The conditions leading to dry lumps and dry granular jets when dispersing grains across a liquid-air interface*

Avoiding the formation of dry lumps when dispersing powders into water is key for many industrial processes.
This study identifies experimentally the conditions leading to i) the dispersion of individual grains ii) the formation of a dry lump or iii) the formation of a dry granular jet when pouring grains onto a liquid-air interface by varying systematically grain size, density, contact angle and the grain flow rate.
Once a granular island is formed onto an interface, it can grow till reaching a maximum number of grains, above which either individual grains leave the interface or the whole island sinks. These two phenomena are governed by a different scaling law and a critical Bond number can be identified, above which grains disperse. The latter depends on two dimensionless numbers: the relative grain density and on their contact angle. Conversely, the formation of dry granular jets depends on the dimensionless kinetic energy of the grains when impacting the interface and on their contact angle.
A physical interpretation is provided to explain common practical strategies used to promote grain dispersion in industrial processes.
REFERENCES
Ong X. Y., Taylor S. E., Ramaioli M., Langmuir, 2019, 35, 34, 11150-11156