(380aa) Wall Friction Measurements on Free-Flowing Material at Low Normal Stresses

Recent experiments in gravity-driven hopper flow have necessitated the quantification of wall friction angles between the fill material, mustard seeds between 2.00 and 2.36 mm in diameter, and three different wall materials. The wall material samples consisted of flat pieces of 3D-printed ABS plastic, one ‘as printed’, one smoothed with acetone vapour, and the other roughened by adhesion of a monolayer of mustard seeds. Angles of wall friction were estimated using two methods: firstly, a translational shear cell was constructed, in which the wall material was moved against a stationary sample of fill material, to avoid issues which could arise from moving cell containing the test sample across a very rough surface, as is usual practice. Forces associated with the movement were related to varying normal forces to establish the wall friction angle for each condition. These values were compared to those obtained by slowly inclining a wall friction sample until seeds on its surface overcame friction and began to move. The latter method resulted in markedly higher angles, though little difference was shown between the ‘as printed’ and smoothed surfaces; also, the angle of friction was significantly higher for the roughened surface, but could not be measured in the shear cell, due to jamming of seeds falling into crevices in the rough surface.