Entrapment of air during imbibition of agglomerated powder beds

Abstract Complete wetting is crucial for efficient recombination of powders. On the powder bed scale, wetting is governed solely by capillary forces and the resistance to flow, i.e. spontaneous imbibition. Slow or incomplete imbibition of the powder bed may lead to gelling of the liquid front, which will stop the recombination process and cause the formation of lumps, which is usually undesirable. In this study, the spontaneous imbibition of powder beds consisting of spray-dried dairy powders with diverse morphologies has been investigated. Uniform radial spreading of the imbibition front was seen in all the beds, but a large amount of air was trapped in the inter-particle free space in the imbibed volume. A positive correlation was found between bed porosity and the fraction of air trapped in the bed after imbibition. Since the amount of trapped air was calculated as a fraction of the porosity of the dry bed, this relation was unexpected. The large fraction of air trapped in the bed and the uniform radial spreading of the imbibition front indicate considerable heterogeneity of bed on the microscopic scale, but a homogenous structure on the macroscopic scale. Possible explanations of the large fraction of trapped air are the presence of local dead-end structures in the bed and film flow of the imbibing liquid.