Effect of Interaction on the Formation of Memories in Paste

A densely packed colloidal suspension with plasticity, called paste, is known to remember directions of vibration and flow. These memories in paste can be visualized by the morphology of desiccation crack patterns. Here, we find that paste made of charged colloidal particles cannot remember flow direction. If we add sodium chloride into such paste to screen the Coulombic repulsive interaction between particles, the paste comes to remember flow direction. That is, one drop of salt water changes memory effect in the paste and thereby we can tune the morphology of desiccation crack patterns more precisely.Comment: 10 pages, 11 figures, Title change

Internal stress as a link between macroscale and mesoscale mechanics

The internal (or residual) stress is among the key notions to describe the state of the systems far from equilibrium. Such stress is invisible on the macroscopic scale where the system is regarded as a blackbox. Yet nonequilibrium macroscopic operations allow to create and observe the internal stress. We present in this lecture some examples of the internal stress and its operations. We describe the memory effect in some detail, the process in which the history of past operations is recalled through the relaxation of internal stress.Comment: 11pages, 2 figures, 7 equations: Refereed notes of the Lectures given at "Morphogenesis through the interplay of nonlinear chemical instabilities and elastic active media" July 2-14 2007. to be published from Springer (NATO series