IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# A drop of Water Makes Slides Stick Together

## Introduction

We have read in books about cohesive and adhesive forces. When two objects made of the same material are in contact with each other, the force acting between the molecules of the surfaces in contact is called cohesive force. However, when two objects of different materials remain in contact, the force between molecules of the surfaces in contact is called adhesive force. How strong are these forces? Sometimes they are so strong that they may appear to challenge even persons of great physical strength. In this experiment we shall use a drop of water to make two thin slides stick to each other to study these forces.

two glass slides

## Procedure

In biology labs or diagnostic shops meant for blood test, thin rectangular glass plates are used. These plates are called slides. Take two such slides. Place a drop of water on any of the slides. Keep the second slide on the first moving the slides over each other, spread the water drop between them. Now by putting the slides with your hands try to separate them out from each other. Remember you are not supposed to move the slides against each other in a sliding manner. Are you able to pull the slides apart? You will not be able to do it as both the slides strongly stick to each other.

## Discussion

Here we have a thin film of water between the two slides. The adhesive force acting between the glass and water is responsible for sticking up of the slides. Greater the surface area larger will be this force. It is the adhesive force that creates surface tension. We can also understand the strong sticking up of the slides by means of the formula for surface tension. The water trapped between the slides has a lower pressure as compared to the atmospheric pressure. The outside pressure of the air tries to push the slides towards each other and so we have to apply more force to separate the slides from each other.

## Note

What is the typical value of force given layer thickness, surface tension, area of slide etc?

A water drop of 0.05 cm is squeezed between two glass plates and spreads into area of $$40\, \mathrm{cm^2}$$. If the surface tension of water is 70 dyne/cm then the normal force required to separate the glass plates from each other will be: Ans 45 N.