Capillary forces between particles caused by the condensation of water from the atmosphere can lead to a significant increase in bulk powder strength and in turn affect the flowability and dispersability of the powder. The effect of relative humidity on powder strength was investigated for a spherical aluminum powder with a mean particle size of 4 pm. The surface hydrophobicity of the aluminum powder was modified with n-octadecyltrimethoxysilane (ODTMS) and tridecafluoro-1,1.2,2-tetrahydrooctyl-1-triethoxysilane (TDFS). Sessile drop contact angle measurements oil ideal aluminum surfaces treated with ODTMS and TDFS respectively. were used to determine optimum conditioning time to produce adequately hydrophobic surfaces. It was found that one hour of treatment resulted in advancing contact angle increase from 67 degrees for bare aluminum to 114 degrees and 113 degrees for ODTMS and TDFS treated surfaces respectively. The powder now properties of the bulk powders were then Subsequently measured at three humidity levels using and automated annular shear cell powder tester. An increase in the unconfined yield strength, f(c), was seen at a relative humidity as low as 50 percent for the untreated aluminum powder. Further increasing the relative humidity to 80 percent resulted in a 100 percent increase in the unconfined yield strength of the untreated aluminum powder. The aluminum powders modified with TDFS and ODTMS demonstrated a smaller increase in the unconfined yield strength of 30% and 20% respectively. Published by Elsevier B.V.