Discovery of factors affecting bubble size in water
By Alan Burris, Ph.D.
December 1999 Water Technology Magazine
(PDF version)
Bubble size is important for aeration, ozonation and
other processes.
Side-by-side comparison of a conventional ceramic
diffuser (left) and a new proprietary diffuser.
Although it is usually assumed that the principal
determinant of bubble size in water and other liquids is
the pore size of the diffuser, experiments have shown
that this is one of the less important factors.
Surprisingly, the most important factor has been
found to be the ratio of the surface energy of the
diffuser material at the water interface to the surface
tension of the liquid. The higher the ratio, the smaller
the bubbles.
There is substantial literature on the role of
bubbles in mass transfer and other applications. Bubble
creation is important for processes such as ozonation;
aeration of drinking and wastewater, aquariums, fish
farms, ponds and lakes; air stripping of volatile
organic compounds, radon and hydrogen sulfide; iron and
sulfide removal by oxidation; and separation by
flotation. However, there is surprisingly little
published about the factors affecting bubble size, or
about methods of controlling the bubble size.
For water with normal surface tension, the higher the
diffuser surface energy (the more hydrophilic), the
smaller the bubbles. Reducing the surface tension of the
water also reduces the bubble size, but it is usually
undesirable to add surfactants to water.
Bubble diameter can critically affect
process efficiency because it determines the surface
area of the bubbles.
Using patented technology, high-surface-energy
materials have been combined with low-surface-energy
materials to obtain bubbles less than 1 millimeter (mm)
in average diameter, compared to 2 to 3 mm for
commercial ceramic diffusers at comparable gas flows.
The small bubbles were obtained with thin, low-cost
materials at low pressure drops.
Pore uniformity provides high turn-down ratios with
even bubble distribution across the diffuser surface.
The diffusers act as check valves by resisting water
intrusion, reducing fouling by waterborne contaminants.
The significance of bubble diameter
Bubble diameter can critically affect process
efficiency because it determines the surface area of the
bubbles. For two different spherical bubble diameters
with the same total volume of gas, the ratio of the
total surface areas is inversely proportional to the
ratio of the diameters. For example, at the same air
flow rate, decreasing the bubble diameters from 2.5 mm
to 0.5 mm would increase the interfacial contact area
between the air and water by a factor of five for
spherical bubbles.
The actual improvement factor would be greater for
bubbles smaller than 1 mm because of the longer rise
time. The improvement factor would be less for larger
bubbles because the surface area of larger bubbles is
increased by frictional distortions as they rise.
Commercial ozone sparging systems with 4 to 7 meters
(m) of water depth are less sensitive to bubble size
than portable ozone water purifiers with less than 0.2 m
depth. Ozone transfer efficiency with a conventional
diffuser's 2 to 3mm bubble diameter is poor at this
depth, and unacceptable for a contact lens disinfecting
device with a liquid depth of less than 0.03 m.
However, few practical methods exist for affecting
bubble size. An example of an impractical way to make
smaller bubbles is adding a surfactant to reduce the
surface tension of the water. Not only it is undesirable
to add a surfactant to drinking water or aquariums, but
in other applications surfactants can create a foam
problem and, used in large quantities for applications
such as ore flotation, would be costly and create a
pollution problem. Mechanical agitation or ultrasonics
would increase the cost, size and complexity of systems.
Methods involving higher pressures or additional pumps
are unappealing for the same reasons.
A finer bubble diffuser appears to be the best
solution for increasing ozone transfer efficiency.
Ceramic diffusers with very fine pores produce smaller
bubbles than the standard types. However, they are not
popular, apparently because of the higher pressure drops
and propensity to clog.
Finer pores, finer bubbles?
There are questions about the theory that finer pores
are the way to make finer bubbles. For example, why is
it that 50-micron-pore-sized ceramic diffusers produce 2
to 3mm bubbles? And why is it that similar pore sizes in
different materials produce different-sized bubbles?
Experiments have shown that the diffuser material
property that is the primary determinant of bubble size
is not the pore size, but the free energy at the
surface, which is a result of intermolecular attraction.
In the bulk of a homogeneous material, molecules are
attracted to each other equally. These forces of
attraction are the same as those that determine whether
a substance is in a gas, liquid or solid phase at a
particular temperature and pressure.
The strength of intermolecular forces depends on the
chemical structure. However, at the boundary between
phases, the forces are unequal because the molecules are
in contact with molecules with different forces of
attraction. This causes the interface to act as if it
were under tension--a condition called surface tension.
