Tracking Spatially Heterogeneous Dynamics of Single Nanoparticles Near Liquid–Solid Interfaces

A classical problem in colloidal physics is the behavior of a spherical particle when it randomly walks close to the interface between a fluid and a rigid wall. Solutions to the two complementary aspects of this problem have been provided by Faxén and Brenner, respectively, based on continuum mechanics. Their results predict that the closer the particle is to the interface the slower it moves, but the pace of the slow-down depends on whether the particle steps parallel with or perpendicular to the interface; that is, the particle's diffusivity divaricates in a distance- and direction-dependent manner. While the theoretical predictions enjoy unequivocal experimental supports for μm-sized particles, their applicability on the smaller length scales remains unclear, however. Here we present the first direct experimental test for the complete Faxén-Brenner solutions on the nanoscale. Our experiment was enabled by a new multiresolution instrument which concurrently and synchronously recorded both the high-resolution lifetime-gated μs 3D tracking of a single diffusing nanoparticle for nanoscale diffusivity and the lower-resolution two-photon laser-scanning microscopy images for the nanoparticle's location relative to the wall interface. The directional diffusivity divarication predicted by Faxén and Brenner was reproduced on the single-nanoparticle level with no adjustable parameters. Our results thus provided experimental supports for the underlying fluid-dynamics physical picture down to ∼ 65 nm, the radius of the nanoparticle sample used in this work, and pointed to next experimental challenges being in the sub-100 nm regime where finite-temperature fluctuations and the molecularity of the fluid are expected to become increasingly noticeable for smaller nanoparticles.