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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.
Emulsions are one of the most popular delivery systems employed in the manufacture of cosmetics. The ability to create o/w, w/o or even w/si emulsions is an integral part of the cosmetic formulator’s skill set. Previously, the theory explaining emulsion behavior was based on the equilibrium contact angle of the particle at the interface; however, Vinothan N. Manoharan, PhD, and his team at Harvard believe the time allowed for the system to reach equilibrium and the force pushing the particle to the interface are equally as important. Manoharan finds that this research, which captures the dynamics of how a particle reaches equilibrium, is not only unique, but also may affect how emulsions are manufactured.
Capturing the Emulsion
The team first set out to study Pickering emulsions, i.e., those stabilized by solvent particles rather than surfactants. “In searching for ways to make new materials, we were interested in what happens to particles as they self-assemble at that interface between oil and water,” said Manoharan. He noted that interactions between solid particles and liquid interfaces were not well understood, so his team looked at those interactions more quantitatively using optical techniques developed in its lab. “We started looking at oil droplets having some micron-sized solid particles on them and trying to image their structure and movement …but the interactions were not at all what we expected, so we decided to look at an even simpler system.”
The team instead investigated what happens when a single, micron-sized, spherical colloidal particle approaches and binds to an o/w interface. To study this, the bottom of a trough was filled with a water/glycerol mixture. Manoharan explained, “The water phase contained a little bit of glycerol, which was incorporated to alter the refractive index so that we did not get any reflections off the o/w interface.” It then layered an oil and alkane decane on top of the water phase. “We chose a standard decane because it is easy to get, pure and it is pretty chemically inert,” Manoharan explained. The chosen particle, poly- styrene microspheres, then sat in the water phase below the interface. “Polystyrene microspheres can be easily purchased at various sizes with various surface groups. They are what we call a model colloid,” added Manoharan.
Two different laser-based methods were then used to capture the emulsion’s behavior. The first, holographic microscopy, was developed in Manoharan’s lab. “In this microscopy, we shine a laser at the colloidal particle, and the particle will scatter or defract the light from that laser. We put a camera somewhere downstream and what we image is the interference pattern between the light scattered or defracted from the particle and the light that passes through,” he said. That interference pattern is called a hologram, and it contains information about the 3-D locations of the particle. Manoharan furthered, “We can process that hologram, which is a single picture, to obtain the location of the particle in all three dimensions-in particular, to find out where it is relative to the interface.”
However, for the particle to reach the interface, another laser called an optical tweezer was required. While the optical tweezer moved the particle, the team took holograms of it to measure its position as a function of time. As expected, the particle slowed as it approached the interface due to a larger drag as the gap for the fluid narrowed at the interface. However, the team was surprised by the rapid increase and slowing down immediately following the initial slowing. “This is the interesting part of the experiment because we observed the first moment when the particle breached the interface,” noted Manoharan. The team also found that once the particle breaches the interface, a long time elapses before it reaches equilibrium.