David Jonsson wrote:
Isn't Teflon a good example?
No. Teflon, like all polymers, is actually an example of the Casimir
attractive force (in addition to Van der Waals, which is different)
http://www.chemguide.co.uk/atoms/bonding/vdw.html
Otherwise Teflon could not "stick" to itself. Teflon's anti-stick
properties are related to the next larger geometric spectrum.
At the molecular level up to a few tens of nm - if we have flat plates
[or long strings of polymers] separated by a small distance we can
measure an attractive force between the objects which varies via a power
law down to the Forster radius of about 1-2 nm.
This is due to the fact that virtual particles (so-called quantum
"foam") whose wavelength exceeds the distance between the real particles
are excluded - causing the vacuum energy (assorted *excluded*
wavelengths and near-fields) to be lower between the objects than
outside them. Since we define the vacuum as zero point energy we can say
we have negative energy density in the region between the plates or
strings.
However when we replace plates or strings with objects with spherical
surfaces in the range of a few nm in dia, we can often (but not always)
observe a small repulsive force - but it is less than the corresponding
attractive Casimir, and it has recently been noted that this could
relate to excitonic properties rather than ZPE exclusion.
Curved surfaces may serve to enhance (or reflect) the number of virtual
particles or waves which are possible between the objects (or on their
surface as near fields) as compared to the vacuum. We also get the
repulsive Casimir if the dielectric constant of the space separating the
plates has a value greater than the dielectric constant of one object
but less than the other object. It is difficult to come up with a simple
explanation which determines the sign of the force, even for perfectly
conducting bodies, and the situation is worse for dielectrics.
There should be lots of info out there on this, if you dig a little
deeper than Wiki.
Jones
