A toy app to calculate Stokes-Einstein diffusion constants

The one formula that I use above all others in my research is that for the diffusion constant of a dilute species in a fluid, called the Stokes-Einstein (or sometimes Stokes-Einstein-Sutherland) equation:

D_SE = kT/(6 π η R_H)

The diffusion constant D_SE of something with a size (hydrodynamic radius) R_H is just the thermal energy kT divided by 6π times the product of the viscosity of the fluid, η, and the hydrodynamic radius, R_H. All it tells you is that large species in viscous fluids diffuse more slowly than small species in less viscous fluids.

This is a simple formula but whenever I want to calculate a diffusion constant I usually have to get Python running and use it like a calculator. This takes less than a minute, but even so this is irritating.

So (together with LLMs Google Gemini and MS Copilot) I have done a simple Streamlit app that calculates the diffusion constant, given the hydrodynamic radius as input – for water at room temperature. It runs on Streamlit Community Cloud which is a doodle to do as it just pulls the Python/Streamlit code from a GitHub repository.

It is very basic and only does this one thing: compute a diffusion constant. But because of this it is very easy to use. If I find it useful may be I will make it fancier but we will see. If you find it useful or feel that a fancier version (eg where you can vary the temperature, viscosity etc), just let me know.