Diffusing Colloidal Probes

Dennis Prieve introduced me to the concept of analyzing microscopy measurements of colloidal trajectories using the Boltzmann and Smoluchowski equations. In this talk, I will present novel analyses that adapt and extend approaches originally implemented by Denis to investigate different shaped polarizable colloids in AC electric fields and confined polymer coated colloids interacting with mucus barriers. In the first problem, polarization of electrostatic double layers on different shaped particles produces a time averaged induced dipole that can be localized in nonuniform AC electric fields. We show that Boltzmann inversions of position and orientation dependent histograms reveal potential energy landscapes that can be modeled by a simple analytical potential for a broad range of superellipsoidal shapes including spheres, disks, ellipsoids, and various rounded prism shaped particles. In the second problem, we measure particles with and without copolymer coatings interacting with mucus barrier interfaces. We show that fitting the Smoluchowski equation to measured particle trajectories self-consistently reveals both conservative forces between macromolecules but also demonstrates how lubrication together with polymer and mucus permeability modify hydrodynamic interactions and position dependent particle diffusivities (“diffusivity landscapes”).