Methods to Determine Surface Tension
The methods to determine surface tension are discussed below:
Capillary Rise Method
This way employs the effect of capillarity, which is the rise or descent of liquid in a thin tube because of the equilibrium of intermolecular cohesion and adhesion forces. The height of the liquid column in the capillary tube is measured, and surface tension can be calculated using the following formula:
T= [2ρgh] / [r cos θ]
where,
- T signifies the surface tension.
- ρ is the density of the liquid,
- g is acceleration due to gravity.
- h represents the height of the column.
- r is the radius of the capillary tube.
- θ is the contact angle between the liquid and the tube.
Example: Understanding of surface tension in water by glass capillary tube.
Drop Weight Method
This method uses a drop of liquid, which is placed on the end of a needle or otherwise small tube. The weight and radius of the drop and the needle are being measured. Surface tension can be calculated using the following formula:
T = 4Mg/ πd
where,
- M is the mass of the drop,
- g is the acceleration due to gravity, and
- d is the diameter of the needle.
Example: Surface tension of a liquid is measured using a precision balance and a fine needle.
Wilhelmy Plate Method
In this technique, a thin plate or wire is placed vertically into the fluid so as to measure the force needed for the plate to be taken out of the fluid surface. Surface tension can be calculated using the following formula:
T = (F/2) × w
where:
- F is the force required to detach the plate, and
- w is the perimeter of the plate in contact with the liquid
Example: Identifying the surface tension of a solution based on a Wilhelmy plate instrument.
Bubble Pressure Method
This is the procedure which allows obtaining a bubble in the solution under study and keeping the volume of the bubble on the present level while measuring pressure. Surface tension can be calculated using the equation:
T= 2[Pinside – Poutside]/ r
where:
- Pinside is the pressure inside the bubble,
- Poutside is the atmospheric pressure outside the bubble, and
- r is the radius of the bubble.
Pendant Drop Method
This approach is characterized by using a tiny droplet of a fluid which is held in place by the tip of a needle or tube. The distortion of droplet shape is identified by means of imaging techniques, and the surface tension value is calculated from its geometry. The surface tension measurement of non-wetting liquid can be done by this method and it is quite effective for the purpose.
Du Nouy Ring Method
This technique is carried out by placing an platinum ring into a liquid up to the point when it is completely wetted. This is then followed by a slow withdrawal of the ring from the liquid and finally the force necessary to separate the liquid film from the ring is measured. Surface tension is measured based on the force and the circumference of the ring.
Example: Finding the surface tension of an organic solvent by means of Du Nouy tensiometer.
Maximum Bubble Pressure Method
Like the bubble pressure method, the method consists in the air bubbles formation in the liquid to be studied. Consequently, the technique relies not on the pressure needed to promote a steady bubble size, but the pressure when the bubble stops to collapse. The Laplace’s equation can be employed for the purpose of calculation of the surface tension.
Example: Measuring the surface tension of a surfactant solution by observing the collapse pressure of generated bubbles.
Spinning Drop Method
In this method, a droplet of one liquid is suspended in another immiscible liquid. The system is then centrifuged so the droplet lengthens in shape by forming a cylinder. Surface tension is calculated according to the shape of the droplet under the centrifugal force.
Example: Spinning drop tensiometer employed in assessing the capillary forces/interfacial tension between oil and water.
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Methods to Determine Surface Tension
Surface tension is the property of a liquid surface that allows it to resist external forces. It plays a vital role in natural phenomena and is used in a wide range of industrial processes. There are various ways to find surface tension. Some of the most common methods are capillary rise method, drop weight method, Wilhelmy Plate Method, bubble pressure method, etc. In this article, we will learn about how surface tension is determined both theoretically and practically.