(78b) Fluid-like Phenomena in Binary Granular Materials: Revealing the Physics behind Rising Granular Bubbles and Splitting Granular Droplets

Agitated granular materials often exhibit similarities to fluids, such as breaking waves in granular shear layers¹, condensation-like droplets on tapped plates², and gas bubbles in gas-solid fluidized beds³. More recently, McLaren et al.⁴ discovered a series of fluid-like phenomena that occur if a cluster of granular material is submersed in a granular material of different size and density and subjected to fluidizing gas and vertical vibration. If the cluster particles are larger and lighter than the surrounding bulk particles, a coherently rising structure forms (“granular bubble”) reminiscent of a gas bubble rising in a liquid. Conversely, a cluster of particles smaller and denser than the bulk particles sinks and performs consecutive splits (“granular droplet”) resembling a liquid drop sinking in a miscible fluid. Despite visual similarities to classical fluid phenomena, the emerging granular bubbles and droplets must be governed by distinct physics due to the ability of granular matter to solidify and the absence of surface tension between dry grains.

The present work elucidates the physics behind rising granular bubbles and sinking granular droplets and further explores the transition from rising to sinking. To this end, experiments in pseudo-2D vibro-gas-fluidized beds are combined with numerical simulations using computational fluid dynamics and the discrete element method (CFD-DEM). This combined approach allows insight into the interstitial gas flow and the network of inter-particle forces⁵.

The results reveal a gas flow heterogeneity in the proximity of a granular cluster, where gas flows preferentially through regions of larger particles due to their increased permeability. This preferential gas flow is the driving force for the rising and sinking of the cluster since the heterogeneity locally affects the fluidization of particles and consequently their motion. Based on these results an analytical buoyancy model is derived to compile a dimensionless regime map that predicts the occurrence of granular bubbles and droplets, as well as additional regimes that have not been reported yet experimentally. By revealing the underlying physics of the motion of a granular cluster subjected to agitation, these findings have implications on the operation of industrial machinery such as vibro-fluidized beds, where binary or polydisperse granular materials tend to segregate.

1. Goldfarb DJ, Glasser BJ, Shinbrot T. Shear instabilities in granular flows. Nature. 2002;415(6869):302-305.
2. Duran J. Rayleigh-Taylor instabilities in thin films of tapped powder. Phys Rev Lett. 2001;87(25):254301.
3. Davidson JF, Harrison D, Carvalho JRFGD. On the liquidlike behavior of fluidized beds. Annu Rev Fluid Mech. 1977;9(1):55-86.
4. McLaren CP, Kovar TM, Penn A, Müller CR, Boyce CM. Gravitational instabilities in binary granular materials. Proc Natl Acad Sci USA. 2019;116(19):9263-9268.
5. Metzger JP, McLaren CP, Pinzello S, Conzelmann NA, Boyce CM, Müller CR. Sinking dynamics and splitting of a granular droplet. Phys Rev Fluids. 2022;7(1):014309.