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.
Surprise at the Interface
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.