Particles Cling to der Waals
CONTAMINATION CONTROL IN AND OUT OF THE CLEANROOM
Barbara Kanegsberg
and Ed Kanegsberg
BFK Solutions LLC
Geckos appear to defy gravity as they run up a wall or across a ceil- ing. How can they do
that? It is not by exuding some
sort of sticky goo. It is due to
force, the same kind of force
that causes a particle or soil to
adhere to the surface of a component being manufactured or
used, or to tooling, or to cleanroom surfaces.
Three molecular forces —
polar, hydrogen bonding, and
dispersion — are associated
with adherence as well as with
solvency and cleaning.1 Geckos
stick to walls by exploiting
dispersion force. 2 Dispersion
force — sometimes referred
to as a non-polar, Van der
Waals, or London force — is
the weakest of the three, but it
may be the most fascinating as
well as the most significant for
particle adhesion and critical
cleaning. Dispersion force is
the mechanism by which non-polar compounds liquefy or
solidify.
Subtle power
Polar and hydrogen bonding forces are associated with
molecules that have dipoles,
permanent positively and negatively charged sides. Dispersion
forces, however, are a property of all molecules whether
or not they have an inherent
dipole. Because the electrons
are always in motion, there
can be a momentary fluctuation during which more of the
electrons are on one side of the
molecule than on the other.
When this happens, the side
with more electrons becomes
momentarily negatively
charged and the other side
becomes positively charged,
creating a momentary dipole.
If another molecule comes
very close to the molecule
with the momentary dipole,
the electrons on the approaching molecule will be induced
to move. For example, if
the momentary dipole has
its positive side nearest the
approaching molecule, electrons will be drawn towards
that side creating a dipole in
the second molecule. This
sets up an attractive force
between these two molecules.
Sometimes, the two colliding
molecules do not have enough
energy to overcome this force
and bounce back away; the
molecules “stick” together.
From this point, the dipoles
are no longer momentary;
they will last as long as the
molecules are bound together.
Moreover, this molecular
duo is itself a dipole that can
attract additional molecules,
in somewhat of a chain reaction or domino effect.
Gecko feet
The feet of geckos have millions of extremely tiny hairs.
The ends of each hair subdivide into many tiny pads
or spatulae (about 0.2 µm
diameter), each of which
induces dipoles and associated
attractive dispersion forces on
molecules of the wall surface.
Although dispersion forces are
exceedingly small, the collec-
tive sum from all the spatulae,
or even those on a single toe,
is sufficient to support the
gecko’s weight, even on a very
smooth surface.
Particles and dried soils
Dispersion force is also a primary mechanism that causes
contamination to stick. For
dispersion forces to be attractive or adhering, the molecules
have to be very close, essentially
within one atomic diameter of
each other — just a few nanometers. Small particles fit more
easily into the pits and crevices
of a surface (most surfaces are
rather rough at a microscopic
level even if they look smooth),
just as gecko feet hairs do, so
that more of the molecules
of the particle are within the
distance for attraction to the
surface molecules. This is why
small particles that stick to a
surface are more difficult to
remove than large ones.
A similar explanation elucidates why soil that is allowed
to dry onto a surface is more
difficult to remove. When a
liquid that has soil molecules
in suspension evaporates, the
soil molecules become close
enough to the surface for
attractive dispersion forces
to hold them there. Once the
attractive bond between molecules is made, it takes additional force to overcome and
break these bonds to allow the
soil to be removed.
This is why we keep reminding you that deferring cleaning
until the product wends its way
through the supply chain over
to your cleanroom is a terrible
idea. Critical cleaning may
need to happen early on in the
build process.
