NSF-BSF: Controlling Droplet Shape and Liquid-Crystal Order via Interfacial Freezing

Many common materials, including foods, medicines, cosmetics, and biological fluids, contain small liquid droplets suspended in another liquid. Although these droplets may appear simple, their surfaces can sometimes organize molecules into very thin ordered layers. When that happens, droplets can change from smooth spheres into shapes with flat faces, sharp edges, or thin tails. Liquid crystals are another class of soft materials, composed of rod-shaped molecules that align with one another and respond to nearby surfaces. This NSF-BSF project will study what happens when these two types of soft matter are brought together. The research will determine how shape-changing droplets steer the internal organization of liquid crystals and whether liquid crystals can, in turn, influence how the droplets change shape. Understanding this two-way interaction will advance basic understanding of how molecular order, surface structure, and shape interact in soft materials. The results could lead to new principles for designing materials whose optical, mechanical, or transport properties can be adjusted by small changes in temperature or other external signals. Potential long-term benefits include adaptive optical components, controlled-release systems, and responsive sensors. The project will also train students through an international collaboration between researchers in the United States and Israel and will support public outreach activities that use visually engaging shape-changing droplets to introduce soft-matter science to broad audiences. This award will establish fundamental principles governing the coupling between interfacial freezing, droplet shape transformations, and liquid-crystal ordering in structured multiphase systems. The central hypothesis is that interfacial freezing creates crystalline boundary conditions that organize liquid-crystal orientation and defect structures, while liquid-crystal elasticity can feed back to modify interfacial freezing behavior, faceting thresholds, and symmetry selection. The work will pursue three objectives. First, the project will determine how interfacial freezing modifies liquid-crystal anchoring at spherical crystalline interfaces by comparing liquid crystals within and outside interfacially-frozen droplets. This comparison will isolate the roles of curvature and interfacial chemistry. Second, the project will establish how dynamically tunable polyhedral droplet shapes prescribe liquid-crystal orientation and defect arrangements under conditions where surface elasticity dominates bulk liquid-crystal elasticity. Third, the project will test whether liquid-crystal order feeds back onto the droplet interface by altering interfacial free energy, freezing transitions, and faceting pathways. The research will combine temperature-controlled emulsions, microfluidic droplet preparation, polarization microscopy, confocal microscopy, fluorescence imaging of interfacial defects, interfacial tension measurements, and continuum elastic modeling. Together, these studies will provide a mechanistic understanding of how interfacial crystallinity, geometry, topology, and molecular alignment interact in multiphase soft materials. 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.