In-situ measurement of the critical stress of viscoplastic soil layers
Predicting the removal of residual layers or soiling deposits from process equipment surfaces requires knowledge of the rheology of the layer material. Removing a sample for analysis in a rheometer is likely to disrupt its structure, thereby changing its rheological behaviour. The millimanipulation device presented by Magens et al. (2017, J. Food Eng, 197, 48–59) was employed here as an in situ rheometer to estimate the critical (yield) stress of layers of viscoplastic food and fast-moving consumer goods (FMCG) products on steel plates using the protocol reported by Tsai et al. (2020, J. Food Eng, 285, 48-59). Measures of the critical stress of 15 materials, including spreads, cosmetics and ointments, were obtained on a rotational rheometer using increasing shear stress ramp, shear stress step, and oscillatory shear stress amplitude sweep testing. Reasonably good agreement (±30%) was obtained between these values (ranging from 70 to 2000 Pa) and those obtained with the millimanipulation device, indicating that the latter could be used to study soil layers in situ. Better agreement was obtained for materials with a high yield stress, where the shape of the accumulated berm was more easily identified. The use of the millimanipulation device to quantify spreadability is compared with the strain-energy-at-yield metric obtained from rotational rheometry.