Tracking Spatially Heterogeneous Dynamics of Single Nanoparticles Near Liquid-Solid Interfaces.

Area of Science:

• Colloidal physics
• Nanoscale fluid dynamics
• Statistical mechanics

Background:

• The behavior of particles near interfaces is crucial in colloidal physics.
• Faxén and Brenner's continuum mechanics solutions predict distance- and direction-dependent particle diffusivity.
• Experimental validation for micron-sized particles exists, but nanoscale applicability is unclear.

Purpose of the Study:

• To experimentally test the complete Faxén-Brenner solutions on the nanoscale.
• To investigate nanoparticle diffusivity near a fluid-wall interface.
• To bridge the gap between theoretical predictions and experimental evidence at the nanoscale.

Main Methods:

• Developed a novel multiresolution instrument for simultaneous nanoscale tracking and interface localization.
• Utilized high-resolution lifetime-gated 3D tracking of single nanoparticles.
• Employed two-photon laser-scanning microscopy for precise nanoparticle-wall proximity measurements.

Main Results:

• Successfully reproduced the predicted directional diffusivity divarication at the single-nanoparticle level.
• Achieved experimental validation of the Faxén-Brenner theory down to approximately 65 nm.
• Demonstrated agreement with theoretical predictions without adjustable parameters.

Conclusions:

• The study provides the first direct experimental support for Faxén-Brenner solutions on the nanoscale.
• Fluid dynamics principles governing particle diffusion are validated for nanoparticles near interfaces.
• Future research should focus on the sub-100 nm regime, considering finite-temperature fluctuations and fluid molecularity.