Measurements and modelling of the ‘millimanipulation’ device to study the removal of soft solid layers from solid substrates

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‘Millimanipulation’ is a mm-scale scraping device for studying the removal behaviour of soft solid layers from solid substrates. A vertical blade is moved horizontally through a sample at controlled speed and the force on the blade and the shape of the berm of deformed material are recorded. Computational fluid dynamics (CFD) simulations of the action were performed using the volume of fluid (VOF) method with dynamic meshes in OpenFOAM, with attention given to capturing the two-phase boundary near the moving blade. Particle image tracking was utilised to capture the velocity distribution within layers while being scraped. Two viscoplastic materials (a commercial petroleum jelly and a pharmaceutical soft white paraffin) were tested using the millimanipulation device reported by Magens et al. (2017) J. Food Eng. 197, 48–59. Separate testing established that these exhibited yield stress behaviour with critical stresses of 280 Pa and 460 Pa, respectively. Acceptable agreement was obtained between the predicted and experimental removal force and berm shape under different scraping conditions. The sensitivity to rheometrical parameters was investigated and confirmed that under the conditions tested (Bingham numbers > 10) the critical stress was the dominant parameter. Interrupted mode testing allowed the contribution from yield behaviour to be quantified and the critical stress estimated from simple, in situ, testing. The results demonstrate that millimanipulation can be used to quantify the rheological characteristics of complex fluids and industrial materials which are difficult to load into standard rheometer geometries or which are only found in the form of layers, disruption of which would disrupt the material immeasurably.