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Viscoelastic second normal stress difference dominated multiple-stream particle focusing in microfluidic channels.
Haidong Feng1, Jules John Magda2, Bruce Kent Gale1
1Center of Excellence for Biomedical Microfluidics, Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah 84112, USA.
Applied Physics Letters
|March 5, 2020
Summary
In viscoelastic fluids, concentrated polymer solutions enable multiple particle stream focusing by utilizing the second normal stress difference (N2). This contrasts with dilute solutions, offering new microfluidic particle separation methods.
Area of Science:
- Fluid dynamics
- Polymer physics
- Microfluidics
Background:
- Particle focusing in viscoelastic fluids is key for microfluidic separations.
- Theoretical studies suggest the second normal stress difference (N2) drives multiple stream focusing.
- N2 is typically small in dilute polymer solutions but significant in concentrated ones at high shear rates.
Purpose of the Study:
- To investigate if the second normal stress difference (N2) can induce multiple particle stream focusing.
- To test this hypothesis by examining particle behavior in microchannels with varying polymer concentrations.
Main Methods:
- Systematic study of fluorescent particle behavior in microchannels.
- Utilizing carrier fluids with increasing polymer concentrations.
- Varying shear rates and dimensionless Weissenberg numbers (Wi).
Main Results:
- Multiple particle stream focusing was not observed in dilute polymer solutions, even at high Wi (~30).
- In concentrated entangled polymer solutions, particle streams bifurcated into two symmetric off-channel streams at higher shear rates.
- This observed focusing behavior differs from prior phenomena and is attributed to N2.
Conclusions:
- The second normal stress difference (N2) in concentrated entangled polymer solutions can induce unique multiple particle stream focusing.
- This N2-driven phenomenon offers a novel approach for particle manipulation and separation in microfluidic devices.
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