Origins of suppressed self-diffusion of nanoscale constituents of a complex liquid

TL;DR

This study uses MHz X-ray photon correlation spectroscopy to reveal microsecond density fluctuation dynamics of semiconductor nanocrystals, showing suppressed self-diffusion due to attractive interactions, contrasting with other colloidal systems.

Contribution

It demonstrates the ability to directly measure nanoscale dynamics in complex liquids and explains the suppressed diffusion through interparticle attractions, supported by coarse-grained simulations.

Findings

Wavevector-dependent fluctuation rates are suppressed in the liquid phase.

Suppressed rates are due to decreased self-diffusion caused by attractive interactions.

Simulations show the interparticle potential explains liquid stability versus gelation.

Abstract

Understanding and ultimately controlling the transformations and properties of nanoscale systems, from proteins to synthetic nanomaterial assemblies, is limited by the inability to uncover their dynamics on their characteristic length and time scales. Here, we nevertheless demonstrate this ability using MHz X-ray photon correlation spectroscopy (XPCS) -- directly elucidating the characteristic microsecond-dynamics of density fluctuations of semiconductor nanocrystals (NCs), not only in a colloidal dispersion but also in a liquid phase consisting of densely packed, yet mobile, NCs with no long-range order. We find the wavevector-dependent fluctuation rates in the liquid phase are suppressed relative to those in the colloidal phase and relative to observations of densely packed repulsive particles. We show that the suppressed rates are due to a substantial decrease in the self-diffusion of NCs, which we attribute to explicit attractive interactions. Using coarse-grained simulations, we find that the extracted shape and strength of the interparticle potential explains the stability of the liquid phase, in contrast to the gelation observed via XPCS in many other charged colloidal systems. This work opens the door to elucidating fast, condensed phase dynamics in complex fluids and other nanoscale soft matter, such as densely packed proteins and non-equilibrium self-assembly processes, in addition to designing microscopic strategies to avert gelation.