Structural programming: balancing external stress with internal structure in shape-responsive emulsions

Date
2018
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University of Delaware
Abstract
Responsive materials, such as shear-thickening fluids, change their properties when acted upon by an external stress or trigger. When these materials are incorporated into composites, it is possible to design new responses that are absent in their component parts. These composites are known as programmable materials and are unique in their ability to significantly change properties \textit{in situ}. In this dissertation, we study a programmable materials platform called endoskeletal droplets. These are emulsion droplets with an internal structure that allows them to hold non-spherical shapes. Critically, this internal structure is responsive, allowing it to resist the surface tension acting on the droplet interface, but allowing surface tension to dominate when triggered by an external stimulus. As a result, droplets change shape on command, reconfiguring from high surface area shapes to spheres when prompted by increased temperature or decreased surfactant concentration. These droplets are programmed by shaping their internal structure to a geometry with a predictable shape-change response. ☐ We present a microfluidic approach to generating these programmable emulsions continuously and implement a new technique for the optical measurement of crystallinity. Once generated, droplets are either released or coalesced into larger, three-dimensional superstructures. These structures are bicontinuous fluids similar to bijels and allow the programmability of endoskeletal droplets to extend to larger structures. We also demonstrate that endoskeletal droplets can respond to additional stimuli---magnetic fields---by introducing magnetite nanoparticles. The suitability of programmable emulsions in a deposition application is also studied, where shape change allows control over droplet deposition and retention to surfaces. ☐ The result is a multiple order of magnitude change in droplet binding affinity to a substrate, which suggests that shape and reconfigurability are highly useful parameters for the design of future programmable materials. Finally, we extend the application of endoskeletal droplets to aerosol systems, demonstrating that the principles behind shape retention and reconfiguration apply outside of emulsion systems. ☐ These results are unique examples of the fundamentals, production, and application of programmable materials. Endoskeletal droplets can serve as a template for developing new composites with similar behaviors or completely novel structures with yet unknown responses and applications.
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