This study considers the consequences of adding grains to an air−liquid interface from a funnel. Depending on the grain contact angle and liquid surface tension, the interface is found to support a single or multiple layers of grains, forming a granular stack. By continuing to add grains, the stacks grow until either the lower grains disperse in the liquid, or the complete stack breaks free from the surface and sinks as a dry powder lump. Herein, the effects of grain contact angle, density, and size on these processes are studied experimentally, and a theoretical analysis is given. The maximum number of grains contained in a floating stack and its critical depth are observed to increase as the grain size decreases. The maximum number of grains scales with the bond number (Bo) as Bo−1.82 when stack detachment is observed and with an exponent −2.0 when grains disperse into the liquid. As a result of these different scaling exponents, a critical bond number above which grains wet and disperse can be identified. Favorable conditions for dispersion are achieved with larger grains and, to a lesser extent, by lower surface tension and contact angle. The critical bond number separating grain dispersion from lump formation increases with an increasing grain contact angle, thus providing a physical justification for increasing grain size with common processes such as granulation or agglomeration. Conversely, a quantitative framework to interpret the limitations in dispersing small grains is proposed, justifying the need for low contact angle or liquids with low surface tensions, both favored by the use of surfactants. The present findings have identified conditions under which lump formation occurs, and hence how these undesired phenomena can be avoided in applications requiring the efficient dispersion of grains across a liquid interface.
Poor dispersion of beverages from dehydrated powders is a defect that obstructs the development of novel dehydrated food powders. We investigate how the physico-chemical properties of the powder grains affect the dispersion process. We used insoluble and non-cohesive grains to form islands of powders at a static water surface. Liquid “wicking” can avoid the formation of lumps, lead to complete sinking of powder grains when the grain contact angle is below a critical value in the range 51°-77°. The effect of grain size on powder sinking, or on the depth of island formed, was experimentally studied to understand the impact of particle size enlargement on powder dispersion. The interplay of grain contact angle and size was also quantitatively demonstrated. We also report the conditions leading to the detachment of a powder island from the interface, forming a powder lump that is wet outside and dry inside. Importantly, introducing flow in the liquid by agitation does not necessarily improve the dispersion process.
This study aims at understanding the interplay between the interfacial properties of the powder grains and the characteristics of the liquid flow used to disperse them, in order to obtain an effective dispersion of a powder in a liquid, avoiding air entrainment. The dispersion of grain “rafts” and powder islands “stacks” was investigated both on a static and on a moving air-liquid interface. Powder wicking prevents the formation of a powder island when the grain contact angle is below a critical contact angle. Above the critical contact angle, a powder island forms and grows to a critical depth that depends on grain radius and contact angle. Imposing a flow on the air-liquid interface can either promote water impregnation, reducing the depth of the powder island or destabilise the whole island. In the latter case, the island sinks, forming a heterogeneous powder structure that is wet outside and dry inside.