Optical Coherence Tomography Velocimetry for In-Line Processing: Velocity Profiles and the Intermittency of Opaque Complex Fluids In Situ

We demonstrate optical coherence tomography (OCT) velocimetry with in-line processing of complex fluids for the first time. The OCT measurements were performed on a perspex section of a test rig containing ∼40 L of complex fluids, analogous to real-world manufacturing conditions. Opaque solutions of lamellar surfactant gel networks (LGNs) and powdered milk were explored. Velocity profiles characteristic of power law fluids were found in the LGNs, in good agreement with independent measurements of the flow rate and off-line determination of viscosity. The velocity fluctuations of 3.4 pL volumes of the fluids in the test rig were also explored. LGNs demonstrated smooth, steady flows, whereas the powdered milk demonstrated marked instability, both showing intermittent behavior and Kolmogorov scaling for fully developed classical turbulence of Newtonian fluids (P(ω) ∼ ω^−5/3, where P(ω) is the power spectral density of the velocity fluctuations, and ω is the frequency). The effects of dynamic changes in formulation on velocimetry measurements could be observed with LGNs during the addition of salt and with the milk powder due to biofouling.