The surface tension of liquids is easily and
accurately measured, but the surface tension (surface
free energy) of solids can only be measured indirectly
and approximately. Measurements are expressed in
dynes/centimeter. Polar and metallic liquids and solids
have high surface tension/energy compared to non-polar
substances.
The significance of such data for bubble formation is
that the degree of wetting of a solid by a liquid is
determined by their relative surface energies. A solid
is generally completely wetted by a liquid that has
equal or lower surface energy. This is why surfactants
that lower the surface tension of water improve the
wetting of greasy surfaces. An interesting phenomenon is
that water does not wet hydrocarbons, such as oil or
paraffin, but oil wets solid ice. Figure 1 illustrates
the forces acting on a drop of water on a lower energy
surface.
Figure 1
The equilibrium between the solid, liquid and vapor
phase interfaces is expressed by the Young equation:
In this equation g is the tension between the phases,
sv refers to the solid-vapor interface, sl to the
solid-liquid interface, lv to the liquid-vapor
interface, and q is the contact angle. The contact angle
expresses the degree of wetting of the solid by the
liquid.
Conventionally, when the contact angle is zero,
wetting is complete and the liquid displaces air to
spread over the surface. For partial wetting or
non-wetting, the contact angle may increase up to 180
degrees when a drop is contacting the solid at only one
point. While the theory applies to the wetting of solids
by all liquids, the primary concern is wetting by water.
This is because hydrogen bonding makes water's surface
tension much higher than the critical surface tensions
at which many common solids can be wetted. In aqueous
systems, high-surface-energy materials are referred to
as hydrophilic, and low as hydrophobic.
To relate this information to bubble formation,
consider why some bubbles leave the diffuser surface
while they are very small, while others grow to a much
larger size before detaching from their surface. This is
explained by the two diagrams in Figure 2, one for a
low-surface-energy diffuser and the other for a
high-surface-energy diffuser.
Figure 2
With the small contact angle, water displaces the gas
from the high-energy diffuser surface pore so that the
tension holding the bubble to the surface is minimized.
A smaller upward force from the gas "balloon" suffices
to detach it to rise through the liquid. In contrast,
the contact angle for a low-surface-energy diffuser is
greater so that the gas displaces the water. The bubble
must grow bigger before the upward force produced by the
density difference between gas and liquid is sufficient
to overcome the surface tension of the larger area and
produce "liftoff."
Other factors that may affect bubble size
It may turn out that pore size is a more important
factor for diffusers with a high-energy surface. In
other words, bubble size may proportionally vary more
with the pore size of high-energy diffusers. Lower
surface energy may overpower the effect of pore size.
Needed research in this area will be difficult because
of the problem of holding constant the other factors
involved while varying the pore size.
Uniformity of pore size is an important factor in
average bubble size. Non-uniformity causes a higher gas
flow through the larger pores, which increases the
bubble size from those pores, and reduces the number of
active pores producing bubbles.
Often, to compensate for non-uniformity, diffusers
are made thicker. The greater thickness more uniformly
distributes the gas flow to the pores, but makes the
diffuser more prone to clogging. Clogging from
waterborne pollutants and fouling from biological
growths can change the size and number of active pores,
change the diffuser's surface properties and increase
its pressure drop.
The ideal diffuser would have a uniform small bubble
pattern across its surface over a wide range of gas flow
rates. It would resist clogging, maintain a low pressure
drop and be inexpensive to manufacture.
New technology has been developed that offers
substantial progress toward this ideal. The basic
concept is to use a porous material that has a high
surface energy at the liquid interface, but a low
surface energy below the interface. The
high-surface-energy interface produces fine bubbles, and
the low surface energy material, being hydrophobic,
resists water intrusion, acting as a check valve. Thus,
only a thin flexible layer at the interface is exposed
to waterborne contamination.
The finer the pores of the hydrophobic layer and the
lower its surface energy, the more the water pressure
that can be resisted. And, because the hydrophobic layer
resists water and waterborne clogging, its pores can be
made finer. The fine pores enable a thin layer to evenly
distribute gas flow through the hydrophilic surface.
This combination enables a 0.5-mm-thick membrane to
produce bubbles close to 0.5 mm in diameter at a low
pressure drop, while acting as a check valve.
Experiments have shown that the small bubbles
produced with this technology substantially improve
oxygen transfer efficiency as well as solving problems
with ozone transfer efficiency in very small systems.
The author of this article is President of ALAB,
LLC, and can be reached through e-mail at:
alab@quickpure.com.
-- Water Technology Magazine --
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