Humidity induced interparticle friction and its mitigation in fine powder flow

Humidity can affect the bulk behavior of fine hygroscopic powders at multiple levels and fewer mitigation approaches are presently available to counter its adverse effects. This work reports these approaches while investigating humidity influenced frictional behavior in fine cohesive starch powder flow under confined conditions. Interparticle friction and wall friction were examined under varying relative humidity (RH) using shear and wall friction tests in the quasi-static regime. The measured stress details from the bulk powder testing in the powder rheometer revealed increased stick-slip friction in the form of unsteady stresses at lower RH which gradually decreased with increase in RH. The surface texturing of particles and modifying contact stiffness through the dry coating alleviated the nonmonotonic powder flow at all the RH conditions. The stress oscillations and the consequent increase in measured friction were attributed to the discretely distributed surface moisture causing alternate sticking and slipping of the contacts. The lubricating effect of moisture at higher RH and the surface modification facilitated steady sliding, also accompanied by reduced coefficient of friction and wall friction angle. Furthermore, the different fluid film lubrication regimes were identified and highlighted the importance of fluid-particle interactions leading to undesirable bulk behavior in fine powders.