(708d) Humidity Dependent Stick-Slip Behavior in Fine Cohesive Powder

Humidity is a crucial factor affecting the friction between the particle surfaces. The stick-slip type of friction phenomenon indicates intermittent stop and start flow behavior of powders. This type of distinct phenomenon occurring in powders could lead to undesirable powder behavior during its handling, storage or processing.  In the present study, the humidity dependent frictional behavior of fine cohesive powder and its effect on the flow and packing properties was investigated. The corn starch powders, conditioned at relative humidity (RH) conditions ranging from 30% to 65%, were subjected to compressibility, shear and wall friction tests using powder rheometer. Both interparticle friction (cohesion) and the friction between the particles and stainless steel (SS) surface (wall friction) were examined.  At lower humidity levels, increased stick-slip behavior of corn starch in the form of unsteady normal and shear stresses was observed from the raw data of compressibility and shear tests respectively. This unsteady behavior decreased gradually with the increase in RH. The chaotic oscillations observed in normal and shear forces indicated discontinuous distribution of moisture on the surface. The transition from stick-slip motion of corn starch powder to steady particle sliding was observed at a critical humidity of about 60%. Thus, at higher humidity levels, lubricating effect of moisture layer facilitated better particle rearrangement (better slippage). In the absence of this lubricating layer, the steady sliding motion of particles was switched to stick-slip motion. In addition, the consolidation load dependency of the stick-slip friction was observed at different RH conditions. Consequently, this phenomenon resulted in variation in powder bed packing and increased interparticle cohesion, as observed from the compressibility and shear tests respectively. Furthermore, increased adhesion between particles and SS surface was observed at lower RH levels due to higher frictional forces. The present study showed that the stick-slip phenomenon occurring at lower RH conditions in fine powder could lead to undesirable powder bulk behavior. Also, a critical humidity for stick-slip to steady-state motion transition of corn starch powder was identified.