Van der Waals force
This page is not going to discuss the origin and nature of the VdW force but is focusing on practical applications and an intuitive understanding.
Contents
Practical usage
Bond trustworthiness, bond area and temperature (energy)
The question: VdW forces are "weak", so are they sufficient to hold stuff trustworthily together?
At room temperature a C-C bond practically does not break due to thermal motions. So a VdW bond with an area big enough to provide the same bonding energy will too practically not break at room temperature. As it turns out, this area is not all that big (relative of the area of a single C-C bond), so one might rely on VdW forces for reliably holding things together quite early on in the size scales of crystolecules.
So to prevent thermal motion from knocking VdW bonds open it might not be necessary to do some clever form closure designs (that are then strongly locked at a bigger size scale) except maybe for very small parts at very high temperatures. (TODO: ckeck that)
VdW bonds – stronger than expected (force = energy per length)
Two coplanar atomically flat surfaces attract each other quite a lot.
The attractive pressure from VdW forces is in the low nN range per square nm.
Here are two quite different values:
- ~1nN per square nm. Note that this equates to no less than around ~10,000 bar.
Original Source: (Nanosystems 9.7.1.)
indirect source: [1] (beware: the noted binding energy is mistakenly taken from a covalent interface - Nanosystems 9.7.3.)
double indirect source: [2] - ~2.7nN per square nm. Note that this is about 1/20 of the tensile strength of diamond
Source: (Nanosystems 3.5.1.b) (And this is more than titanium and low grade steel. These are just two flat surfaces contacting. VdW forces are by no means weak from an intuitive pespective)
Especially if there is superlubrication a flat surfaces can still slide effortlessly on each other (that is - in case of small parts - relative motion may even be triggered by thermal motion) so depending on the use case male protrusions penetrating female indents may be needed to prevent that random 2D diffusion motion.
(related: intuitive feel)
Comparison in energy, force and stiffness (stiffness = force per length)
(wiki-TODO: Make a proper table comparing energy, force and stiffness of a single covalent C-C bond to a surface to surface contact VdW bond by showing the areas that are necessary such that the VdW bond can provide the equivalent values than the single C-C bond. Note that these are three different areas.)
(wiki-TODO: find the missing value(s) for stiffness of VdW bonds)
Note: Force is the first spacial derivative of energy and stiffness is the second.
Theory
Please use external sources - there are plenty out there.
Wikipedia: [3]
