Suspending a liquid within a liquid, within a liquid… (Day 280)

When I read through scientific journals, the articles that grab my attention aren’t always the ones describing the most novel ideas. Sometimes it’s enough to just make something easier. That’s why today’s story appealed to me.

Many everyday products including medicines, beauty products and foodstuffs contain emulsions: liquids with tiny droplets of another liquid suspended within them (see my blog ‘food, glorious, food…emulsions‘).  A classic example that we all can create at home is vinaigrette (salad dressing), which is an emulsion of oil and vinegar.

MIT researchers designed these complex emulsions to change their configuration in response to stimuli, such as light, or the addition of a chemical surfactant.

Photo Credit | Christine Daniloff\MIT
MIT researchers designed these complex emulsions to change their configuration in response to stimuli, such as light, or the addition of a chemical surfactant.

Vinaigrette is a straightforward two component mixture. However, things get far more interesting when you suspend a liquid within a liquid, within a liquid. These complex emulsions (in this case a double emulsion) can be tailored for use in specific applications.

A team of chemists and chemical engineers from Massachusetts Institute of Technology (MIT), US, have found a way to simplify the process of creating complex emulsions. Their method offers possibilities for rapid production at scale.

This work was recently published in Nature under the title: Dynamically reconfigurable complex emulsions via tunable interfacial tensions – one of those impenetrable descriptions, but don’t let that put you off!

Complex emulsions offer a way of fine tuning the properties of the liquid droplets. Dr Lauren Zarzar, a chemist and lead author of the article said: “We believe that by having this precise and easy way of controlling the morphology of the complex emulsion, we may be able to tune the physical and chemical properties to use them to our advantage.”

Lauren worked with a team of chemists and chemical engineers to develop this work. With Vishnu Sresht (a chemical engineering graduate student), Lauren created this excellent little YouTube video in which they explain how they control their complex emulsions:

A complex emulsion can be made with a microfluidic device that squeezes bubbles of oil into droplets of water, which, in turn float in a stream of oil; but this only works well for small-scale production.

Lauren, Vishnu and the team set out to find a simple way to create larger volumes of this type of complex emulsion, with precise control over the composition of the resultant droplets. To do this they devised a two-step process.

The first step is dependent on combining two liquid hydrocarbons that  are only miscible above a certain temperature; in this case, hexane and perfluorohexane. When heated above 23ºC, the liquids will mix and form an emulsion when water is added. On cooling, the hexane and perfluorohexane inside each droplet separate and a complex emulsion is created.

The second step involves adding a mixture of surfactants, which alter the interfacial tension between the two hydrocarbons and the water. These surfactants engage in a tug of war, where one pulls on the perfluorohexane-water interface and another pulls on the hexane-water interface.

Vishnu said: “By playing with the relative quantities of these two surfactants, we were able to directly control the relative strengths of the two interfacial tensions.

“And the interplay between that, depending on which interfacial tension is larger and which is smaller, forces the droplet to take a specific configuration. This allows us to control which liquid is exposed and which is hidden inside the droplet.

“You can use these emulsions for delivery applications, clean up applications, anything where you need to protect something, shield something, or pick up and deliver something. It’s like a package that you can open and close at will.”

The use of complex emulsions offers tremendous possibilities. That’s why I find this research so exciting.  It’s also a great example of multidisciplinary collaboration.  Chemical engineering matters, but it’s the combination with chemistry that makes the difference here.


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