Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paul.fr on 22/07/2007 22:30:49

Title: Adhesive power of water
Post by: paul.fr on 22/07/2007 22:30:49

I was cleaning some old microscope slides yesterday. After rinsing them in a bowl of water, i noticed that some of them had pretty much adhered to eachother. The only way to separate them, without breaking, was to slide them apart.

It was impossible to pull them apart, why is that? what makes water so adhesive?

Title: Adhesive power of water
Post by: DoctorBeaver on 22/07/2007 23:27:40

Isn't it something to do with forming a vacuum as you try to pull them apart?

Title: Adhesive power of water
Post by: lyner on 23/07/2007 00:28:37

Exactly. Atmospheric pressure is much higher than we  fully appreciate.

Title: Adhesive power of water
Post by: neilep on 23/07/2007 04:12:07

Ewe didn't accidently happen to be in the Antartcitc or floating in deep space ?......if you were...ewe may not have noticed that the water had frozen and therefore would have been difficult to pull the slides apart !!.......

...it happens y'know !!......next time, check your orientation !

Title: Adhesive power of water
Post by: daveshorts on 23/07/2007 13:38:57

It is a combination of atmospheric pressure and surface tension. if there is a film of water between the slides, the water will stick to the glass. As you try and pull them apart air will try and get in between them to take up the space you are trying to create, the water will create a meniscus and surface tension will resist the air coming in. Because the distance between the two slides is small the force air pressure can exert is very small (the area is small) and the force the surface tension can exert is large (the water can be very curved) so the air can't get in between the slides, so when you pull them apart you are fighting the whole of atmospheric pressure, which is very hard work...
[diagram=262_0]

Title: Adhesive power of water
Post by: lyner on 23/07/2007 14:38:38

Yebbut the pressure over the sides is proportional to the square of the dimension - the surface tension is only proportional to the circumference.

Title: Adhesive power of water
Post by: paul.fr on 23/07/2007 20:10:12

Glad Dave is back, nice diagrams have been lacking lately.

Title: Adhesive power of water
Post by: daveshorts on 23/07/2007 22:35:01

Quote from: sophiecentaur on 23/07/2007 14:38:38

Yebbut the pressure over the sides is proportional to the square of the dimension - the surface tension is only proportional to the circumference.

Yes so as you make the two plates closer together the more difficult it is for the air to get between them and counteract the air pressure from the other side of the plate as you pull them apart.