The bubble
clouds formed underneath breaking waves have been shown to
have a significant influence on air-sea gas transfer rates,
and current models for bubble clouds exist. However, no
models to date have included the changes to the bubble
population caused by surfactants, and consequently the
total effect of surfactants on gas transfer rates. The
fellowship research proposed here will build such a model,
based on insights into bubble dynamics resulting from
laboratory, wave tank and oceanic experimental work.
There is strong evidence to suggest that bubbles scavenge
natural surfactants very quickly: the rapid reformation of
the surface microlayer after an ocean wave breaks is
mediated by larger bubbles bringing surfactants up to the
surface and optical scattering evidence suggests that most
small bubbles are coated in surfactant. To date, very few
studies have considered the details of how surfactants
could affect bubble formation dynamics underneath breaking
waves (particularly fragmentation and coalescence),
although surfactants have been shown to influence bubble
dynamics. However, an understanding of the mechanistic
origin of ocean bubbles is essential to guide effective
parameterizations of any upper ocean process affected by
bubbles.
This project will begin with laboratory studies of the
effects of various natural and artificial surfactants on
fragmentation and coalescence events. Simultaneous
high-speed photography and acoustics have been shown to
produce an impressive amount of information about
individual events, and these techniques will be combined to
study the dynamics and acoustics of fragmentation and
coalescence events. In the second and third years of the
project, the insights gained from the small-scale
laboratory experiments will be used to tune models of the
bubbles produced by breaking waves with different
surfactants present, and these model outcomes will be
tested in a wave tank and at sea.
================================================
Detailed
dynamics of bubble production by
raindrops.
Last year a
large dataset of stereo high-speed photography accompanied
by the simultaneous acoustical signals was gathered for the
bubbles generated by fresh raindrops falling on seawater.
The surfactants present in the tank were varied, and we are
looking for changes that these surfactants may cause in the
bubbles produced. The data was collected at SIO with the
help of Grant Deane and Dale Stokes, and preliminary
analysis of this data is being undertaken by my intern,
James Wallis.