We study the bursting of a bubble on a liquid free surface under critical conditions, i.e. those leading to the minimum (maximum) size (velocity) of the first-emitted jet droplet. We consider the effect of a surfactant remaining in the monolayer during the cavity collapse and jetting (the surfactant is considered as insoluble). Our experiments show that a tiny amount of surfactant considerably increases (decreases) the droplet radius (velocity). The volume of the first-emitted droplet increases by a factor of 20 for a concentration that produces an insignificant reduction in the bubble surface tension. The total liquid volume ejected by the bubble increases with the surfactant concentration. Surfactant accumulates at the bubble base due to the shrinkage of the cavity bottom and surfactant convection. The resulting reduction in surface tension narrows the region of free surface reversal. Despite this effect, the droplet size increases because Marangoni stress widens the jet and slows the liquid jet interface, delaying droplet detachment. More liquid flows into the droplets, increasing the mass and energy transfer to the resulting spray. A significant increase in the droplet size is also observed with a weak surfactant. This indicates that natural water contamination can substantially alter bubble bursting under critical conditions. Our results may explain the size of the particles emitted by bubble bursting in seawater.
Rodríguez-Aparicio et al. (2026) studied this question.