Critical bubble bursting in real water. Effect of surface-active contaminants

Abstract

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. 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 cavity bottom shrinkage and surfactant convection. The resulting reduction in surface tension narrows the region of free surface reversal. Despite this effect, the size of the emitted droplet increases due to the Marangoni stress acting on the jet surface. Marangoni stress slows down the interface of the liquid jet, delaying the detachment of the droplet. More liquid flows into the droplet, 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 the bursting of bubbles under critical conditions. Our results may explain the size of the particles emitted by bubble bursting in seawater.