Rheology of Soft Particle Pastes

Lavanya Mohan lavanya@che.utexas.edu Office: CPE 5.472 Phone: 512-471-5082

Soft particle pastes (SPPs) consist of soft and deformable particles packed densely together. Thus, they have a large volume fraction (Φ_c ≤ Φ ≤ 1) where Φ_c is 0.63. Their microstructure resembles a disordered array of spheres compressed together. Concentrated suspensions of soft particles include emulsions, foams, microgels and slurries. Several cosmetic and food products that we use in our everyday lives and many biological fluids also fall under this category.

Applications of SPPs include rheological additives for screen printing,film coating etc. They are also used as stimuli responsive materials for encapsulation drug delivery

The soft and deformable nature of the particles and the high volume fraction causes the particles to develop flat facets at contact. This gives rise to some interesting rheology which I intend to study and understand.

Simulations have been performed to determine the rheological properties , such as osmotic pressure, low and high frequency moduli. The behavior at the facets determine the rheology, and so we intend to find these rheological properties by calculating the pairwise distribution function g(r) using a mean field theory.

In mean field theory (MFT) the n-body system is replaced by a 1-body problem with an effective external field. The external field replaces the interaction of all the other particles to an arbitrary test particle.

Thus I am considering a single test particle which faces a force field equal to the effective interaction of all the other particles.

MODEL

For a stationary SPP

Where     g(r)     is the radial distribution function     V(r)     is the velocity     M       is the Mobility Tensor     FH     is the hertzian force due to elastic contact     FE     due to elastohydrodynamic interactions     U      is the mean thermal energy

For a SPP in shear: A simple shear flow is assumed

I am using a commercial partial differential equation solver (COMSOL Multiphysics) to solve the above. Once I get the pairwise radial distribution function, I will be computing the rheological properties using the following formulae: