  In this study, we report the use of digital holography microscopy (DHM) for
3D-resolved flow kinematics and shear rheometry of viscoelastic polymeric
fluids. We computationally reconstruct the recorded holograms to visualize the
tracer imbued flow volume in microchannels, followed by 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 complex fluids, we studied the effect of 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 condition or
constitutive equation. The viscosity versus shear rate data for Newtonian and
polyethylene oxide solutions could be measured in the range of ~ 0.05 - 20,000
s-1 with just four input flow rates. This data from holographic shear rheometry
was found to be in good agreement with computational fluid dynamics simulations
and macrorheometry. The holographic shear rheology technique remained
unaffected by wall-slip events and instead provided an avenue to quantitate
slip severity. Finally, we discuss holographic visualization of particle
migration in microfluidic flows which can limit flow field access while at the
same time provide a fingerprint of the suspending fluid rheology.
