First Passage Time Distributions to Charged Solid-Liquid Interfaces

Diffusion of a molecule or colloidal particle to a surface is a fundamental problem with applications to problems in targeted drug delivery, diffusion-controlled surface reactions and separation processes to name a few. Often, these processes are controlled by the first passage time distribution, which describes the probability that a particle first reaches the surface from a specified starting point. Near an interface, anisotropic forces can act on the particle in addition to Brownian motion and modeling these interactions is an active area of research. Furthermore, the interface can perturb the structure of heterogeneous fluids (e.g. blood), thereby altering particle dynamics in the near-interfacial region.

Towards the goal of understanding the relationship between interfacial properties and first passage times, three-dimensional super-resolution particle tracking was used to follow individual nanoparticles diffusing near and to solid surfaces in the presence of charge repulsion. This permitted a direct measurement of the first passage time distribution as a function of ionic strength in glycerol-water mixtures to quantify the effect of charge repulsion. Experimental results were compared to theoretical predictions in the limits of either constant charge or constant potential. Finally, the behavior of the glycerol-water system was compared to that of blood at constant ionic strength to identify the effect of heterogeneous structure in the fluid phase.