Rheometric Study of Complex Systems

Abstract

Rheology measures the system response towards a given loading to study the mechanical property of the system. It plays a vital role in soft matter physics and adapted to polymer, petroleum, and food industries as a well-established protocol. But classical rheology is subject to limitations. The first limitation comes from the scale; a rheometer requires milliliters of samples, where the volume of a single human cell is typically less than 10^-8 milliliter. But understanding the rheological property of a single cell is crucial, as each cell's differentiation, division, migration, and apoptosis all have cell mechanics involved. A second limitation comes from the nonlinearity of the system. Rheology only reflects the system property in the laminar flow regime, but the suspension, for example, can interact with the flow and leads to nontrivial flow profiles. In this dissertation, I introduce two works focusing on the frontier of rheology.

In the first work, I focus on the novel interaction between the viscoelastic liquid and the suspended particles in the viscoelastic suspension. By carefully investigating the viscoelastic suspension flow beyond the laminar flow regime, we find the suspended particles interact with the viscoelastic liquid at high shear rates, disrupt the original flow, and lead to an unexpected flow profile. We employ rheoflow visualization techniques to quantify the phase transition and the flow change in-situ. We find the particles self-assemble and crystallize under shear flow. The assembled structure rotates like a rigid body, leading to oscillatory flow motion. As validation, we find the change of the flow is reversed if the crystallization is inhibited.

In the second part of the dissertation, I use a homebuilt electromagnetic tweezer as a microrheology tool to measure the yielding and post-yielding properties of the cytoplasm of the cell as a soft gel. I capture the onset of the solid-to-liquid transition and the post-yielding property of the cytoplasm. We find the deformation rate of the cytoplasm saturates when applied force increases. Later, we reveal the fact that the cytoskeleton dynamics dictate the creeping rate from control experiments with alternated cytoskeleton density and cytoskeleton dynamics. The role of the cytoskeleton is further validated with live-cell fluorescent visualization, where we can directly quantify how the cytoskeleton deforms under large deformation.