The impact of liquid drops on powder bed surfaces

The phenomena of liquid drop impacts on powder bed surfaces plays an important role in wet granulation processes. The liquid binder is sprayed onto the tumbling powder mass where it forms nuclei by binding particles together by a combination of viscous and capillary forces. In many processes, there is a strong correlation between droplet size and granule nuclei size. If the liquid droplets break-up during impact, then this will increase the number and decrease the size of granule nuclei produced. If drops spread across the surface before sinking in, then this will increase the surface coverage and incidence of caking and nuclei coalescence. Although liquid drop impacts on solid and liquid surfaces have been studied extensively, there is little work concerning drop impacts on powder surfaces. This paper reports the results of preliminary experiments studying the behaviour of liquid drops impacting a static powder bed. Water, methanol and silicone oil were used in order to vary liquid viscosity and surface tension. The powder studied was glass ballotini with sizes ranging from 60 to 850 mum. The amount of drop surface spreading increased with impact velocity, and there was a critical drop impact Webber number (Wecrit) required for drop breakup in the order of 1000. This critical Webber number appeared to increase with increasing particle size. Drop impact behaviour displayed behaviour with analogies to impacts with both solid and liquid surfaces, depending on whether or not the liquid spread across the surface before a significant amount had soaked in. It was concluded that drop break-up was unlikely to occur under most practical granulation conditions.