The Physics of Toothpaste

Have you ever wondered why it is that your toothpaste flows like a liquid when you squeeze the tube hard but pulls back like stretched elastic if you squeeze too gently? Scientists from France now have a partial answer: they have found in a series of experiments that pastes behave mechanically much like glass. The insight, reported in this week's issue of Physical Review Letters, may help researchers develop better versions of industrial pastes, such as the additives commonly used in inks. These pastes let the ink flow through printers but stay put on, say, patterned fabrics, like the polka-dot cloth at right. Moreover, this knowledge may shed more light on the physics of glass and other so-called disordered systems in general.

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, tested a paste of tightly packed 200-nanometer-diameter gel particles in a concentrated solution. They placed a thin sample between two plates and applied a twisting force large enough to make the sample flow to the upper plate. Then they removed the stress and watched how the material recovered. Almost immediately, the paste began to spring back, although slowly, and continued to do so for hours on end. This slow recovery is, in fact, typical of glasses. The team also noted that the sample "aged" in that it grew increasingly stiff with time during recovery--another common trait of glass materials. The scientists suggest that the particles are distorted under high stress, and swell and jostle for position upon relaxation, again trying to reach equilibrium. It is because the relief of one particle may come at another's expense, they say, that the process is so slow.