(82f) Forming Lumps and Jets when Pouring Particles through a Liquid Interface

The dispersion of dehydrated powders in water is a key step in many process industries. During the rehydration process, the powder grains can form heterogeneous powder lumps that are wet outside and dry inside [1, 2]. It is often difficult to predict and control the conditions leading to lump formation since dissolution is hindered subsequently by entrained air. Phenomena associated with the balance of forces responsible for controlling whether small objects float at an interface or sink have been the subject of recent studies [3, 4]. A theoretical condition for the formation of granular jets was also proposed [5], but a comprehensive experimental validation is still lacking.

The focus of this contribution is therefore on studying experimentally the behaviour of a powder poured on a static liquid interface and the formation of a granular jet, understanding the effect of grain flow rate, contact angle, density and size. The wetting properties of the system were varied by using ethanol-water mixtures. Two types of grains, of different density, were used and the size of the grains varied between 50 microns and 1 mm.

The conditions under which powder grains form a ‘jet’ through the interface, or alternatively float to form a powder island, were quantitatively investiaged. The relationship between the kinetic energy of the grains generated as they flow through a circular orifice under gravity and the surface energy created by the grains at the interface per unit time was also quantitatively studied. The experimental results indicate that the powder island size grows monotonically with decreasing grain density and increasing contact angle. These finding can be used to promote powder dispersion and avoid lumps.

References

[1] J. Dupas, L. Forny, M. Ramaioli, Powder wettability at a static air-water interface, Volume 448, p. 51–56, JCIS, 2015.

[2] J. Dupas, E. Verneuil, M. Van Landeghem, B. Besson, L. Forny, M. Ramaioli, F. Lequeux and L. Talini, Glass transition accelerates the spreading of polar solvents on a soluble polymer, Phys. Rev. Lett. 112, 188302, 2014.

[3] M. Abkarian, S. Protière, J. M. Aristoff, and H. A. Stone, Gravity-induced encapsulation of liquids by destabilization of granular rafts., Nature Commun., 4, 2013.

[4] D. Vella, P. Metcalfe, and R. Whittaker, Equilibrium Conditions for the Floating of Multiple Interfacial Objects, J. Fluid Mech., 549, 215-224, 2005.

[5] G. Lagubeau, Interfaces à grains, et autres situations de mouillage nul, PhD Thesis, 2010 (https://www.pmmh.espci.fr/fr/gouttes/Equipe_files/TheseGuillaume.pdf