Microfluidic shear rheology and wall-slip of viscoelastic fluids using holography-based flow kinematics

In this study, we report microfluidic shear rheology and wall-slip using the 3D-resolved flow kinematics obtained from digital holography microscopy (DHM). We computationally reconstruct the recorded holograms to visualize the tracer imbued flow volume in linear microchannels, followed by the implementation of particle tracking velocimetry (PTV) to quantitate spatially resolved velocity fields in 3D. In order to select optimal parameters for DHM-PTV characterization of viscoelastic fluids, we studied the effect of the hologram recording distance, seeding density, and particle size. Using the optimal parameters, we show quantitative characterization of the shear rheology from the velocity fields without any a priori assumptions of wall boundary conditions or constitutive equation. The viscosity vs shear rate data for Newtonian and polyethylene oxide (PEO) solutions could be measured in the range of ≈0.05 to 20 000 s^-1 with just three input pressures using sample volumes as low as 20 μl. These data from holographic shear rheometry were found to be in good agreement with computational fluid dynamics simulations and macrorheometry. With respect to the wall-slip, we find that highly viscoelastic PEO solutions can show slip lengths in the order of few microns. Finally, we discuss holographic visualization of particle migration in microfluidic flows, which can limit flow field access, whereas at the same time provide a fingerprint of the suspending fluid rheology.