Oscillatory elastic instabilities in an extensional viscoelastic flow.

Area of Science:

• Fluid Dynamics
• Polymer Physics
• Non-Newtonian Flows

Background:

• Dilute polymer solutions exhibit elastic instabilities even at low Reynolds numbers (Re).
• Previous studies in similar geometries reported steady asymmetric flows due to spatial symmetry breaking.
• The role of geometry, particularly recirculating cavities, in elastic instability dynamics is not fully understood.

Purpose of the Study:

• To quantitatively investigate elastic instabilities in dilute polymer solutions within a T-junction with a long recirculating cavity.
• To characterize the nature of the observed instabilities and compare them with existing literature.
• To explore the influence of the recirculating cavity geometry on flow bifurcations.

Main Methods:

• Utilizing a T-junction microfluidic device with a long recirculating cavity.
• Employing quantitative measurements of fluid discharge rate.
• Simultaneously imaging the interface between dyed and undyed polymer solutions to visualize flow dynamics.

Main Results:

• Observed two consecutive oscillatory elastic instabilities in the polymer solution flow.
• The first instability was identified as a forward Hopf bifurcation, leading to a uniformly oscillating state due to broken time translational invariance.
• The second instability manifested as time-aperiodic transverse fluctuations at the channel junction, advected downstream.

Conclusions:

• The presence of a long recirculating cavity promotes time translational invariance breaking, leading to oscillatory instabilities, contrasting with previous reports of steady states.
• The geometry of the flow system, specifically the recirculating cavity length, is crucial in determining the type of elastic instability observed.
• These findings offer new insights into the complex dynamics of viscoelastic flows and elastic instabilities in microfluidic devices.