Cleaning of soft-solid soil layers on vertical and horizontal surfaces by stationary coherent impinging liquid jets

• Removal of PVA and petroleum jelly from glass and Perspex monitored by photography.
• Removal of Xanthan gum from stainless steel monitored by luminescence method.
• All soils exhibit growth of circular region as liquid film detaches soil layer from substrate.
• Quantitative model gives good agreement with experimental data for all three soils.
• Lumped cleaning kinetic parameter depends on soil layer thickness and rheology.

The cleaning action of stationary coherent liquid jets impinging (a) vertically downwards on horizontal plates, and (b) horizontally on vertical plates, was investigated using three soft-solid model soil layers: (i) PVA glue on glass and polymethylmethacrylate (Perspex) substrates; (ii) Xanthan gum on stainless steel; and (iii) petroleum jelly on glass. The liquid stream nozzle sizes, mass and volumetric flow rates and mean jet velocities investigated were: PVA, 2 mm, 17–50 g s−1 (0.06–0.139 m3 h−1), 5.3–15.9 m s−1; Xanthan gum, 0.39–3.3 mm, 2.1–148 g s−1 (0.008–0.53 m3 h−1); 4.5–31.7 m s−1; petroleum jelly, 2 mm, 7.8–50 g s−1 (0.06–0.139 m3 h−1); 2.5–15.9 m s−1. For all three soils, rapid initial removal of soil from the jet footprint was followed by the growth of a nearly circular, clean region centred at the point of jet impingement. The rate of removal of soil decreased sharply when the cleaning front reached the hydraulic or film jump. The data for the radial growth removal stage were compared with a mathematical model describing removal of the adhesive soil layer, where the force on the cleaning front was evaluated using the result reported by Wilson et al. (2012): their theory gave the momentum of the liquid film; this momentum was balanced against the soil strength, giving a simple relation between the cleaned radius and time. All three soils showed reasonable agreement with the model, across the range of flow rates and temperatures studied. The kinetic constant in the model was sensitive to soil layer thickness and the nature of the soil. Cleaning tests on the petroleum jelly soils at different temperatures, and separate rheological measurements, showed that the kinetic time constant for coating removal was proportional to the (critical shear stress)−1.8. There was good agreement between results obtained with vertical and horizontal plates for the PVA and Xanthan gum soil layers. The petroleum jelly results differed, which is partly attributed to differences in preparing the layers of this rheologically complex material.

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