Assessment of droplet self-shaping and crystallization during temperature fluctuations exceeding the melting temperature of the dispersed phase

Oil-in-water emulsions with crystalline dispersed phase are widely used formulations in life sciences. Ideally, during cooling each droplet crystallizes in its original size and shape set by chosen process parameters. Recently it was shown, that droplets may spontaneously break their shape symmetry crystallizing in non-spherical shape, which strongly depends on the surfactant molecular structure and cooling profile. In this contribution, we focus on the assessment of droplet self-shaping, crystallization and melting behavior during repeated cooling and heating cycles, and the influence of small changes in the temperature-time profile on these processes. These temperature cycles are typical conditions to which crystalline dispersions are subjected during storage and transport. Shape changes and crystallization of n-hexadecane droplets stabilized with the surfactants Brij (R) S20 and Tween (R) 60 were investigated by means of a thermo-optical method. In both model systems, self-shaping was observed during the first cooling cycle. Subsequent melting of particles led to the breakup of droplets into smaller, sub-micrometer droplets due to self-emulsification. When a second cooling step followed directly after melting at 20 and 22 degrees C, spontaneous self-shaping was again observed for n-hexadecane droplets stabilized with Brij (R) S20, while droplets stabilized with Tween (R) 60 crystallized in spherical shape. However, these samples showed self-shaping of droplets, when given longer times at 22 degrees C (incubation of 1 h) or being heated to the bulk Tween (R) 60 melting temperature (similar to 24 degrees C) before the second cooling step. During cooling of pure 1 wt% Tween (R)

Automated summary

This study focuses on the assessment of droplet self-shaping, crystallization, and melting behavior during repeated cooling and heating cycles of oil-in-water emulsions. The results show that the shape symmetry of droplets can be broken during crystallization, depending on the surfactant molecular structure and cooling profile.