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incidental observation of a particular fluid dynamical phenomenon

(most experiments don’t work most of the time)



2021

failed experiment documented by videos

materials:
  • aq.dest.
  • silver nitrate
  • ammonia
  • sodium hydroxide
  • dextrose
  • window sill
  • dust
  • sunlight

phenomena:
  • formation of elemental silver by Tollens reaction (expected)
  • ammonium dissolving glue and leading to leakage allowing chemicals to drip down window sill (unexpected)
  • peculiarly stable eddies, swirling for ~10min, visible via the silver particles dragged along (unexpected)
  • aesthetically pleasing constellation of late afternoon sunlight with leaking reagent, silver particles and surface of windowsill (unexpected)


Physics is a technique of effects (Gaston Bachelard).
To experiment is to create, produce, refine and stabilize phenomena.  If phenomena were plentiful, summer blackberries there just for the picking, it would be remarkable if experiments didn’t work. But phenomena are hard to produce in any stable way. [...] Most experiments don’t work most of the time. (Ian Hacking)

This work emerged from an incidental observation when preparations for some other experiment failed. I was trying to metallize part of a window using Tollen’s reaction - a chemical process which was formerly used to coat mirrors by precipitation of silver from a solution of silver nitrate. That this didn’t work out as planned I found less remarkable than what I observed as my eyes followed the leaking reagent down the window sill: a particluarly stable fluid dynamical phenomenon. In a trickle of a width of 10mm the precipitating silver particles were twirling for minutes, visualizing surprisingly stable eddies in the flow of the liquid over the uneven - because repeatedly painted over and infrequently cleaned - surface of the window sill.
This incidental observation amidst the chaos of my failed experiment seemed significantly more interesting to me than pursuing my original plan.
The observation posts several questions: What exactly happened? Which properties of the reagent, which of the ambient conditions are significant for the occurence of this phenomenon? Was there some driving force constantly supplying energy to the circulation? It could be the downward flow of the liquid hitting the tiny edge resulting in the velocity stream lines being deflected. Another possible driving force are density gradients. Those could arise from temperature gradients due to radiation of the sun or from a change of fluid composition due to the chemical reaction. Also, local change in surface tension caused by contamination could drive flow dynamics.
This is not at all a controlled experimental setup apt for the isolation of a phenomenon and its generating conditions. The window as well as the window sill are covered in contaminants such as dust or household cleaner residuals. Those can be highly influential on surface tension or the chemical precipitation reaction.