Granular fragility under thermal cycles

We study the effect of thermal cycling of granular materials on both the granular packing fraction and the position of an intruder object within a granular sample. We demonstrate that the packing fraction consistently increases with thermal cycling, regardless of the relative coefficients of thermal expansion for the grains and their containers. The exact magnitude of the packing fraction increase appears to be influenced by factors other than the thermal expansion coefficients, with the intergrain friction and the friction between the grains and their container likely to be important. We also examined the motion of spherical intruders with thermal cycling as a function of the size and density of the intruders, and find that such intruders sink within the grains when the pressure from the intruder exceeds a threshold. These data demonstrate the fragility of dense granular ensembles to changes in the ambient temperature.