Research

Shear Rheology of Particle Suspensions in Viscoelastic Fluids: Suspensions of solid particles in polymeric fluids appear in many industrial settings. Understanding the physics of their stress-strain response provides a guide for formulating these suspensions. The presence of small amounts of particles in the suspension causes an overall increase in the suspension shear viscosity, driven by increased polymer stretch in the fluid around the particle. Further increasing the number of particles in the suspension increases the suspension shear viscosity due to particle-particle interactions.

Locomotion in Complex Fluids: Active “swimmers” are often immersed in complex biofluids that exhibit non-Newtonian rheology (e.g. spermatazoa in cervical mucus, cilia in the upper respiratory tract of the lungs). It was found that undulatory swimmers (e.g. C. elegans) swim slower in viscoelastic fluids due to resistance from regions of increased polymer stretch created near the head while swimming. Swimmers that create “swirling” flow have also been observed to speed up in elastic fluids, or even create propulsion that would not be possible in Newtonian fluids. In the latter, polymers stretched in the swirling flow generate stresses that propel the swimmer.

Rheology of Particles in Shear-thinning Fluids: The injection of inert micro-particles into the knee joints of mice were found to alleviate symptoms of arthritis. The fluid within joints, synovial fluid, is a highly shear-thinning viscoelastic material. Arthritic synovia have much lower viscosities and less shear thinning than healthy synovia. The rheology of a suspension of rigid particles in a suspending medium approaching mock synovial fluid is measured, and the results compared to predictions from simulation. The addition of particles leads to initial shear-thickening followed by shear-thinning in the suspension shear viscosity. From simulations, it was observed that the presence of particles create spiral-like streamlines when the suspension is sheared, leading to more diffuse stress.

Modeling gas phase etching in high aspect ratio stacked nanostructures for semiconductor processing: The need for precise control of the nanoscale features in gate-all-around (GAA) nanotransistors poses a challenge in manufacturing them. The high material selectivity makes gas phase etching well suited to fabricate such designs in comparison to liquid phase and plasma-based etching techniques. An etching configuration that is of particular interest is one consisting of alternating layers of Si and SiGe from which the SiGe layers are selectively etched using F2 gas. In the etching of these structures, it is important to have a uniform etch-rate for SiGe layers from top to bottom, to maintain consistency of the internal features. In this study, we have developed a simulation tool to predict the etch profile evolution over time in a gas phase etching process, which considers the transport processes (Knudsen diffusion) and surface interactions involved. Our results show that the re-emission of etchants (determined by the sticking coefficient) at the etching interface affects the net flux of etchants anywhere on the interface. Hence re-emission plays a vital role in determining the differential etch rates observed across layers at different depths in the stacked feature.

High-throughput Measurement of Red Blood Cell (RBC) Shear Moduli: The dynamics of RBCs in blood flows is strongly associated with the deformation of RBCs in flow. Reduced deformability of RBCs can affect microcirculation and reduce oxygen transport efficiency. It is also well known that reduced RBC deformability is a signature of various physical disorders, including sepsis, and that the primary determinant of RBC deformability is the membrane shear modulus. To measure the distribution of an individual’s RBC shear modulus, a high-fidelity computational model of RBCs in confined microchannels was developed to inform the design and operation of a novel microfluidic flow, imaging, and image-analysis system. The simulation platform was used to construct the appropriate deformability figure(s) of merit to quantify RBC stiffness based on an experimentally measured, steady cell shape in flow through a microchannel constriction of specific dimensions. An automatic image processing routine then tracks individual RBCs and analyzes their shape to retrieve the distribution of shear moduli for the population.

Thin Liquid Films over Curved Substrates: Thin films are ubiquitous. When exposed to air, the air-liquid interface can establish surface tension gradients that lead to Marangoni flows. In tandem with other factors such as gravity, evaporation, and curvature, the stability of the thin firm can increase or decrease. In the Shaqfeh group, we develop theoretical models that help us understand the driving forces behind experimentally observed thin film dynamics.

Modelling novel additive manufacturing processes: In additive manufacturing, it is imperative to increase print speeds, use higher viscosity resins, and print with multiple different resins simultaneously. To this end, the Shaqfeh group is modelling the fluid mechanics governing a new UV-based photopolymerization 3D printing process, injection continuous liquid interface production (iCLIP), which accelerates printing speeds 5 to 10-fold over current methods such as continuous liquid interface production (CLIP), can utilize resins an order of magnitude more viscous than can CLIP, and can readily pattern a single heterogeneous object with different resins in all Cartesian coordinates. iCLIP exploits a continuous liquid interface—the deadzone—mechanically fed with resin at elevated pressures through microfluidic channels dynamically created and integral to the growing part, which can effectively be described by lubrication theory.