Cohesion of Granular Matter in Subzero Humidity

Atmospheric humidity has an important effect on granular media by increasing the cohesion of powders and grains and is hence of significance for the industrial processing of finely dispersed materials. Water bridges around the contact points of hydrophilic particles lead to an increased cohesion at higher humidities, and this has been studied at ambient temperatures in model systems. Because liquid water bridges may form below the freezing point of water, we decided to study the cohesion of millimeter-size glass beads in humid atmospheres at subzero temperatures. We find that the angle of repose of a pile of glass beads in a saturated atmosphere of water vapor below 0 °C decreases as the temperature falls, and closely mirrors the behavior as the humidity at room temperature is reduced from saturation; that is, the angle of repose of the glass beads is similar for water bridges that are of a similar size. These results show that both temperature and moisture determine the cohesion of granular media in subzero environments, and confirm that surface roughness acts to decrease the area over which capillary forces between particles act. Our work also illustrates how interfacial curvature reduces both the vapor pressure and the melting temperature of a substance.