# Memories of paste

Original entry: Becca Perry

Second Entry: Haifei Zhang, AP 225, Fall 2009

## Overview

Key Words: Paste, Microgel, Stress, Relaxation, Ageing, Rheology

Weitz describes toothpaste as a paste we are all familiar with. S. Ehardt, Wikimedia Commons

## Summary

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. In a solid-like state, the paste's constituent particles are jammed together in a disordered structure. Under sufficient stress, the structure breaks, and the particles flow. The response of pastes to stress is complex, and experiments are hard to reproduce.

Another interesting property of pastes is that small thermal fluctuations can cause the particles in the material to reach a more stable configuration. This means that the material changes or "ages" with time. The material properties depend on its history.

Cloitre, et al. studied the aging of their paste and found a memory for the direction of applied high stress. The researchers applied a high shear stress to turn the paste into a fluid and then removed the stress and allowed the material set like a solid. The material did not simply set up in the configuration left when the high shear stopped. The material remembered the direction of the high shear and pressed back in the opposite direction even though one might have expected the fluid flow at high shear to create a completely disordered system that would have no way to "remember" where it had been.

The discovery of paste memory led Weitz to pose some additional questions:

• 1) What causes the memory effect at a microscopic level?
• 2) Is the memory 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 al. 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.

Weitz seems surprised that the paste responds like a glass or gel, so I must assume that pastes, gels, and glasses are all separate classes of soft matter.

Experimental Methods

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. Cloitre et al. scale their data and plot it on a logarithmic master curve.

## Rheological Aging and Rejuvenation in Microgel Pastes

Strain recovery following flow cessation for different conditions of preparation Solid Triangle: $\sigma_P$ = 60 Pa; Solid Circle: $\sigma_P$ = 180 Pa; Hollow Circle: $\sigma_P$ = -180 Pa (the flow direction is reversed). $\sigma_P$ is applied for 60 s.

In the left figure, data measured for various experimental condiditons are plotted. The strain origin and the time origin are taken at the end of preparation. It is observed that the strain recovery after flow cessation does not depend on the magnitude of $\sigma_P$ provided that it exceeds the yield stress $\sigma_y$.

## References

[1] Weitz, D., Nature 410, 32-33 (2001).

[2] Cloitre, M., Borrega, R. & Leibler, L. Phys. Rev. Lett. 85, 4819-4822 (2000).

[3] Cipelletti, L., Manley, S., Ball, R. C. & Weitz, D. A. Phys. Rev. Lett. 84, 2275–2278 (2000).