Contact‐mediated nucleation in melt emulsions investigated by rheo‐nuclear magnetic resonance

Increasing the efficiency of disperse phase crystallization is of great interest for melt emulsion production as the fraction of solidified droplets determines product quality and stability. Nucleation events must appear within every single one of the μm‐sized droplets for solidification. Therefore, primary crystallization requires high subcooling and is, thus, time and energy consuming. Contact‐mediated nucleation is a mechanism for intensifying the crystallization process. It is defined as the successful nucleation of a subcooled liquid droplet induced by contact with an already crystallized droplet. We investigated contact‐mediated nucleation under shear flow conditions up to shear rates of 457 s−1 for a quantitative assessment of this mechanism. Rheo‐nuclear magnetic resonance was successfully used for the time‐resolved determination of the solids fraction of the dispersed phase of melt emulsions upon contact‐mediated nucleation events. The measurements were carried out in a dedicated Taylor–Couette cell. The efficiency of contact‐mediated nucleation λsec decreased with increasing shear rate, whereas the effective second order kinetic constant kcoll,eff increased approximately linearly at small shear rates and showed a linear decrease for shear rates higher than about 200 s−1. These findings are in accordance with coalescence theory. Thus, the nucleation rate is optimal at specific flow conditions. There are limitations for successful inoculation at a low shear rate because of rare contact events and at a high shear rate due to too short contact time. Contact‐mediated nucleation of a model melt emulsion was investigated by Rheo‐NMR. Geometry and flow fields were measured by 1H‐MRI, crystallization was monitored by NMR. The impact of contact time and the number of possible collisions is pointed out very clearly and for the first time within one geometry. Up to a specific shear rate, an increasing solid fraction of the dispersed phase was measured.