Hydrodynamic irreversibility of non-Brownian suspensions in highly confined duct flow

A.H. acknowledges support from the US Department of Energy, Advanced Scientific Computing Research programme, under the Scalable, Efficient and Accelerated Causal Reasoning Operators, Graphs and Spikes for Earth and Embedded Systems (SEA-CROGS) project, FWP 80278. Pacific Northwest National Laboratory (PNNL) is a multi-programme national laboratory operated for the US Department of Energy by the Battelle Memorial Institute under contract no. DE-AC05-76RL01830. F.V. acknowledges funding from the University of Granada through the Brown/CASA-UGR Research Collaboration Fund and MICINN PID2019-104883GB-I00 project (Spain).Supplementary movies are available at https://doi.org/10.1017/jfm.2023.793The irreversible behaviour of a highly confined non-Brownian suspension of spherical particles at low Reynolds number in a Newtonian fluid is studied experimentally and numerically. In the experiment, the suspension is confined in a thin rectangular channel that prevents complete particle overlap in the narrow dimension and is subjected to an oscillatory pressure-driven flow. In the small cross-sectional dimension, particles rapidly separate to the walls, whereas in the large dimension, features reminiscent of shear-induced migration in bulk suspensions are recovered. Furthermore, as a consequence of the channel geometry and the development and application of a single-camera particle tracking method, three-dimensional particle trajectories are obtained that allow us to directly associate relative particle proximity with the observed migration. Companion simulations of a steadily flowing suspension highly confined between parallel plates are conducted using the force coupling method, which also show rapid migration to the walls as well as other salient features observed in the experiment. While we consider relatively low volume fractions compared to most prior work in the area, we nevertheless observe significant and rapid migration, which we attribute to the high degree of confinement.US Department of Energy: FWP 80278, DE-AC05-76RL01830University of GranadaMICINN PID2019-104883GB-I00 project (Spain