Abstract
Viscoelastic fluids, including particulate systems, are found in various biological and industrial systems including blood flow, food, cosmetics, and electronic materials. Particles suspended in viscoelastic fluids such as polymer solutions migrate laterally, forming spatially segregated streams in pressure-driven flow. Viscoelastic particle migration was recently applied to microfluidic technologies including particle counting and sorting and the micromechanical measurement of living cells. Understanding the effects on equilibrium particle positions of rheological properties of suspending viscoelastic fluid is essential for designing microfluidic applications. It has been considered that the shear-thinning behavior of viscoelastic fluid is a critical factor in determining the equilibrium particle positions. This work presents the lateral particle migration in two different xanthan gum-based viscoelastic fluids with similar shear-thinning viscosities and the linear viscoelastic properties. The flexibility and contour length of the xanthan gum molecules were tuned by varying the ionic strength of the solvent. Particles suspended in flexible and short xanthan gum solution, dissolved at high ionic strength, migrated toward the corners in a square channel, whereas particles in the rigid and long xanthan gum solutions in deionized water migrated toward the centerline. This work suggests that the structural properties of polymer molecules play significant roles in determining the equilibrium positions in shear-thinning fluids, despite similar bulk rheological properties. The current results are expected to be used in a wide range of applications such as cell counting and sorting.
Highlights
Microfluidics-based analytical tools have recently attracted much attention, as very small volumes of samples are typically required and the whole process from sample preparation to analysis can be potentially integrated onto a single chip [1]
The PS beads in XG solution in PBS solution (XGPBS) migrated toward the channel corners, while the beads were focused along the channel centerline in Figure 3a,b,d,e) are blunter than the quadratic profile of the viscoelastic fluid with constant shear viscosity (PVP; Figure 3c,f)
The present results demonstrate that the equilibrium particle positions in the shear-thinning fluid can be tuned by varying the ionic strength of the suspending medium
Summary
Microfluidics-based analytical tools have recently attracted much attention, as very small volumes of samples are typically required and the whole process from sample preparation to analysis can be potentially integrated onto a single chip [1]. We investigate lateral particle migration in a square microchannel (Figure 1a) in an aqueous solution of xanthan gum (XG) as a shear-thinning fluid. We demonstrated that the equilibrium particle locations in the shear-thinning fluid are clearly affected by the ionic strength (or structural properties) of the XG molecules (Figure 1b).
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