Holographic Rheology of Viscoelastic Fluids in Microfluidic Geometries

To explore the kinematics and rheology of viscoelastic flows it is critical to map three-dimensional (3D) velocity fields. The 3D flow information can be used for understanding phenomena such as elasticity-driven instabilities, slip and shear-banding. Digital holography microscopy (DHM) with particle tracking velocimetry (PTV) is an interferometry technique which can provide real-time 3D flow information by recording holograms of the flow. We demonstrate the robustness of holography in characterizing the kinematics of polymeric and Newtonian fluids in rectilinear channels, contraction-expansion and curved microchannels, which provide for shear, extensional and curvilinear flow behavior respectively. Using DHM-PTV, we recover the full 3D velocity field which is in good agreement with analytical results and flow simulations. Additionally, we demonstrate a digital holography driven rheology (DHR) approach for quantifying shear viscosity curves of complex fluids in thin-slit geometries. The DHR approach does not make a-priori assumptions about material properties and is thus independent of slip which has been found to affect narrow gap rheology. In sum, we conclude that digital holography microscopy has powerful applications for investigation of complex fluids in complex geometries.