Thermal cycling leads grains to more homogeneous force networks and energy repartition

With the help of molecular dynamics simulations of a realistic granular system, here it is first reported that thermal cycling is able to induce the total system energy to be reallocated to grains, and this makes grains gradually evolve to the energy equilibrium, especially under larger cyclic temperatures. At the same time, the stress in force chains (and on grains) will also get much relieved, and this does not need the decrease in applied pressure. Consequently, the statistical probability distribution of both internal particle forces and energy will become narrower around the most likely value, and their spatial distribution will also become more and more homogeneous. The primary cause behind the above phenomena is that grains within the initial granular system are not in sufficient energy equilibrium, and they were rearranged by thermal excitations to a new steadier packing where grains are closer to energy equilibrium. Besides, it was also observed that for the macroscopic-scale grains, the internal particle energy distribution within the granular system obeys a more general Maxwell-Boltzmann (M-B) distribution which has a more uniform distribution profile than the classical M-B distribution for gases, suggesting that the direct-contact collision may be a more effective channel for the energy interchange between particles than the indirect-contact collision. The results showed here demonstrate that more homogeneous energy reallocations may be a universal phenomenon for granular matter and may have important practical implications for the handling of granular materials. (C) 2018 Elsevier B.V. All rights reserved.