Granular convection in a monolayer induced by asymmetric horizontal oscillations

Granular systems subjected to vibrations might exhibit convection due to uneven frictional contacts or unbalanced external perturbations. Gravity's ubiquitousness in most practical applications makes it the predilected selection to trigger the mentioned imbalance. However, we demonstrate that asymmetric horizontal shaking generates convection in a granular monolayer experimentally and numerically by varying the dimensionless acceleration r without gravity influence. Convection is evidenced through non-uniform particle density and granular temperature distributions. Two distinct convection patterns are manifested depending on r. Two roles are observed for low r values, whereas a four-vortex pattern is displayed when r reaches significant values. Results suggest that wall friction influences the resulting convective pattern but is not responsible for global material circulation. Instead, the asymmetry in the energy injection is reflected in asymmetries of the macroscopic gradients of granular temperature, particle density, and momentum transfer, correlating with the overall circulation of the grains.