Optical coherence tomography velocimetry of colloidal suspensions

Abstract

Optical coherence tomography velocimetry combined with a rheometer and optical modulation techniques provides increased sensitivity to the low shear rate motion of complex fluid systems. Optical modulation coupled with a new interferometer design yields improved signal to noise ratios and is demonstrated with optically opaque colloidal suspensions. Thus the measurable range of shear velocities with complex fluids can be as low as ∼40 μm s⁻¹, more than an order of magnitude improvement on the previous lower limit of ∼700 μm s⁻¹. Furthermore the apparatus demonstrates improved sensitivity to the measurement of velocity. The instrument was used to study two hard sphere colloidal systems, sterically stabilized PVP spheres of 1 μm radius and sterically stabilized polystyrene spheres of 600 nm radius, which display shear banding behavior due to shear induced concentration gradients. OCT velocimetry also allows the velocity fluctuations of the system to be quantified as a function of the distance across the rheometer gap to help classify underlying unsteady or turbulent phenomena.