(533g) Microrheology of Soft Particle Pastes: Forced Motion of a Tagged Particle in a Jammed Suspension

Soft particle pastes like microgels and compressed emulsions are densely packed, disordered suspensions of soft and deformable particles. The particles are jammed beyond random close packing (φ >0.64). They behave like weak elastic solids at rest and low stresses but flow like liquids above the yield stress and this unique feature makes them useful as advanced rheological additives to process materials including high-performance coatings, textured food and personal care products. To relate the particle scale behavior to the macroscopic rheology we study the forced motion of a tagged particle through jammed suspensions with the help of particle scale simulations. A random particle is chosen and a known constant external force is applied on it and its motion in response is tracked. At small forces the particles are trapped in cages which resists the external force whereas at larger forces the tagged particle breaks free of the cage. This provides the microscopic evidence of yielding behavior in these materials. The average velocities of these particles in response to the applied forces are computed and the microscopic mobilities of these particles are extracted from them. The maximum distance through which the effect of the disturbance caused by this single tagged particle travels is also computed to determine the effectiveness of the particle-particle facet network present in these materials in transmission of localized microscopic forces. The simulations are performed with different packing fractions to understand the effect of volume fraction which gives information about the “microscopic cage strength” at different packing densities. The simulations thus provide means to probe the mirco and nano scale microstructure and properties of these materials including particle scale mobilities, transmission of localized disturbances and cage strength.