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Reconfigurable Emulsions Might Find Use As Chemical Sensors

Surfactants tune the surface tension between liquids, cause droplets to flip inside out

by Celia Henry Arnaud
February 26, 2015

Droplets in an emulsion change morphology in response to a fluorosurfactant.
Credit: Nature
An emulsion of fluorohexane (gray) and hexane (red) in water changes morphology in response to a concentration gradient of a fluorosurfactant. The droplets on the right are hexane in fluorohexane, the ones in the middle are equally divided, and the ones on the left are fluorohexane in hexane.

By judicious selection of components, researchers can fabricate complex emulsions that can be reconfigured on demand. The technology, say its developers, could find use in developing high-precision chemical sensors.

An emulsion’s composition is usually fixed. But now researchers at Massachusetts Institute of Technology have developed a method that allows them to reconfigure droplets in complex emulsions (Nature 2015, DOI: 10.1038/nature14168). These so-called tunable emulsions would be sensitive to changing environmental conditions, the chemists say.

In the new method, Timothy M. Swager, Daniel Blankschtein, Lauren D. Zarzar, Vishnu Sresht, and coworkers harness the temperature-sensitive miscibility of hydrocarbons, fluorocarbons, and silicone to form their emulsions. In one example, hexane and fluorohexane don’t mix at temperatures below 23 °C, but with gentle heating, they do. The researchers then emulsify the mixture with water. The result is droplets of fluorohexane inside hexane dispersed in water.

But the initial morphology doesn’t have to be permanent. By incorporating surfactants in the system, the researchers can change the droplet morphology.

For example, in response to a fluorosurfactant, the droplets switch from fluorohexane-in-hexane to hexane-in-fluorohexane. The droplet composition flips because the surfactant changes the surface tension between the layers of the droplet. In the transition, the liquids become what is known as a Janus particle at the midpoint of the reversal.

“You can add surfactants to the composition when you’re making the particles, or you can add surfactant after the particles are made to dynamically change them,” Swager says.

By using surfactants that change their properties in response to the presence of certain molecules, the researchers believe they could use the emulsions as chemical sensors. “I think we can figure out how to get large morphology changes from very small perturbations,” Swager says. The morphology changes would be visible, providing easy readout, and because the compositions are controlled thermodynamically, such sensors should be quantitative.

Physicist David A. Weitz, an expert on designer emulsions at Harvard University, says the work “is a very elegant example of the control over the morphology of triple emulsions that can be achieved through the control of the surface tension between the fluids.” The morphology changes also could prove useful for the controlled release of materials encapsulated in the double emulsion, Weitz says.



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