The Physics of Cleaning, Part 3: Gecko Feet
Ed Kanegsberg
If you have ever been to Hawaii or tropical Asia, you probably have observed
geckos, lizards that can appear to defy gravity and run up a wall or across
a ceiling. Have you ever thought about how they can do that? It is not
by exuding some sort of sticky goo. It is due to forces, the same
kind of forces
that cause a soil to adhere to the surface of a component being manufactured
or used.
As a physicist, I view the processes of cleaning, washing, rinsing, and
drying, in terms of physical actions - force and energy. The dispersive
force, the
gecko feet force, is the weakest of the three molecular forces associated
with solvency and cleaning (i.e. weaker than polar or hydrogen bonding),
but at the same time it is perhaps the most significant to the processes
of achieving high quality, cost-effective cleaning. The mechanism for the
dispersive force is really fascinating.
The other two forces
In the first
part of “The Physics of Cleaning,” I explained that soil
is generally held to the product by intermolecular forces. Once you understand
the forces and how to overcome them, you can
often make simple and profitable improvements to your industrial or critical
cleaning process.
In Part 2, I describe two of the three types of intermolecular forces. These are the polar and hydrogen bonding forces. Both these forces involve molecular shapes that result in a dipole, a displacement of positive and negative charges. The attractive force between unlike charges then causes a negative side of a molecule to “stick” to a positive side of a neighbor.
Dispersive force – subtle power?
The remaining type of force is the dispersive force, sometimes referred
to as a “non-polar” or “Van der Waals” force.
Dispersive forces, however, are present even with molecules that do not
have positive
or negative sides. This is because the electrons on atoms are always
in motion and there can be a momentary fluctuation in which more of the
electrons are
on one side of the molecule than on the other. So 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 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.
Dispersive force is the mechanism by which non-polar compounds liquefy
or solidify, resulting in oceans, mountains – the world as we know
it. Dispersive force is also the mechanism that causes contamination
to stick.
Dispersive force – as subtle as gecko feet
Foot of a Tokay Gecko
Photo: David Clements, From Wikipedia
Geckos stick to walls by exploiting dispersive Van der Waals forces(1).
The confirmation of this effect opens the way for the development
of synthetic
gecko feet that might enable a non-Hollywood “Spider
Man” or
a dry “fly paper” to reduce cleanroom contamination.
The feet of geckos have millions of extremely tiny hairs.
The ends of each hair subdivide
into many tiny pads or spatulae each of which induces dipoles
and associated attractive dispersive forces on molecules
of the wall surface. Although dispersive
forces are exceedingly small, the collective 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.
Gecko feet, particles, and dried soils
For dispersive forces to be attractive or adhering, the molecules
have to be very close. This is why small particles are
more difficult to
remove than
large ones. The 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.
A similar explanation elucidates why soil that is allowed
to dry onto a surface is more difficult to remove. 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.
More physics lessons? Absolutely!
In the next installment of this series, we will discuss
solvency. Most molecules of interest in cleaning technologies
involve
all three kinds
of force. For
a given molecule, the strength of each force and the
balance of forces determine solvency properties. You
can exploit
these physical
properties
to achieve
optimum cleaning.
1) K. Autumn et. al. “Evidence for van der Waals adhesion in gecko
setae”. Proceedings of the National Academy of Sciences of the
USA 2002, 99, 12252-12256. Available online at http://www.pnas.org/cgi/content/full/99/19/12252
