Active microrheology of soft particle glasses

L Mohan and M Cloitre and RT Bonnecaze, JOURNAL OF RHEOLOGY, 58, 1465-1482 (2014).

DOI: 10.1122/1.4887535

We study the forced motion of a tagged soft particle through a jammed suspension of identical soft particles using particle simulations. We relate the local particle dynamics, microstructure, and microrheology to the bulk rheological properties of the suspension. At small forces, the tagged particle is trapped in a cage of other particles that resist the external force whereas at larger forces the tagged particle breaks free of its cage. This is indicative of the macroscopic yielding behavior in these materials. The threshold force from microrheology is quantitatively related to the macroscopic yield stress. The computed microviscosity also follows the shear thinning nature of these glasses and matches the exponent of 0.5 observed in bulk measurements. The semiquantitative agreement between the micro and bulk viscosity is discussed by comparing the microstructural processes involved in both. In microrheology, at moderate forces, the probe displacement causes an accumulation of particles in the direction of motion and a depleted wake behind. In bulk rheology, there are accumulation and depletion of particles along the compression and extension axes, respectively. Despite these differences, the average number of contacts and the elastic contact force on the particles are similar, which justifies why microrheology measurements match bulk rheology data. (C) 2014 The Society of Rheology.

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