  Bubble bursting on water surfaces is believed to be a main mechanism to
produce submicron drops, including sea spray aerosols, which play a critical
role in forming cloud and transferring various biological and chemical
substances from water to the air. Over the past century, drops production
mechanisms from bubble bursting have been extensively studied. They usually
involve the centrifugal fragmentation of liquid ligaments from the bubble cap
during film rupture, the flapping of the cap film, and the disintegration of
Worthington jets after cavity collapse. Here, we show that a dominant fraction
of previously identified as 'bubble bursting' submicron drops are in fact
generated via a new mechanism underwater, inside the bubbles themselves before
they have reached the surface. These drops are then carried within the rising
bubbles towards the water surface and are released in air at bubble bursting.
Evidence suggests that these drops originate from the flapping instability of
the film squeezed between underwater colliding bubbles. This finding
fundamentally reshapes our understanding of sea spray aerosol production and
establishes a new role for underwater bubble collisions regarding the nature of
transfers through water-air interfaces.
