What is Surface Tension?
Surface tension arises from cohesive forces between the liquid molecules. In the bulk, molecules are interacting equally with each other in all directions. However, at the surface, molecules don’t have the same neighbours on all sides therefore, unequal cohesive, intermolecular forces. Thus, a net inward force pulls the molecules towards the bulk. The force is proportional to the cross sectional length of surface. This gives rise to a property called surface tension. How high the surface tension is, is dependent on the type of molecular interactions present. For example, the surface tension of water is high, as the water molecules interact with each other through hydrogen bonds which are relatively strong.
There are several different units for surface and interfacial tension; typically N/m is used.
Surface tension measurements are widely used in many industrial areas as it has a direct effect on the spreading of the coating formulation etc.
Figure 1. Water Surface Growing (Wikipedia, 2013)
In figure 1, imagine the film of fluid is stretched out in the x-direction. A force (F) is required to overcome the cohesive intermolecular forces present within the film of fluid.
Here the force required is:
F = Force (N)
𝛾 = Surface Tension (N/m)
L = Length (m)
Work done in the moving distance dx is:
Equation 1.13 represents the change in surface area of the fluid film. The number 2 is used in the equations because there are 2 surface which are the front of the fluid film and the back of the fluid film. As both sides are being stretched, both of the sides contribute to the work being done.
What Is Surface Tension A Measure Of?
Surface tension is a measure of the average force pulling the molecules in the interior of a liquid to the surface. The surface tension of water is comparable to the tension of a thin elastic film, such as the skin on a bowl of water, relative to the weight of the water. The value of the surface tension can range from fractions of a dynes/cm for highly refined oils, to hundreds of dynes/cm for coarse surfactants.
For extended surfaces, the value of surface tension is linearly related to the surface area. For example, a flat plate has a larger surface area and hence larger surface tension than a sphere of radius equal to the plate thickness.
Du Noüy Ring Method
The Du Noüy ring method is a simple, precise, and sensitive method for determining surface tension, and has been in use for over a century. It is one of the oldest methods in surface tension determination, and was introduced in 1838 by Du Noüy of the École Polytechnique (France) 1.
You will need the following materials in order to carry out a test:
- A flask, partially filled with water.
- A flask which holds a water drop precisely in its centre.
- A Du Noüy ring.
- A steady hand.
- A force balance.
Place the flask containing partially filled water at the same level as the horizontal base of the Du Noüy ring. Observe the surface tension as you adjust the flask. Using a force balance; calculate the surface tension of water.
- DO NOT adjust the flow rate.
- DO NOT TOUCH the glass plate.
- DO NOT TOUCH the plate during your experiment.
The height of the horizontal base of ring is 10 cm
Place 9 cm. of water in the flask.
Place the Du Noüy ring at the centre of the drop with the horizontal base of the ring exactly on the drop.
Carefully move the ring downwards and observe.
Record the reading.
Wikipedia. (2013). Surface growing. Retrieved from Wikipedia: