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Researchers at Harvard's School of Engineering and Applied Sciences (SEAS) have discovered that stabilized emulsions may take months to years to reach equilibrium. This research has important implications for the manufacturing processes used in pharmaceuticals, cosmetics and foods, among other chemical industries.
In "Physical ageing of the contact line on colloidal particles at liquid interfaces," which appeared on Dec. 4, 2011, in Nature Materials, the researchers studied Pickering emulsions using holography to observe a 3D view of microscopic polystyrene balls while they approached an interface between oil and water.
The researchers used light from a focused laser (optical tweezers) to gently push a particle toward the interface, hoping to watch it settle into its predicted equilibrium point, straddling the oil-water boundary. Contrary to previous research, which has suggested instantaneous equilibrium in emulsions, none of the particles reached equilibrium during the experimental time frame, which lasted a few minutes. Instead, they breached the interface quickly, but then slowed down more and more as they crossed into the oil. After mathematically extrapolating the logarithmic behavior, the researchers concluded that the process could take months to years to reach equilibrium.
This finding, according to the researchers, suggest that manufacturing emulsions may have more to do with time and the energy applied to the system rather than the conventional rules based on the properties of the materials. Therefore, the researchers believe that current models to predict and optimize these systems may be too simplistic.
Researchers on the project, which was funded by the National Science Foundation and the NSF-supported Materials Research Science and Engineering Center at Harvard, included: David Kaz, PhD; Ryan McGorty; Madhav Mani; Michael Brenner; and Vinothan N. Manoharan.