Dynamic (flow - induced) expansion (dilation) of fine powders inside a rotating drum is investigated. Most previous work on powder dilation is based on two-phase effects, where the presence of air either drives the flow (fluidized bed), or air needs to be displaced,for the powder to flow (hoppers). Experimental results show that for many common powders the bed dilates visibly up to 25%, depending on powder composition, particle size, and the rotation speed of the drum. Discrete element simulations were performed in parallel for various values of powder cohesion, rotation rates, and drum size. Results show qualitative agreement between experiments and simulations. In experiments, after the initial transient behavior, the density reached at equilibrium for all materials tested is lower than the static "bulk density" reported in the literature. In both experiments and simulations, increase in powder cohesion enhances dilation. DEM simulations are used to analyze local density fluctuations and coordination numbers, which decrease with increasing cohesion. As cohesive forces become dominant, large pores are formed in the bed, leading to substantial fluctuations in local (micro) density that can have tremendous consequences for product uniformity and quality. (c) 2006 American Institute of Chemical Engineers.