(571j) Measurement of Particle Adsorption to and Diffusion on an Oil/Water Interface Using Bessel Beam Microscopy

Recent studies of particles adsorbed to an oil/water interface have found a range of interesting behaviors including slow relaxation times to reach equilibrium positions, variability in contact angles, and non-uniform inter-particle forces. These behaviors impact interfacial particle interactions, which regulate particle laden interface’s interfacial mesostructure and rheology.  In turn, these changes impact the bulk stability and rheology of particle stabilized Pickering emulsions.  Hence, understanding the particle behavior on an interface has important repercussions.

To characterize the behavior of particles at an oil/water interface requires resolution of particle location to the 10’s of nanometers.  This necessary resolution can be difficult to achieve for anything more than a single particle, which makes it difficult to acquire statistically relevant data sets. Using a new technique known as Bessel Beam Microscopy, we are able identify the location of hundreds of particles in a single image with resolutions down to 65 nanometers normal to the interface and 20 nm in the plane of the interface.  When this system is used with a high speed digital camera, highly accurate, 3D particle time traces can be made. Using this technique, we have analyzed the motion of a large and statistically significant number of particles adsorbing to and diffusing on an oil/water interface.

Using the motion of the particles before they adsorb to the interface has allowed us to evaluate whether particle diffusion normal to the interface is affected by the interface. Preliminary results suggest that the particles do not feel the presence of the interface until adsorbed.  As particles adsorb, their positions are tracked, and compared to recent results measured using different methodologies. We find similar long time scales for relaxation as previous work, but also find particle motion normal to the interface continues to oscillate about the slowly evolving mean position. The normal diffusion of particles is restricted by the interface; however, we do see significant oscillation, indicating that although particles are trapped, their position normal to the interface varies in a non-negligible manner with time. We do not observe significant deviation of in plane interfacial diffusion in comparison to the bulk behavior of particles. Using time averaged position of adsorbed particles, we find the distribution of contact angles based on particle properties not diffusion; a large contact angle distribution is observed despite particle monodispersity.

Previous examination of these behaviors have been unable to capture a similar number of particles in a single measurement as reported or been capable of decoupling particle motion effects from particle properties.  Our results indicate that particle position on an interface varies due both to normal diffusion and significant variation in particle properties of monodisperse particle populations.  These results have significant impact on the understanding of inter-particle interaction and the mesostructure developed by particle laden interfaces.