Chemical Partitioning and the Role of Asymmetry in the Design of Droplet Microswimmers
Open Access
- Author:
- Meredith, Caleb
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 26, 2022
- Committee Members:
- John Mauro, Major Field Member
Darrell Velegol, Outside Unit & Field Member
John Mauro, Program Head/Chair
Lauren Zarzar, Major Field Member
Ayusman Sen, Chair & Dissertation Advisor - Keywords:
- droplets
active matter
solubilization
emulsions
active droplets - Abstract:
- Solubilizing, self-propelling droplets have emerged as a rich chemical platform for exploring the properties of active matter, but isotropic droplets rely on spontaneous symmetry breaking to sustain motion. In this dissertation, I explore alternative methods for introducing asymmetry leading to the propulsion of droplets without requiring spontaneous symmetry breaking and establish chemical design principles that may be used to extend similar active behaviors to many solubilizing emulsion systems. In contrast to active materials composed of solid colloidal particles, my findings suggest the unique capability of liquids to dynamically repartition chemical components between different microscopic fluid phases unlocks additional pathways for constructing and controlling active matter. First, chemotactic signaling between microscale oil droplets of different chemistries in micellar surfactant solutions is used to produce asymmetric solute gradients leading to predator-prey-like nonreciprocal chasing interactions. The interactions and dynamic self-organization result from the net directional micelle-mediated transport of oil between emulsion droplets of differing composition and are powered by the free energy of oil mixing. Elucidation of the chemical design rules for tuning the interactions between droplets is studied systematically by varying oil and surfactant chemical structure and concentration. Through integration of experiment and simulation, the active behavior and dynamic reorganization of multi-droplet clusters is also investigated. Second, the introduction of permanent asymmetry in the form of biphasic Janus oil droplets is used to bring about propulsion under conditions of solubilization. Through an investigation of how active behavior is influenced by the degree of oil mixing, droplet shape, and oil solubilization rates for a range of oil combinations, a chemomechanical framework underlying the propulsion of droplets is uncovered. In addition, spatiotemporal control over droplet swimming speed and orientation is demonstrated through the application of thermal gradients applied via laser directed joule heating. The interactions between collections of Janus droplets, including the spontaneous formation of rotating multi-droplet clusters is also explored. Finally, a perspective of future directions for research in the field of active droplets is presented.