(98bc) High Deborah Number Elastic Instabilities Around Microfluidic Confined Cylinders

Flow of viscoelastic fluids around a confined cylinder is a hallmark fluids problem because it resembles industrial processes and is easily simulated. This flow displays multiple regimes with elastic and inertio-elastic instabilities that occur at critical Deborah (De) or Reynolds (Re) numbers.   Practical limitations have restricted the experimentally explored range of De; using microfluidic channels, these limitations have been overcome, and allowed the study of high De flows.  We have systematically characterized the flow around symmetrically placed, highly confined\ microfluidic cylinders at 10^-2 < De < 10³, using low viscosity Boger fluids.   By changing fluids, flow rates and geometry, non-dimensional parameters are independently tuned, allowing a controlled and detailed study.  

At low De, the nature of the flow around a confined cylinder displays inertio-elastic instabilities forming downstream of the cylinder at critical values of Re and De consistent with predictions in literature.   At larger values of De attainable due to microfluidic channels, new elastic instabilities are observed that have not been previously reported.  We first identify a downstream instability of temporally and spatially varying streamlines at De~50.  This instability is a precursor to a new class of upstream instability at De > 100.  During this instability, the upstream stagnation point separates from the cylinder face up to ~ 5 diameters upstream of the cylinder, and its position oscillates in a growth and collapse cycle.  We characterize flow patterns and onset of both instabilities as functions of the Elasticity number and the viscoelastic Mach number, and conclude that the instability is purely elastic in nature but affected by inertia.  Furthermore, we characterize the role of geometry on the onset of both instabilities.   We have identified two new and unique elastic instabilities around confined cylinder flows and provide a basic description of their behavior and underlying physical mechanisms.