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Mechanism
Foam
formation is the result of dissolved molecules in a liquid.
The dissolved molecules alter the surface tension of the
liquid, and can be viewed as surface active agents (surfactants).
The surfactants can be nonionic, cationic, anionic, or amphoteric.
The liquid can be either aqueous, nonaqueous, or both (some
industrial systems may contain dissolved organics which
require special consideration). Different surfactants will
generate different types of foam, and foam stability. When
agitated, bubbles will form, which will immediately encounter
gravitational effects pulling liquid along the bubble walls
back down into the liquid beneath the bubble. A simplified
picture of a bubble can be described as spherical, having
both an outer wall and inner wall.
Nonionic
surfactant generated foam is generally depicted as having
a hydrophobic head (water insoluble portion) at the air-liquid
interface, and the hydrophilic tail (water soluble portion)
at an aqueous solution. Its orientation would be reversed
in nonaqueous liquids.
Anionic
surfactant generated foam would have a negative charge on
the hydrophilic tail. As aqueous liquid is pulled down over
the bubble's surface, the negative charges reach a concentration
at the bubble at the liquid interface. Most often the negative
charge serves to stabilize the bubble, and will begin to
repel each other at the interface. This phenomenon is known
as an electrostatic repulsion.
Cationic
surfactant generated foam would have a positive charge on
the hydrophilic tail, and would exhibit similar behavior
as the anionic surfactant in an ideal aqueous liquid.
When
the surface tension is high enough, bubble formation becomes
more rigid and stable. If a bubble is subjected to mechanical
agitation, bubbles caused by entrained air, would form very
stable lamellar structures. The Marangoni effect is a major
stabilizing factor in foam, and is driven by osmotic pressure.
In some cases, the aqueous liquid is being pulled through
the bubbles' walls creating regions of low and high surfactant
concentrations, which sets up a gradient along the bubbles'
surface. The gradient would pump liquid back onto the bubble
walls, where this phenomenon is referred to as a surface
transport.
The
bulk viscosity also contributed to foam stability. As the
viscosity of liquids increase, entrained air, now a bubble,
can be trapped below the liquid's surface. Increasing viscosity
of the system also reduces the coalescence capability of
smaller bubbles merging to become larger bubbles. If the
bubbles become large enough (increasing the diameter), bubble
stability decreases. The surface viscosity is also important,
as it effects the coalescence formation between bubbles.
The higher the bulk viscosity becomes, the lower the coalescence
formation is between bubbles.
When
the surface tension is lowered on the bubble, it will burst.
The resulting interaction of the defoamer to disperse the
foam and its bubble formation is a physical interaction
with the aqueous liquid. We engineer defoamers to work with
specific applications and their systems. We use a check
list to describe the criteria in which form is generated
and from this check list, a defoamer selection is made to
begin testing.
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