Optical Coherence Tomography Velocimetry for In-Line Processing: The Spherical-to-Wormlike Micelle Transition

Abstract

Personal care products are often dynamically formulated in situ. Variations in the chemistry of the base components (e.g., their polydispersities or ionic contents) require extensive off-line rheological analysis to ensure the products meet benchmarks for performance and thus consumer satisfaction. An in-line alternative for rheological quality control and monitoring thus has the potential to improve efficiency on industrial pipelines. Therefore, we demonstrate optical coherence tomography velocimetry (OCT-V) for the in-line processing of a shampoo based on SLES and CAPB with varying concentrations of salt. OCT-V is a noninvasive quasi-elastic light scattering technique, capable of spatially resolved velocity measurements with an axial depth resolution of 9 μm and a penetration depth of 1.5 mm into the samples. Our in-line apparatus uses infrared light (the wavelength is 1315 nm) and has been optimized for live manufacturing with a 200 L test rig at the Unilever R&D lab. The addition of salt to the shampoo is used for in situ formulation to increase the viscosity, inducing the spherical-to-wormlike micelle transition. To create in-line rheological benchmarks, we measured time averaged velocity profiles and transient velocity fluctuations in the shampoo formulations at imposed flow rates of Q = 500 and 1000 L/h. For a shampoo formulation with 1.1% NaCl salt, fits of the Hagen–Poiseuille equation to velocity profile data give flow rates that are in close agreement with the imposed values, QHP = 502 ± 2 L/h and 944 ± 4 L/h, corresponding to a 0.4% and 5.6% error, respectively. Combined with measurement of the longitudinal pressure drop, this could be used to calculate constitutive properties, such as the viscosity. The standard deviation of the distribution of transient velocity fluctuations is a decreasing function of salt concentration due to the increasing viscosity. If calibrated to the desired end product, the velocity fluctuations could also be used as a reproducible indicator of product quality on industrial pipelines.