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Name:  Michael L.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2000-2001

When two glass slides are placed together with a drop of water between them they are hard to pull apart, but can be slid apart.

a) Are only cohesive and adhesive forces involved or does atmospheric pressure come into it?

b) How can the forces be so strong?

c) Do the water molecules rotate when you slide the glass slides, thus lessening the forces required to separate the glass slides?

Hi, Michael !!

In this case, atmospheric pressure does not come into it because it acts in all directions. In other words, the atmospheric pressure that acts upon both external surfaces of the glass slide also acts against the surface of water between them, so that the pressure action will be neutralized.

What really comes into action are the adhesive forces. As you know, adhesive forces are considered that between two different bodies; cohesive forces are internal forces of a body, resulting from attraction between the molecules of it. The attractive forces between water and glass ( the glass contain Silicium atoms, negatively charged and water is a polar molecule so that the positive side of water is attracted and causes part of the bound) keep them firmly together.

Due to the big surface of the glass slide, the resultant force is also big. As you know, it is all dependent on how big the surface is. And, in this case, the surface is big enough, so that the force is also big. You say in your question that when you slide the glasses this seems to lessen such forces. Well, this happens because the surface in contact are smaller than before, and the adhesive forces will be smaller.

Alcir Grohmann
Beschaffung SAM

Hi, Michael !!

If the surfaces in contact do not change, than we can figure out the situation like this : a stationary surface, a layer of water over it, and over it, the other surface of glass that moves. Considering the newtonian fluids, you know that : F/A = k . V / L where F = force parallel to the surface of glass A = area in contact; V = velocity of the moving glass in relation to the stationary glass; L = distance between both surfaces k = constant (viscosity). For a given A and L, this force will be bigger depending on how great the velocity becomes. There is a velocity gradient between both surfaces. The water molecules in both glass surfaces are firmly attached to the glass and move together with the moving glass, with velocity V, and have velocity zero, because they are attached to the glass surface that does not move. Between these extremes, the other water molecules move from zero velocity till the bigger velocity, in a linear relationship. This is what is called "limit layer movement".

Now, considering the force necessary to separate both surfaces.

The forces that are put into action depend on how big the surface is. For a given surperficial tension ( F/L ) the bigger the area, the bigger the force to cause separation. It is not possible to say that "to slide is easier than to separate", because the first is dependent on velocity, distance between the surfaces, viscosity ( that is also dependent on temperature ) and the second is dependent on surface ( among other factors ).

To compare extreme cases, it can be - for instance - easier to separate than to slide the surfaces, if you have a high velocity of movement and a small area of contact. In normal situations, you will have the contrary, as you propose in your question when you say " is much easier to slide than separate".

So, the forces will be bigger depending on how the situation is to be considered.

Alcir Grohmann

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