Development of a method to determine normal and shear stress necessary to remove a swollen soil from a surface
To simulate cleaning processes is of constantly growing importance with regard to optimising the cleaning efficiency in food production lines. This requires suitable parameters to describe the cleaning behaviour of different soils. Therefore, a method was developed to determine the normal and shear stress, which are necessary to remove a swollen soil layer from a surface, by an impinging jet. The method consists of a cleaning procedure in combination with normal force measurements. For the cleaning tests, a stainless steel plate (AISI 304 with a 2B finish) with a defined soil layer was soaked reproducibly for a defined time. Subsequently, a very short impulse of a liquid jet impinged on the surface in order to determine the gauge pressure limit state, which is necessary to remove the soil from the surface in the impact area. At this determined limit state, the measurement was repeated with the same parameters, but with a piezo-based force sensor instead of the soiled surface. Out of this measured forces, it was possible to calculate the target values: the normal and the shear stress. Measurements were performed with petroleum jelly and egg yolk as a food based model soil at different jet angles and for different swelling times. As a result, it was possible to draw the calculated stress as a function of the jet angle. By that, it is possible to quantify and compare the cleaning behaviour of different soils and at different swelling states.