INTERDROPLET HETEROGENEOUS NUCLEATION OF SUPERCOOLED LIQUID DROPLETS BY SOLID DROPLETS IN OIL-IN-WATER EMULSIONS

The crystallization of oil droplets in rt-hexadecane oil-in-water emulsions was monitored by pulsed nuclear magnetic resonance (NMR). The emulsions initially contained an equal mixture of solid droplets and supercooled liquid droplets at either 6 or 8 degrees C. The degree of crystallization in the droplets was determined by measuring the NMR signal 30 mu s after a 90 degrees radio frequency pulse was applied to the sample. The signal from the solid droplets decayed rapidly after the radio frequency pulse, allowing the measured signal to be related to the fraction of liquid droplets. No crystallization was observed in a sample that contained only supercooled liquid droplets, but crystallization was observed when solid droplets of the same material were present. This indicated that crystallization was induced in the liquid droplets by the solid droplets and was most likely caused by interactions between solid and liquid droplets - that is, by interdroplet heterogeneous nucleation. The rate of induced crystallization decreased as the viscosity of the continuous phase was increased and the size of the droplets was increased, but was independent of droplet concentration (20-40%).