The discussion centers on the strong attractive force observed between two smooth surfaces, specifically referencing the Van der Waals force and its practical applications. Participants mention phenomena such as cold welding and friction stir welding, highlighting the importance of surface smoothness and cleanliness for effective bonding. The Johansen Blocks are cited as a practical example of this effect, demonstrating how perfectly machined metal pieces can adhere to each other without the presence of air. The conversation also touches on the Casimir Effect, suggesting a broader context for understanding these forces.
PREREQUISITESStudents, physicists, engineers, and anyone interested in material science and the principles of adhesion between smooth surfaces.
If the surfaces are smooth and flat enough and free from contamination (oxides, dirt etc.), you should be able to join to pieces of metal or glass together by putting the surfaces together. If no air is trapped, the molecules of the two surfaces start sharing electrons, and you end up with a sort of weld where the two surfaces disappear. I am not sure what the name is for this.LENIN said:I was just serching true the local library and I fount this qouit old book on physical expriments and meserments. Smewhere in inside the avtor talks about a strong attractive force that accourse when you put two bodys with very smooth surfices close together. I wonder if any of you ever heard of something like theat?
Danger said:Lenin, what you're referring to is a demonstration of the Van der Waals force. I remember doing it in high-school with a couple of perfectly machined steel pieces called Johansen Blocks (sp?). When you slide them into contact, rather than just push them together, they become essentially one piece of metal. You can hold one, shake it around, whatever, and the other will remain stuck to it. The bonds are not as strong as regular molecular ones, though, so you can with some effort slide them apart again, or maybe even snap them in two. As Andrew explained, it's due to electron sharing.
This sounds like a parlor trick. Anything with moisture like food will naturally stick together. I'll bet it wouldn't stand up to the shear force from a stiff feather.Gelsamel Epsilon said:Reminds me of the Anime Rurouni Kenshin, where he gets a kitchen knife made by an extremely skilled sword maker and cuts some vegetable in half so precisely that he just puts it back together and it's whole again! xD
It wasn't an experiment, just a demonstration. The experimental part would be in seeing if you could machine the parts accurately enough. There can't be a variance greater than something like .0001 " over the entire contact surface of each block. At that time (35 years ago), it was quite expensive to have them made.TuviaDaCat said:can u give me the experiment details? sound interesting, and if u did that at high school, I am sure that i could make the experiment at home...
Danger said:It wasn't an experiment, just a demonstration. The experimental part would be in seeing if you could machine the parts accurately enough. There can't be a variance greater than something like .0001 " over the entire contact surface of each block. At that time (35 years ago), it was quite expensive to have them made.