THE NEAR-SURFACE LAYER OF THE OCEAN
The observational data given by Clift et al. (1978) suggest that the rise
velocity for seawater bubbles with radii larger than approximately 0.6 mm is
close to that of hydrodynamically clean bubbles. An explanation is that
larger bubbles rise quickly to the surface and thus do not have sufficient time
to collect surfactants (Keeling, 1993; Woolf, 1993). Small bubbles (less than
approximately 0.3 mm radius) are assumed to perform as hydrodynamically
dirty bubbles (Woolf and Thorpe, 1991). Their surface becomes covered
with surfactants almost instantaneously.
Patro et al. (2001) proposed to treat small bubbles in seawater as dirty
and large bubbles as clean with a transition at circa 0.6 mm. The transition
radius somewhat increases with the increase of the time period the bubbles
are in the water column. The assumption about clean and dirty bubbles is
however, somewhat uncertain. Adding to the uncertainty is the collective
behavior of bubbles in bubble clouds.
6.1.3 Bubble size distribution function
The bubble-mediated component of the air-sea gas exchange, production
of spray droplets and aerosols, optical properties of the sea surface,
generation of ambient noise and sound transmission within the oceans, and
scavenging of biological surfactants also essentially depend on the size
distribution of bubbles. The size distribution of entrained bubbles is also an
important factor in controlling turbulence and dynamics in wave breakers. In
addition, bubbles of different sizes behave differently in the turbulent surface
layer. Bubble size distribution data potentially lead to a dynamical
description of air-sea interaction based on near surface turbulence,
advection, and other properties.
Different approaches for measuring bubble size spectra, from acoustical
to optical, have been developed. Each technique has advantages and
disadvantages and is effective over different size ranges and bubble density
regimes (Leifer et al., 2003). Acoustic methods have played a definite role in
observing bubble size distributions because bubbles have a high resonance
quality factor leading to an acoustical cross-section some three orders of
magnitude greater than their geometrical cross-section (Farmer et al, 1998).
This effect reaches a maximum, usually at a frequency of 30 kHz, which
corresponds to a bubble radius of 100 Pm. Different frequencies excite
bubbles of different radii, which allow an estimate of the bubble size
distribution from acoustic measurements. However, acoustic methods have
difficulties with large bubbles (r > 0.5-0.7 mm), which are non-spherical in
turbulent flow, and at high bubble concentrations. Laser techniques are
noninvasive but, in common with acoustic methods, have problems when
multiple bubbles enter the beam (high bubble density) and with large
400
The observational data given by Clift et al. (1978) suggest that the rise
velocity for seawater bubbles with radii larger than approximately 0.6 mm is
close to that of hydrodynamically clean bubbles. An explanation is that
larger bubbles rise quickly to the surface and thus do not have sufficient time
to collect surfactants (Keeling, 1993; Woolf, 1993). Small bubbles (less than
approximately 0.3 mm radius) are assumed to perform as hydrodynamically
dirty bubbles (Woolf and Thorpe, 1991). Their surface becomes covered
with surfactants almost instantaneously.
Patro et al. (2001) proposed to treat small bubbles in seawater as dirty
and large bubbles as clean with a transition at circa 0.6 mm. The transition
radius somewhat increases with the increase of the time period the bubbles
are in the water column. The assumption about clean and dirty bubbles is
however, somewhat uncertain. Adding to the uncertainty is the collective
behavior of bubbles in bubble clouds.
6.1.3 Bubble size distribution function
The bubble-mediated component of the air-sea gas exchange, production
of spray droplets and aerosols, optical properties of the sea surface,
generation of ambient noise and sound transmission within the oceans, and
scavenging of biological surfactants also essentially depend on the size
distribution of bubbles. The size distribution of entrained bubbles is also an
important factor in controlling turbulence and dynamics in wave breakers. In
addition, bubbles of different sizes behave differently in the turbulent surface
layer. Bubble size distribution data potentially lead to a dynamical
description of air-sea interaction based on near surface turbulence,
advection, and other properties.
Different approaches for measuring bubble size spectra, from acoustical
to optical, have been developed. Each technique has advantages and
disadvantages and is effective over different size ranges and bubble density
regimes (Leifer et al., 2003). Acoustic methods have played a definite role in
observing bubble size distributions because bubbles have a high resonance
quality factor leading to an acoustical cross-section some three orders of
magnitude greater than their geometrical cross-section (Farmer et al, 1998).
This effect reaches a maximum, usually at a frequency of 30 kHz, which
corresponds to a bubble radius of 100 Pm. Different frequencies excite
bubbles of different radii, which allow an estimate of the bubble size
distribution from acoustic measurements. However, acoustic methods have
difficulties with large bubbles (r > 0.5-0.7 mm), which are non-spherical in
turbulent flow, and at high bubble concentrations. Laser techniques are
noninvasive but, in common with acoustic methods, have problems when
multiple bubbles enter the beam (high bubble density) and with large
400