In this work, we investigate the influence of channel structure and fluid rheology on non-inertial migration of non-Brownian polystyrene beads. Particle migration in this regime can be found in biomedical, chemical, environmental and geological applications. However, the effect of fluid rheology on particle migration in porous media remains to be clearly understood. Here, we isolate the effects of elasticity and shear thinning by comparing a Newtonian fluid, a purely elastic (Boger) fluid, and a shear-thinning elastic fluid. To mimic the complexity of geometries in real-world application, a random porous structure is created through a disordered arrangement of cylindrical pillars in the microchannel. Experiments are repeated in an empty channel and in channels with an ordered arrangement of pillars, and the similarities and differences in the observed particle focusing are analyzed. It is found that elasticity drives the particles away from the channel walls in an empty microchannel. Notably, particle focusing is unaffected by curved streamlines in an ordered porous microchannel and particles stay away from pillars in elastic fluids. Shear-thinning is found to reduce the effect of focusing and a broader region of particle concentration is observed. It is also noteworthy that the rheological characteristics of the fluid are not important for the particle distribution in a randomly arranged pillared microchannel and particles have a uniform distribution for all suspending fluids. Moreover, discussion on the current discrepancy in the literature about the equilibrium positions of the particles in a channel is extended by analyzing the results obtained in the current experiments.

Information on the viscoelastic response to extensional deformation of polymeric solutions and other complex fluids is crucial for understanding their behavior in a wide variety of industrial applications. While the characterization of complex fluids using simple shear experiments is well established, a complete characterization of their extensional rheological properties is still challenging. Currently, methods such as CaBER, ROJER, and microfluidics are used for this. The downside of these techniques is that fluids with low viscosity and/or short relaxation times are still hard to characterize. Here we propose an alternative approach for measuring extensional rheological properties by using a microscopic air bubble as an extensional rheometer. We pursue this idea experimentally by submerging a single bubble in a pool of liquid and studying the dynamics of the bubble under influence of a change in hydrostatic pressure. This situation is analogous to a stress-controlled shear rheology experiment, where a time-dependent shear stress is applied to a sample and the resultant strain is measured. We test our approach on a range of polymeric fluids with different relaxation times and find that the bubble microrheology experiments provide results and trends that are indeed consistent with theoretical expectations and with previous data acquired using established methods. The concept of bubble microrheology is thus very promising. While the experimental technique is straightforward, practical improvements can still be made on the experimental setup and the data analysis, which could extend the range of investigable materials and the amount of information that can be extracted from the measurements. A chief advantage of our methodology is that it ensures the measurement of only extensional properties and that fluids with very short relaxation times can be measured. Finally, by applying a sinusoidal pressure variation instead of a pressure jump, our method could potentially be extended to oscillatory measurements.