Memories of paste
- Weitz, D., Nature 410, 32-33 (2001).
In his article, Weitz primarily describes the following paper:
- Cloitre, M., Borrega, R. & Leibler, L. Phys. Rev. Lett. 85, 4819-4822 (2000).
Key Words: Paste, Glass, Microgel, Stress, Relaxation, Ageing
Weitz's article draws attention to the findings of Cloitre, Borrega, and Leibler and goes on to list related questions for future exploration.
Cloitre, Borrega, and Leibler studied the rheology of a paste consisting of pieces of microgel suspended in a fluid. A paste acts like a solid under low stress, but high stress makes the material flow like a liquid. When in a solid-like state, the paste's constituent particles are jammed together in a disordered structure. When sufficient stress is applied, the structure is broken, and the particles flow. The response of pastes to stress is complex, and experiments are hard to reproduce.
Another interesting property of pastes under stress is that the material typically reaches a more stable state after recovering from an imposed stress. This means that the next time a stress is applied, it will be harder to induce the fluid-like state. In this sense, the paste has a memory and changes or "ages" with successive stresses.
Cloitre, et al. found a specific type of memory in the paste that they studied. The researchers applied a high sheer stress to turn the paste into a fluid, and then removed the stress and allowed the material set like a solid. When they went on to test the newly-set material's response to stress, they found that the material had a memory for the direction of the previously applied high shear.
The related questions Weitz lists are:
- 1) What causes the ageing affect at a microscopic level?
- 2) Is the ageing affect specific to the particular microgel paste studied by Cloitre et al., or does it apply to other pastes or even other kinds of soft matter?
Soft Matter Details
Types of Soft Matter
As the title suggests, Memories of Paste focuses primarily on a class of soft materials called pastes. However, the author remarks on a similarity between pastes, gels, and glasses. "...the way a paste recovers from an applied stress is remarkably like the behaviour of glasses and gels" (Weitz p.32). This made me wonder:
- What is the difference between pastes, glasses, and gels? What about colloids?
"Pastes typically consist of a suspension of small particles in a background fluid. These particles are crowded, or jammed together like grains of sand on a beach, forming a disordered, glassy, or amorphous structure" ( Weitz p.32). In the experiment described above, the particles in the paste are made of micro-gel. At low particle concentrations, the particles act like hard colloidal particles; however, at high concentrations the material acts like a paste (Cloitre et a. p. 4819).
A colloid is a suspension of solid (or sometimes liquid) particles dispersed in a liquid, so it seems that a paste is a type of colloid where the volume fraction of solid particles is quite high.
The author was seems surprised that the paste responded like a glass or gel, so I must assume that a paste is a separate classification from gels and glasses.
Cloitre et al. made bulk rheological measurements using a rheometer to study a paste's response to stress. See their paper Rheological Aging and Rejuvenation in Microgel Pastes for more details.
author: Becca Perry