Active motion of nematic oil emulsions in a nematic aqueous environment

This paper was a final report summarizing my research through a summer-long REU program at the University of Wisconsin-Madison, supported jointly by the University of Puerto Rico-Mayagüez.

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Abstract

In this study, we explored the behavior of an uncommon emulsion wherein both the oil and water phase are liquid crystalline. 4-Cyan-4’-pentylbiphenyl (5CB), which is in a nematic phase at room temperature, is used as the oil phase. This oil phase is dispersed in an aqueous solution of disodium cromoglycate (DSCG) at weight fractions that lead to its formation of nematic phase at room temperature. The aqueous phase also contains the surfactant sodium dodecyl sulfate (SDS) at a concentration above its critical micellar concentration (CMC). The limited but finite solubility of the oil in the micelles leads to the dissolution of the oil from the droplets into micelles. This results in a disruption of the surfactant distribution throughout the surface of the emulsion, generating a Marangoni flow, which propels the droplet. Surprisingly, we observed that the motion of the 5CB droplets occurs significantly more on the axis perpendicular to the DSCG director. Additionally, there was an inverse relationship between strength of the DSCG alignment and the magnitude of the active motion of the 5CB droplets. Both these trends could potentially serve as mechanisms for controlling the motion of oil-in-water emulsions for uses in medicine or environmental remediation.

An image of the liquid crystal emulsions (golden spheres). The nematic aqueous solution in which the emulsions swim appear as a flurry of rainbow streaks since the entire environment is viewed through a light polarizer.

See the oil emulsions in motion! They almost look alive, but I can guarantee they are just balls of oil swimming in water. The emulsions are tugging along blobs of a surfactant (basically a soap) as they go. I called these “knapsacks”. I noticed this phenomenon near the end of my experimentation, so was unable to investigate the utility of these “knapsacks” in detail, but I imagine this could be a way to transport drugs if we could figure out how to control the direction of the oil’s swimming.