DROPLET BREAKUP MECHANISMS DURING EMULSIFICATION IN COLLOID MILLS AT HIGH DISPERSED PHASE VOLUME FRACTION

In this article the time scale of the emulsification process of concen trated emulsions in colloid mills is considered. The breakup process w as modelled using a model based on a population balance equation. The shear-thinning character of the concentrated emulsion was incorporated in the model. Two possible breakup mechanisms are considered: one in which droplets are divided a number of times into two fragments, until this cascade of 'binary' breakup events leads to droplets of a size s o small that further breakup cannot occur under the prevailing hydrody namic conditions;a second mechanism in which droplets are stretched in to long filaments that break up by capillary waves into a much greater number of fragments during one breakup event. Only one or a few steps are then necessary to obtain a sub-critical size. The comparison of e xperimental and calculated mean droplet sizes showed the importance of the capillary mechanism. The binary breakup model overestimates the t ime scale of the emulsification process. It was seen that at high flow rates the binary model led to overestimated droplet sizes, whereas th e capillary model agreed with experimental observations.