Magnetite nanoparticles program the assembly, response, and reconfiguration of structured emulsions.

Endoskeletal droplets-non-spherical emulsion droplets that respond to external stimuli with shape change-are modified with ferromagnetic iron oxide nanoparticles to make them susceptible to magnetic fields. The resulting droplets can be manipulated using static or oscillating magnetic fields, each producing a different response. Static fields control the orientation and position of the droplets, important in driving assembly into organized structures. Oscillating fields are shown to cause magnetic hyperthermia in ferrofluid nanoparticles, leading to droplet heating and forcing droplet reconfiguration. We demonstrate the use of static and dynamic fields to affect the organization and stability of endoskeletal droplets and their assemblies, producing highly-tunable programmable colloids that adapt to changing environmental conditions.

Model of Structured Emulsion Droplet Stability and Reconfigurability.

A model of internally structured emulsion droplets is presented that accounts for the traction forces generated by interfacial tension and the von Mises yield criterion of the internal supporting network. For symmetric droplets, the method calculates the total stress acting on a droplet locally, allowing droplet stability and location of failure to be predicted. It is not regions of high interfacial curvature that prompt droplet reconfiguration, rather regions transitioning from high to low curvature. The model enables the design of emulsion droplet response and reconfigurability to external triggers such as changes in surface tension (surfactant concentration) and temperature.

Crystallization Kinetics of Partially Crystalline Emulsion Droplets in a Microfluidic Device.

We measure the crystallization kinetics of petrolatum-hexadecane emulsion droplets as they are produced in a microfluidic device. After droplets form, they are cooled, causing an interior network of wax crystallites to grow. Polarized light microscopy is used to quantify the droplet crystallinity as a function of residence time in the device. Two wavelengths and two polarization orientations are used to decouple the wavelength dependence of the optical retardation, the crystallite orientation, and the crystallite number density. The droplet crystallinity follows the Avrami kinetic model with parameter values in agreement with the theoretically expected values. These results provide a means to engineer the crystallization kinetics, stability, and arrested coalescence of partially crystalline emulsion droplets.