Came across this terminology in a 2017 paper.

https://ieeexplore.ieee.org/document/8046760

" The constant residue level at small sample size was attributed to the surface adsorption phenomenon, or “Flux Shell” phenomenon, where a layer of flux was strongly adsorbed on the solder powder surface"

This isn't a phenomenon I have seen referred to elsewhere, does anyone have a description of the formation of this shell and its impact?