Crystal Breakage Due to Combined Normal and Shear Loading

Combined normal and shear stress on particles occurs in many devices for solid–liquid separation. Protein crystals are much more fragile compared to conventional crystals because of their high water content. Therefore, unwanted crystal breakage is to be expected in the processing of such materials. The influence of pressure and shearing has been investigated individually in the past. To analyze the influence of combined shear and normal stress on protein crystals, a modified shear cell for a ring shear tester is used. This device allows one to accurately vary the normal and shear stress on moist crystals in a saturated particle bed. Analyzing the protein crystals in a moist state is important because the mechanical properties change significantly after drying. The results show a big influence of the applied normal stress on crystal breakage while shearing. Higher normal loading leads to a much bigger comminution. The shear velocity, however, has a comparatively negligible influence.