ASHRAE Journal - November 2019 - 74

COLUMN BUILDING SCIENCES

Brownian Motion
The oldest known deposition mechanism will be
described first-Brownian motion. A long dead English
botanist (yes, botanist not physicist), R. Brown, noticed
tiny particles behave much like molecules with respect
to vibration. Under the kinetic theory of gases, molecular vibrations are directly related to temperature. The
higher the temperature the greater the molecular vibration. Brown observed that tiny particles in fluids get
jostled or bounced around depending on temperature.
The higher the temperature, the greater the jostling or
movement of the tiny particles. He believed that the
vibration of molecules in the fluid bounced the particles
around. He was correct and became famous-hence the
term Brownian motion. Okay, so far this is an interesting
story, but how does this relate to our problem?
Well, air is a fluid and particles in air get jostled around
based on the temperature of the air. When air with all
of these bouncing particles brushes against the interior
of an exterior wall it gets cooled. Not all of the air gets
cooled, only the air right against the interior surface of
the exterior wall. We call this layer of air the boundary
layer. Now it gets a little complicated. Not all of the air in
this boundary layer cools at the same rate. Some spots
are colder than others. Where there is higher heat loss
such as at a wood framing member, say a 2 × 4, the air
in the boundary layer right at this spot is colder than
the air in the boundary layer just inches away. Well, the
air doesn't bounce around as much in these cold spots
which means the tiny particles in the air at these cold
spots also don't bounce around very much. If the particles are not bouncing around very energetically, it is
easier for them to get stuck to the surface of the wall. The
particles in the air next to the wall tend to "plate out" on
colder surfaces. Since every stud is a "thermal bridge"
the outlines of the studs can be sometimes seen due to
the accumulation of tiny particles at the cold spots. This
is often called ghosting and was very common in the
old days, before MTV, even before Ozzie and Harriet. It
was common back then because we typically had poorly
insulated walls (real cold spots) and lots of people that
smoked. The smokers supplied the particulates. The
tobacco particulates plated out on the cold spots, usually
at the studs due to Brownian motion. Those of us in New
England and old enough remember the days of coal,
oil and gaslights and all the associated markings. OK,
nobody's old enough to remember gaslights.
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Why doesn't this particular phenomenon happen as
much today? Two reasons: our houses are better insulated-continuous exterior insulation makes thermal
bridging go away and there are fewer smokers-inside
anyway. Fewer people smoke inside and those that do
typically accept the fact that smoking not only causes
their house to smell but smoking also makes their house
dirtier due to the tobacco particles plating out on cold
surfaces. We point this out since many units I have seen
problems in contain heavy smokers and the problems in
those units are usually due to tobacco particulates. Don't
smoke inside and your white walls won't go brown, nor
will your light colored carpets discolor-same for your
blinds. You should accept the consequences when you
live in an ashtray. Folks caught on. Then cannabis came.
We have to go through the same education process again.

Electrostatic Attraction
Another well-known deposition mechanism is electrostatic attraction. When tiny particulates become electrically charged they will become attracted to other electrically charged surfaces. The particulates can become
charged due to many reasons. Friction and proximity
to electrical and magnetic fields are some of the most
common. Us old school engineers loved electrostatic
filtration on furnaces and air conditioners; they charged
these bastard particles big time and created deposition
paradise inside. Plastic surfaces and surfaces that contain electrical equipment also become easily charged.
The particulate depositions observed on window blinds,
inside of kitchen cabinets and on appliance surfaces
(microwave ovens, and refrigerators) are due to electrostatic attraction.
Remember home ionizers? They create-wait for
it-electrically charged particles. Folks purchased ion
machines for health reasons and got dirty walls. Home
ion machines cause particles to become charged and
plate out on surfaces. The air is clean, but your walls are
not.

Impaction
Impaction occurs when moving air carrying tiny
particulates flings or throws the particulates against a
surface. This typically happens when air moves under a
door due to an air pressure difference. As the air rushes
under a closed door it has to "bend" under the door.
When air in the middle of the room (height-wise) has to

ASHRAE Journal - November 2019

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