What is a van der Waal’s force? How would it apply to a gecko’s feet?
I’m paraphrasing here, maybe someone else read the same article in Discovery magazine.
What is a van der Waal’s force? How would it apply to a gecko’s feet?
I’m paraphrasing here, maybe someone else read the same article in Discovery magazine.
The van der vaals force is a weak attraction that exists between all molecules and atoms. It is for instance what keeps petrol molecules together as a liquid and not flying off into space, However it is fairly weak (e.g petrol vapourise easily).
If you are a gecko and need some adhesion (but not too much otherwise your feet would be stuck), the van der waals interactions are ideal.
It arises due to temporary attraction between the negative electrons on one atom and the positive nucleus on another
To add to scm1001:
There are 2 types of VDW forces: dispersion and dipole-dipole. The VDW forces lead to the stickyness of molecules to each other.
The dispersion forces are due to fluctuating dipoles in the molecule (or atom). Larger atoms feel bigger effects of fluctauting dipoles. Adjacent molecules have dipole fluctuations induced by neighbours and around it goes.
Dipole - dipole interactions can exist on top of dispersion force. This is due to one part of a molecule having a greater number of electrons, thus giving it a lopsided charge or dipole.
Geckos feet apparently increase the intimacy of contact of their feet with surfaces through the millions of tiny hairs on their feet. Presumably the hairs deform easily to increase surface contact. This allows the dispersion forces to really have an effect, since the dispersion forces only act over a very short distance.
To further expand on what has already been said, remember that everything is made of atoms, whic consist of a positive nucleus surrounded by a negatively-charged electron “cloud.” Molecules are larger structures that arise from atoms forming bonds between one another. One way they can do this is by the transfer of electrons from one atom to the other. This is an “ionic bond” since in the process, the atoms become electrically charged ions. The atom which loses electrons becomes positively charged, while the atom which gains an electron becomes negatively charged. These bonds are very strong.
The other major type of bond arises when two atoms “share” electrons. In this type of “covalent” bond, the shared electrons are distributed in a “shell” which surrounds the two atoms. There is no net charge change.
The above are bonds within a molecule. There are some additional forces which act between molecules. Most molecules (the interesting ones, anyway) are made up of different atoms attached to each other, and different atoms have different “electronegativities,” which is a measure of how much a particular atom likes electrons. This means that the shared electron(s) shared between two atoms in a molecule will tend to spend more time around one atom and less time on the other. This results in making one portion of the molecule slightly negative, another part slightly positive. This is the “dipole” Antechinus talks about. Water is a good example; the Oxygen is slightly negative, while the Hydrogen atoms in the molecule are slightly positive. These charged regions attract one another (positive to negative) generating an attractive force, and giving water many of its unique properties.
What about molecules which lack a permanent dipole structure? They should repel each other, right? I mean, they are surrounded by a negatively-charged electron cloud, and similar electrical charges should repel one another.
Two things should be remembered. First, the electron(s) which form the bond (depending on how you view it) can only be in one place at a time. At any given instant, it will be one one atom or the other, creating for that instant, a transitory charge difference. This is the basis of the Van der Waals force. It consists of the sum of these minute, fleeting interactions between molecules. It is the weakest of all intermolecular interactions, but when you’re talking about the billions and trillions of molecules in, say the hairs on a gecko’s foot, well, it adds up.
The second thing to remember is that the electron clouds of two molecules do, in fact, repel each other. However, instead of driving the two molecules apart, what results is that the electron clouds surrounding the molecules are distorted, driving the electron(s) into an assymetric distribution around their respective atoms. This is what Antechinus is talking about when he mentions “dispersion.” Dispersion leads to a transient charge distribution within the molecules which can attract or repel, depending on the geometry of and between the two molecules.
Is this why the last two Cheerios float together in the bowl of milk? :wally
very indirectly. In this case the cheerios are held together because of surface tension in the milk. In liquids surface tension is due to van der waals forces (and hydrogen bonding in water) However, note it is not the cheerios attracting each other. It is the fluid trying to adopt the lowest energy.
Hey, FX45
Just curious, what merited the :wally ?
Generally, the “wally” is reserved for for use when driving home a sacastic or derogatory response. Was there something in my or others’ posts which offended you?