Watching emulsions freeze

As a liquid turns to a solid, getting a picture of events at the interface between what’s already solid and what’s still liquid can be difficult. Sylvain Deville and Dmytro Dedovets at the Ceramic Synthesis & Functionalization Laboratory and Cécile Monteux of ESPCI Paris report an imaging approach based on confocal fluorescence microscopy that allows them to watch an emulsion of oil droplets in water as it freezes (Science 2018, DOI: 10.1126/science.aar4503). They use a temperature gradient to control where and how quickly the emulsion freezes and a flow cell to keep the interface within the focal plane of the microscope. Surfactants in the mixture stabilize the oil droplets and the images show that the surfactants help determine the microstructure of the resulting solid. The surfactants induce long-range interactions between the solidifying surface and the droplets that cause the droplets to be redistributed as the emulsion freezes. This redistribution—whether, for instance, the droplets form clumps or are spread out evenly—is what determines the solid’s microstructure. The imaging approach should be useful in many applications, including metal alloy formation, single-crystal growth, and food engineering. Deville plans to use the approach to study problems in cryopreservation of biological cells.