NSF-BSF: Interfacial freezing and shape transformations in surfactant/particle-co-stabilized emulsions

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Emulsions are made of droplets of one liquid suspended in another, immiscible, liquid such as oil droplets suspended in water. Liquid droplets adopt spherical shapes due to surface tension. It was recently discovered that, for over 60 different chemical combinations, the surface of droplets, which is covered with molecules called surfactants, can undergo freezing, which induces drastic changes in the shape of the droplets. This interfacial freezing (IF) phenomenon leads to the formation of faceted liquid droplets, thin platelets or rod/hair-like emulsion droplets. On one hand, these shape transformations may lead to major complications in industrial processes, such as pipe clogging by hair-like droplets. On the other hand, these shape transformations provide a powerful method of synthesizing particles with complex shapes for advanced applications. In numerous industrial and natural settings, the interfaces of emulsion droplets are covered by mixtures of surfantants and solid microparticles, known as colloids. However, the influence of surface-adsorbing colloids on the IF and droplet faceting phenomena has not been extensively investigated, although such colloids are present in many real-world emulsions. The proposed work aims to understand the effect of colloids on the shape transformation of emulsion droplets undergoing IF. Developing a deep understanding of the effect of particle size, concentration, shape and surface chemistry on the shape transformation of emulsions will enable strategies to prevent deleterious impacts of such phenomena in the food, oil and gas, pharmaceutical and cosmetic industries and lead to novel synthesis techniques to create new materials.The composition, structure and elasticity of the droplet interface play a crucial role in determining the stability, encapsulation capability and processability of emulsions in industry and in common chemical and biological systems. In many emulsions, the interfacial molecular layer can undergo a freezing transition, dramatically modifying the emulsion properties. This interfacial freezing (IF) transition drastically changes the shape of the emulsion droplets for sizes spanning 13 orders of magnitude in volume, and for over 60 different oil-surfactant combinations. Such shape transitions may lead to the formation of faceted liquid objects, high aspect ratio platelets or rod/hair-like emulsion droplets, changing the flow properties of the emulsions and potentially causing gelation leading to process complications. Engineering shape transformations of droplets also provides a powerful method of synthesizing highly shape-anisotropic particles with unique functionality. In numerous industrial and natural settings, the interfaces of the emulsions are decorated by mixtures of particles and surfactants, with the particles either added intentionally for emulsion stabilization, or being present as a contaminant. This project aims at understanding the interplay between the surface-adsorbed particles and the IF phenomena, with a particular focus on the effect of isotropic and Janus particles on the shape transformations. Using particle tracking and recently developed microfluidic methods, this project will investigate the effect of wetting properties, size, shape and interfacial concentration of particles on IF-driven shape transformations of droplets. While conventional isotropic colloids may be easily expelled from a droplet interface undergoing IF due to extremely low interfacial tension, Janus particles will strongly adsorb to such an interface due to their intrinsic surface activity, potentially enabling control over the shape transformation. New methods to control the IF-driven self-shaping of droplets will have a potentially transformative impact for oil and gas, pharmaceutical, food, agricultural and cosmetics industries, where emulsions are frequently exposed to surfactant- and particle-containing media. A new demonstration illustrating the interplay between the interfacial curvature and crystallization will be developed by undergraduate and graduate students for local high school teachers and students. To broaden participation of students from underrepresented groups, students with diverse backgrounds will be recruited by hosting students from the University of Puerto Rico-Humacao and by collaborating with Advancing Women in Engineering and Louise-Stoke Alliance for Minority Participation programs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.