Imaging the Nanoscale Dynamic Structuring of Flow-Induced Gold Nanoparticle Superlattices in a Microfluidic Channel

Liquid cell transmission electron microscopy (LCTEM) has made great progress over recent years, overcoming initial challenges such as sample thickness, vacuum compatibility and the implementation of electrodes for electrochemistry or heating, and offers a unique access to the nanoscale structure of sample systems in an aqueous environment [1].<br/>A large area of interest in LCTEM involves the behavior of nanoparticles (NPs) in a liquid medium, with investigations ranging from particle growth processes to particle diffusion and assembly. However, NPs in solutions are typically not visible in LCTEM due to rapid Brownian motion and only particles bound to the walls of the liquid channels can be imaged.<br/>In our experiments, we have now been able to study the turbulent behavior of a dense cloud of citrate-capped Au NPs in aqueous solution with the adjustable liquid flow as an external control parameter for the NP density. The experiments were conducted in a JEOL JEM-F200 TEM with a cold-field electron source and an Insight Chips liquid-cell sample holder with well-defined microchannel flow geometry [2]. Utilizing the electron-beam induced deposition of NPs, we selectively create a nanoparticle sieve within a liquid cell channel, which effectively filters NPs from the flowing liquid. Up-stream of the sieve, NPs pile up and create a dense particle cloud. The density of the cloud is controlled by temperature dependent particle diffusion and the flow velocity of the liquid through the sieve. Comparison of the electron image contrast before and after the sieve yields a direct measure of the local particle density.<br/>For such an experimental configuration, we observe two intriguing phenomena: Firstly, the particle cloud shows millisecond dynamics akin to the turbulent motion in a smoke cloud with indications of vortex formation. We putatively attribute this behavior to a dynamic rearranging of the sieve structure resulting in a change in the local fluid flow profile across the microchannel. Secondly, at the highest particle densities, the particles within the cloud start to form a stable, spatially periodic arrangement which breaks up once the liquid flow velocity is reduced.<br/>Our experiments yield access to the dynamic nanoscale structure formation in densely packed liquid environments with some similarities to the crowded structures inside cells.<br/><br/>[1] F. M. Ross (Ed.), Liquid Cell Electron Microscopy, Cambridge University Press (2017).<br/>[2] M. N. Yesibolati, S. Laganà, H. Sun, M. Beleggia, S. M. Kathmann, T. Kasama, K. Mølhave, Mean Inner Potential of Liquid Water, Phys. Rev. Lett. 124 065502 (2020).