again.
The paste in the experiments was a concentrated solution of 200-nm-diameter gel particles, packed tightly against one another. The particles become compressed and distorted under high stress, according to Cloitre and his colleagues, and after the stress is released, the relaxation and aging effects occur as particles slowly swell and jockey for less strained positions and shapes. But this search for equilibrium can be very slow, since one particle’s stress relief often comes at the expense of other particles. Likewise, glass relaxations in other materials are often explained by competition among atoms or molecules trying to reach more comfortable configurations.
Cloitre says the properties of similar materials, such as foams, emulsions, and slurries, should also be cleanly measurable, once researchers understand the physics of glasses and use appropriate experimental protocols. He also expects the aging properties of industrial materials can be better accounted for in the future, by allowing more time for a coating to cure, for example. David Weitz of Harvard University says the experiments clarify the previously murky status of pastes, showing that they are not normally in equilibrium and that they age in the same way many glasses do. Aging “seems to be a ubiquitous behavior for disordered systems,” he adds.