The rheological response of a colloidal system concentrated to near the glass concentration has been investigated using a cone and plate geometry. The prime purpose of the work was to interrogate the nonlinear response using the so-called "tickle" experiment in which small deformations are superimposed on a single large deformation to probe "rejuvenation" and "reinitiated aging" behaviors as has been done in polymer glasses and solutions. Counterintuitively, we find that the superposition of a small positive probe onto the large deformation gives different results from the superposition of a small negative probe. Such a result is contrary to simple fading memory ideas of viscoelastic materials. We also find that the rejuvenation and reinitiated aging behavior in the positive probe case is dependent on whether one considers the probes at "first" probe or at "second" probe, in spite of following the protocol first suggested by Struik [Physical Aging in Amorphous Polymers and Other Materials (Elsevier, Amsterdam, 1978)], which avoids sequence effects in viscoelastic materials. It is suggested that colloidal systems, while exhibiting some features of fading memory behavior that is expected in molecular (polymer) glasses, in fact also undergo a mechanically induced structural change, perhaps similar to that found in highly filled materials such as rocket propellants and carbon black filled rubber.
- Fading memory
- Structural change