Liquid-Phase Transmission Electron Microscopy (LP-TEM) is a convenient experimental technique for the imaging of dynamic processes at the nano-meter scale (biological, chemical, bio-cehmical, etc.) in wet-chemical environment through electron (e-) beam irradiation in enclosed liquid cells. Over the last years, various stimuli (e.g., temperature, bias, composition of reaction media) were integrated into experimental setups promoting the evolution of LP-TEM towards a quantitative imaging technique. Enabling fluid flow has unleashed several aspirations awaited by the community, i.e., precisely controlling the supply of reagent to and/or reliably removing radiolytic species from the field of view, respectively. However, little work was carried out to calibrate these dynamics for complex LP-TEM setups, nevertheless being a prerequisite to compare in situ results to other analytical techniques.

Here, we present a general approach combining experimental and theoretic methods (contrast variation & numeric convection diffusion modelling) for the hydrodynamic characterization of LP-TEM flow systems. We demonstrate the method by characterizing two commercial LP-TEM flow reactors (Poseidon Select & Poseidon 200, Protochips Inc0.) representing premixing and onsite mixing concepts. Besides fundamental understanding of the fluid dynamics, i.e., interplay of diffusion & convection, inside the complex channels; we quantified the timescales of solution mixing (i.e., replacement) to be in the range of minutes, several orders of magnitude slower than those of typical ex situ mixing experiments.

The acquired understanding allows to develop LP-TEM flow reactors with improved hydrodynamic properties. New diffusion cell setupss with accelerated solution replacement were developed through virtual prototyping based on previously validated numeric models. Physical prototypes were manufactured by top-down fabrication methods (high precision milling and reactive ion etching) and contrast variation methods were applied to quantify the replacement dynamics; for the first time, we report flow reactors that allow the replacement of solute within less than 10 seconds corresponding to typical ex situ mixing scenarios.

We anticipate that the hydrodynamic characterization of LP-TEM flow systems will allow for better planning of in situ experiments and more reliable interpretation of results. The rational design of flow reactors will enhance correlatability to ex situ experiments and open new fields of LP-TEM research.