The response of lunar regolith simulants of varying particle size ranges to vertical and horizontal vibrations

Granular fluidization is a method currently being investigated to mitigate the strong frictional and electrostatic interparticle forces that the material on the surface of the Moon, known as lunar regolith, exhibits. The reason for this ongoing line of inquiry is the eventual utilization of this material for numerous applications, e.g., feedstock for 3D-printed concrete structures or as a solid-support substrate for plant growth and/or oxygen and propellant extraction. It is known that when lunar regolith simulants are housed in a vessel vibrated at a given amplitude and frequency, the material can display fluidlike attributes and convective motion, which can be exploited for its excavation and/or transportation. In this study, three simulants were vibrated at different frequencies and amplitudes. Each simulant had a different particle size range (0.04−250, 0.04–90, and 0.04–35 μm) and accordingly a different particle average size. It was discovered that when applying vertical vibrations, by increasing the average particle size, convective motion can be strengthened. In the case of horizontal vibrations, the opposite occurs, whereby the overall convective movement generally increases on reducing the average particle size. It was also observed that different (transverse, longitudinal, and “hybrid”) convective modes can be excited according to the considered frequency, strength, and direction of vibrations; moreover, for the simulant with the lowest average particle size, a specific frequency exists at which particle aggregation phenomena show up regardless of the direction of vibrations. In such conditions, particles form macroscopic clusters due to the excitation of electrostatic forces.