Lavanya Mohan

Rheology of Soft Particle Pastes

Soft particle pastes like microgels and compressed emulsions are densely packed, disordered suspensions of soft and deformable particles. In these the soft particles are jammed, and they behave like weak elastic solids at rest and low stresses but flow like liquids above the yield stress. This unique feature makes them useful as rheological additives to process high-performance coatings, ceramic pastes, drilling muds, textured food and personal care products. A quantitative bridging of the microscopic parameters, microstructural changes and macroscopic properties would enable customized design of the fluid.

My research is about creating tools to understand and design these materials. We have developed micromechanical 3D simulations and built the microstructural analysis to quantitatively predict their nonlinear rheology during steady shear [1]. Large Amplitude Oscillatory Shear (LAOS) is a widely used characterization technique for these complex fluids and we have built particle scale simulations to predict their macroscopic properties and determined the underlying microscopic changes that give rise to their macroscopic oscillatory rheology [2].

lm1

Left: Simulation box for soft jammed glasses. Center: Particle contact interactions. Right: Simulation predictions for the flow curve (line) and comparison to experiments (symbols)

We have also developed a pairwise interaction theory to predict the pairwise distribution function and elastic properties of quiescent pastes. The short ranged pair distribution function was predicted using a transport equation that includes a concentration dependent mean force that captures the effect of the bulk suspension on the pair interaction [3]. I am currently working on extending the pairwise theory to predict the sheared microstructure and flow properties of these pastes.

lm2

Left: AFM image of a microgel paste. Center: Schematic representation of the forces used in the pairwise theory.
Right: Theoretical predictions of pair distribution function (solid lines) and comparison to simulations (dashed lines)

[1] Seth, J.R., Mohan, L., Locatelli-Champagne, C., Cloitre, M., Bonnecaze, R.T. (2011), A micromechanical model to predict the flow of soft particle glasses. Nature Materials, 10 (11) 838-843

[2] Mohan, L., Pellet, C., Cloitre, M., Bonnecaze, R.T. (2013), Local mobility and microstructure in periodically sheared soft particle glasses and their connection to macroscopic rheology. Journal of Rheology (submitted)

[3] Mohan, L., Bonnecaze, R.T. (2012) Short-ranged pair distribution function for concentrated suspensions of soft particles. Soft Matter, 8 (15) 4216-4222