Stabilized emulsions take longer time reach equilibrium

Stabilized emulsions take longer time to reach equilibrium

1:53 AM, 12th December 2011
Stabilized emulsions take longer time to reach equilibrium
Researchers at Harvard have discovered that the properties of emulsions cannot be characterized simply by Young's law. Image courtesy of Flickr user Daniel Kulinski

CAMBRIDGE, US: By studying the behavior of tiny particles at an interface between oil and water, researchers at Harvard have discovered that stabilized emulsions may take longer to reach equilibrium than previously thought.

"We were looking at what we thought would be a very simple phenomenon, and we found something very strange," said Vinothan Manoharan, Associate Professor, Chemical Engineering and Physics, Harvard School of Engineering and Applied Sciences (SEAS).

"We knew that the particle would stick to the interface, and other researchers had assumed this event happened instantaneously," said Manoharan. "We actually found that the timescale for this process was months to years."

An emulsion is a mixture of two or more insoluble liquids, usually oil and water. A simple emulsion takes energy to create (for example, by shaking it), and over time it will separate out.

When the oil and water in these types of emulsions are completely mixed and stable, the particles are said to be at equilibrium.

“There are certain rules for making different types of emulsions,” explained Manoharan. To study Pickering emulsions, Manoharan and his colleagues used holography to gain a three-dimensional view of microscopic polystyrene balls while they approached an interface between oil and water. The researchers used light from a focused laser to gently push a particle toward the interface, hoping to watch it settle into its predicted equilibrium point, straddling the oil-water boundary.

To their surprise, none of the particles reached equilibrium during the experimental timeframe. Instead, they breached the interface quickly, but then slowed down more and more as they crossed into the oil. Mathematically extrapolating the logarithmic behavior they did observe, Manoharan’s team discovered that the particles would stabilize on a time frame much longer than anyone had predicted.

“Our experiments only went on for a few minutes, but for the system to reach equilibrium would take at least weeks to months, and possibly years,” explained lead author David Kaz, Ph.D, Harvard’s Graduate School of Arts and Sciences.

In biomedical engineering, Pickering emulsions are used to create colloidosomes- microscale capsules that could deliver precise concentrations of drugs to specific targets in the human body. Understanding the behavior of particles at liquid interfaces is also relevant to many aspects of chemical engineering, water purification, mineral recovery techniques, and the manufacture of nanostructured materials.

The new research suggests that the models currently used to predict and optimize these systems may be too simplistic.

“It has always been assumed that the particles moved almost instantly to their equilibrium contact angle or height, and then Young’s law would apply,” said co-author Michael Brenner, Professor of Applied Mathematics and Applied Physics at SEAS. “What we found, though, is that equilibrium might take much, much longer to achieve than the time scale at which you’re using your product.”

“If you’re really stirring hard, maybe you can get the particles to reach equilibrium faster,” Brenner adds, “But what we’re saying is that the process matters.”

 

© Harvard School of Engineering and Applied Sciences News

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