In suspension photoreactors, light and chemistry both distribute unevenly without help: the beam brightens the near wall, settling clears the far side, and products accumulate where they form. Stirring is the standard answer, but how you stir decides whether you fixed the problem or decorated it.
Suspend the catalyst uniformly, no settled bed, no floating raft; homogenize temperature against the heating of absorbed light; and move products away from the catalyst surface before they re-adsorb or over-react. Each goal has a different threshold speed, and the binding one is suspension, which you verify visually.
Magnetic bars suit small flat-bottomed vessels and modest viscosities. Overhead stirring handles larger volumes and dense slurries. Orbital shakers serve parallel formats, sealed vials in multichannel holders. Gas sparging agitates and adds a reactant at once, but bubbles scatter light, a trade to account for.
Above the suspension threshold, additional speed buys little mixing and costs real problems: vortexing draws gas and changes the light field, shear degrades fragile catalysts, and hot magnetic couples add unlogged heat. Find the threshold, add margin, and stop there.
Watch the suspension under full irradiation, settled zones and surface rafts are visible failures. For critical work, verify with a tracer or replicate sampling from different depths, aliquots that agree mean the volume is honest. Log the stirring method and speed with the run, they are conditions, not ambiance.
See Perfectlight's stirred photoreactor configurations under reactors and solutions.
