Creating bubbles:
Anytime sufficient energy impacts
a liquid, the liquid will change from a liquid to a gas. In a
kitchen, for example, heating water on a stove adds energy, which
causes water to boil, changing the water from a liquid (water)
to a gas (steam). The technical term for this is "vaporization."
If the process of vaporization occurs
on the top of a liquid, the gas that was created by the vaporization
process simply mixes with air above it. This might happen if the
energy is being added by a source above the liquid. A heat lamp,
for example, might cause steam to form at the top of a cup of
water. On the other hand, if the heat is applied beneath the liquid,
the liquid at the bottom (near the heat) is the first to vaporize.
This forms gas at the bottom of the liquid. Since the gas that
is formed is lighter (less dense that the water above it), the
gas forms the familiar bubbles that float to the top of the liquid.
In fact, any time a liquid is vaporized under its surface, a bubble
will form.
Energy density also effects where vaporization occurs. For example,
a heat lamp above a liquid would cause steam to form roughly uniformly
on the liquid's surface. Similarly, a pot on a gas burner will
generate a roughly even distribution of bubbles on the bottom
of a pot. A laser (which has a very dense energy "beam"
) that is aimed at the surface, however, would cause evaporation
in a fairly narrow area on the liquid's surface.
The process of vaporization does not have to happen on the edge
of a liquid. It can happen in the middle of a liquid, too. Depending
on how the energy is added to the liquid, vaporization could be
stimulated in any part of a liquid. What matters most is where
the energy is added and the density of the energy. For example,
a heating element in the middle of a pot would cause vaporization
in the middle of the pot, next to the heating element.
The energy that causes a liquid to vaporize does not have to come
through physical contact. For example, a magnifying glass focusing
sunlight in the middle of a liquid could cause the liquid to vaporize
where the sunlight is focused, even in the middle of a pot of
water. So controlling where energy is most dense, even if not
through physical contact, allows one to control where liquid vaporizes
and, thus where bubbles form.
The energy that causes water to vaporize does not have to come
from heat alone. Other forms of energy, such as pressure and mechanical
force, if sufficiently dense and powerful, can also cause water
to vaporize and thus create bubbles. Power boaters and the Navy
know this well, because their vessels constantly create a stream
of bubbles, created through this same phenomenon, as their propeller
blades turn under water. Small, dense pressure waves are vaporizing
little bits of water, creating a gas and thus making bubbles behind
the propeller.
Bubbles can form in different sizes and different locations. Sometimes
bubbles float to the surface, as in a boiling pot, and sometimes
they are reabsorbed back into the liquid or collapse back on themselves
(click on the video to watch this happening).
Under certain, specific conditions, if a bubble collapses, like
in this video, but very near a surface, the corresponding collapse
is exceptionally violent, in a very specific direction, through
a process that is know as a microjet.
Characterizing the bubbles:
Download Movie Cavation
of Bubbles
Some variables we want to control in bubble formation (dependent
variables):
> Exactly where will a
bubbles form?
> How big will the bubble
become?
> What happens to the bubble
after it is formed?
Some variables that we use to control the bubble formation (independent
variables):
> Energy power, density
and location used to vaporize the liquid.
> Composition of liquid
> Pressure of the liquid
> Temperature of the liquid
