41236-AC9
The Rheology of Soft Particle Pastes
We have studied the rheology of dense colloidal suspensions of soft particles. The behavior of these “jammed” and glassy materials near boundaries and in the bulk was investigated and the key material parameters (namely, the solvent viscosity and paste elastic modulus) and external conditions such as the nature of the confining boundaries (the physical roughness and surface chemistry), that govern soft particle paste (SPP) rheology were identified. A summary of the individual problems addressed and the novel contributions to current research on complex fluids are discussed in the following.
Control of wall-slip near smooth surfaces by manipulating “stickiness” of the boundary Careful measurements of the apparent flow of pastes under sub-yield stresses have been performed. It was found that the sliding yield stress, a critical stress below which there is no slip and the paste is stuck to the wall, is sensitive to the nature of the wall and the extent of wall-slip can be promoted or suppressed by increasing either repulsion or attraction between the paste and the wall, respectively. A model based on the concept of elastohydrodynamic lubrication was augmented to account for the surface chemistry at the paste-wall boundary. It has successfully explained the observed experimental trends in the sliding yield stress and the slip velocities for varying paste-wall interactions. The experimental observations and the modified elastohydrodynamic lubrication theory together form a first systematic study of the dependence of slip near smooth boundaries on the chemical nature of the surface. Tribology and heterogeneous flows of soft colloidal glasses Combined fluorescence microscopy and particle tracking velocimetry techniques was used to observe paste flow-profiles under steady shear. Flow visualization of microgel pastes and compressed emulsions sheared near smooth surfaces confirmed under for sub-yield stresses the layer of paste particles next to the wall is slipping while the rest of the material is solid-like. It also showed flow beyond yielding is either shear-banded or uniform depending on whether the paste-shearing boundary combination is attractive or repulsive, respectively. The non-linear velocity profiles measured for the attractive combinations suggest a duplicity of flow curves (and hence flow mechanisms) for pastes sheared near adhering boundaries. A few recent studies have looked at non-homogeneity as a function of interactions between particles or of physical roughness and confinement. By comparison, this study has helped discover the role played by the chemical nature of smooth boundaries on paste tribology and these findings uncover a new perspective on inhomogeneous flows in glassy colloids. Elastic properties of dense packings of Hertzian spheres
The aim of this project was to investigate the solid-like response of SPPs to small deformations. The suspension was modeled as a dense and disordered packing of elastic spheres and the pairwise interactions were estimated using the Hertz theory for small deformation of elastic spheres. From static simulations of packings sheared at the high and low frequency limits the shear moduli (which measure stiffness of the paste) were calculated as a function of packing density and particle stiffness. From various experiments on SPPs the low-frequency modulus of the paste was already known to govern their flow, wall-slip and elastic properties and simulations were helpful in computing this quantity for various SPPs. Approximate equality of the shear modulus and the net compressive pressure of pastes was also established.
Microstructure of the model packings was studied by computing the pair distribution function for undeformed and sheared packings. The long-ranged pair-distribution function for the unsheared packings was similar to that of liquids. The first neighbor peaks at different packing densities were similar and could be collapsed with semi-empirical scaling of the distribution function. Knowledge of the first peak to the pair distribution function is particularly useful as it can used to predict different macroscopic rheological properties of the packings.
Bulk dynamics and non-linear rheology
Dynamic simulations of SPPs under shear were begun with an initial conjecture that cage-effects (due to dominant steric restraints) must lie at the origin of yielding, and viscous drag from pairwise elastohydrodynamic lubrication between particles must dominate at higher shear rates and lead to the experimentally observed power-law dependence of the shear stress on the shear rate. Results from the simulations completely capture the experimental trends but show that in fact viscous contributions to stress are usually negligible for the shear rates studied experimentally and have highlighted the importance of deformation of paste microstructure under shear.
Unequal normal stresses generated in a glassy colloid under shear have been calculated which showed that the excess normal stresses in the gradient and vorticity directions are equal and opposite. The simulations predicted that the normal stress differences must increase with the shear rate raised to half power and subsequently motivated experimental measurements of the normal forces generated during shear which confirmed the simulation findings.
