Colloids and interfaces

Using digital holographic microscopy, we study how colloidal particles bind to oil-water interfaces. Small particles have a natural affinity for interfaces, and this affinity can be used to control their self-assembly and make some interesting materials. However, the dynamics of the particles before and after they breach the interface are not well-understood. We do experiments to observe these dynamics and find some surprising results.

If you shake up a mixture of oil and vinegar, you will disperse droplets of one into the other only very briefly before the vinegar and oil separate into distinct layers. Colloidal particles can bind to the interface to reduce the total oil-water surface area (and energy). Once these particles bind to the interface it's virtually impossible for them to detach. This means the interface can be used as a scaffold for assembling the particles, or the particles can be used to coat the interface and stabilize it, forming what is called a "Pickering emulsion".

The affinity of colloidal particles for liquid interfaces has been studied for more than a century. In 1903, W. Ramsden published a paper about solids accumulating at the interface between oil and water. Since then, there have been thousands of studies on micron-scale particles at immiscible fluid-fluid interfaces, but the dynamics of such systems are still quite poorly understood.

We study the non-equilibrium behavior of these systems using  digital holographic microscopy, which can track the particles in three dimensions at high speed and with high spatial precision. Dave Kaz and Ryan McGorty, the first students to work on this project, built a holographic microscope outfitted with an optical tweezer that could be used to exert a force on the particles. They pushed particles toward a planar water-oil interface and found something surprising happened when the particles started poking through the interface. 

Once the particles breach the interface, they relax logarithmically in time towards equilibrium. The velocity of the particles gets smaller and smaller the closer they get to equilibrium, despite the huge driving force. In fact, if we extrapolate our results, we find that a 1-micrometer particle could take months or even years to reach equilibrium. The explanation most consistent with this observation, proposed by Madhav Mani (working with Michael Brenner), is that nanoscale surface features on the particles pin the three-phase contact line and hinder the progress of the particle toward equilibrium. We have since seen this behavior in a wide variety of different colloidal particles.