Polymer solution flow transitions and scaling laws for changing contraction ratios in planar constriction...

Area of Science:

• Fluid dynamics
• Rheology
• Microfluidics

Background:

• Pore-scale flows through contractions and expansions are crucial in geoengineering, microfluidics, and material processing.
• Polymer solutions can exhibit flow instabilities due to nonlinear rheological properties, even in creeping flows.
• Flow geometry significantly influences the interplay between shearing and extensional kinematics.

Purpose of the Study:

• Investigate flow responses to changing constriction width and contraction ratio (CR) in planar constriction microchannels.
• Examine the behavior of water and three distinct polymer solutions: xanthan gum, polyethylene oxide (PEO), and polyacrylamide.
• Analyze the impact of CR on flow destabilization and vortex dynamics.

Main Methods:

• Experimental investigation of pore-scale flows in planar constriction microchannels.
• Systematic variation of constriction width to alter the contraction ratio (CR).
• Testing with water and three polymer solutions with different rheological properties (shear thinning, viscoelastic).

Main Results:

• Contraction and expansion flows generally destabilize with increasing CR for most fluids.
• Polyethylene oxide (PEO) solutions show a nonmonotonic dependence of instability onset and vortex length on CR.
• Other fluids exhibit a fixed-order dependence of vortex length on CR, determined by fluid rheology.

Conclusions:

• Channel geometry, specifically constriction width and CR, plays a critical role in pore-scale flow stability.
• The unique nonmonotonic response of PEO solutions highlights the complex interplay between viscoelasticity and geometry.
• Findings are valuable for designing microfluidic devices and optimizing processes like enhanced mixing, material recovery, and sequestration.