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Methods & Processes
New Perspectives in Emulsion Formation
By: Katie Anderson (Schaefer), Cosmetics & Toiletries magazine
Posted: January 6, 2012, from the February 2012 issue of Cosmetics & Toiletries.
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The researchers attributed the large time scale for the particle to reach equilibrium to its surface chemistry. “Any solid particle will have some small rough patches or chemical groups on the surface where the contact line gets snagged as it moves down the particle.” The team utilized different polystyrene spheres stabilized by different chemical groups, such as sulfate or carboxyl groups, on the surface. “We found that the rate of relaxation is different depending on what group was on the surface. The slow relaxation seems to be controlled with the surface groups, but in all cases it fits this general model,” added Manoharan. The team therefore hypothesized that the relaxation rate slows with an increase in particle roughness. The team also believes that the time scale can be reduced with more vigorous agitation.
The researchers knew that at equilibrium, the particle should sit between the o/w interface at some particular height relative to the interface, with that height defining a contact angle. They expected the contact angle to be larger than 90 degrees, meaning that it would sit more in the oil phase than in the water phase. “What we found was that the height of the particle [i.e., the contact angle] was quite a bit smaller than we expected,” said Manoharan. Then when the researchers looked at the trajectory after the particle breached the interface, they found that the height increased logarithmically with time. “We found that it can take a long time to reach that equilibrium contact angle, and that angle is always changing. “What it means, I think, is that if you are trying to get a certain type of Pickering emulsion (o/w, w/o), it may depend not only on the equilibrium contact angle but also on the time allowed for the system to reach equilibrium and on the force pushing on the particle to get it to the interface.”
The team has not yet shown any direct implications of this work for Pickering emulsions, as the research was conducted on a plenar oil/water interface, although they may be revealed in future work. “We want to use the holographic technique to see if Pickering emulsions are slowly relaxing at the interface of the emulsion droplet,” added Manoharan. The team is also interested in working with other colloids used in other applications, such as the industrial industry. “We are particularly interested in rough particles because many of the colloidal particles in industrial products are not smooth. If our model is correct, then the effect should be more pronounced with the rougher particle,” noted Manoharan. The team has also been talking with a few different industries about the practical applications of this work; it is excited to develop this research further.