Distribution nucleation: Quantifying liquid distribution on the particle surface using the dimensionless particle coating number

Control of nucleation in wet granulation often determines the size and structure of the final granules. After a decade of research, our knowledge of “immersion nucleation”, where a large droplet engulfs fine particles, is relatively well established. However, far less is known about the second mechanism called “distribution nucleation”, where the powder particles are gradually coated by a layer of small droplets. In this paper, the similarities between particle coating and granulation were used to define the five steps of distribution nucleation. A Bernoulli model was developed to describe the fractional surface coating F, and a new dimensionless parameter, the particle coating number Φₚ, was defined as the ratio of the theoretical area coated by the drops, assuming no overlap, to the total surface area of the particle. The particle coating number was experimentally validated by adding drops randomly over the surface of a particle and measuring the fractional surface coating using image analysis. Standard ping pong balls (40mm diameter) plus four sizes of foam balls ranging from 20 to 50mm diameter were coated with either small or large drops of melted PEG1000. The results demonstrated that the particle coating number Φₚ can be used to predict the fractional surface coverage F using simple, known parameters (without any fitting coefficients), and was able to account for differences in particle size and drop size. Deviations in the experimental data from the theoretical predictions were observed, and attributed to drop spreading and merging and heat conduction into the particle, which all affect the experimental drop footprint area. When the effective footprint area was used as a fitting parameter all the experimental data collapsed onto a single line, demonstrating that the particle coating number captures the key coating coverage behaviour. The particle coating number can also be used to predict the effect of changing particle size, surface area, liquid level, or drop size on the coating fraction, which is in turn known to be linked to granulation kinetics. The particle coating number opens up new options for granulation process control, and is expected to be valuable in a variety of particle wetting and coating applications where small drops are distributed over larger particles.