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Methods of measurement

1. Surface tension by the ring method (Du Nouy method)

The du Nouy ring method is one technique by which the surface tension of a liquid can be

measured. The method involves slowly lifting a ring, often made of platinum, from the surface of

a liquid. The force required to raise the ring from the liquid's surface is measured and related to

the liquid's surface tension.

This technique was proposed by the French physicist Pierre Lecomte du Noüy (1883–1947) in a

paper published in 1925

2. The Du Nouy Padday method

Is a minimized version of the Du Nouy rod replacing a large platinum ring with a thin rod that is

used to measure equilibrium surface tension or dynamic surface tension at an air–liquid

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interface. In this method, the rod is oriented perpendicular to the interface, and the force

exerted on it is measured. Based on the work of Padday, this method finds wide use in the

preparation and monitoring of Langmuir–Blodgett films, ink & coating development,

pharmaceutical screening, and academic research

Du Nouy Padday rod going into liquid showing the maximum force pull of water.

Portable tensiometer that can be used in the field

3. Wilhelmy plate

A Wilhelmy plate is a thin plate that is used to measure equilibrium surface or interfacial

tension at an air–liquid or liquid–liquid interface. In this method, the plate is oriented

perpendicular to the interface, and the force exerted on it is measured. Based on the work of

Ludwig Wilhelmy, this method finds wide use in the preparation and monitoring of Langmuir–

Blodgett films.

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Illustration of Wilhelmy plate method

4. Pendant drop test

In the pendant drop test, a drop of liquid is suspended from the end of a tube by surface tension.

The force due to surface tension is proportional to the length of the boundary between the

liquid and the tube, with the proportionality constant usually denoted γ. Since the length of this

boundary is the circumference of the tube, the force due to surface tension is given by

where d is the tube diameter. The mass m of the drop hanging from the end of the tube can be

found by equating the force due to gravity (Fg = mg) with the component of the surface tension

in the vertical direction (Fγ sin α) giving the formula

where α is the angle of contact with the tube, and g is the

acceleration due to gravity.

The limit of this formula, as α goes to 90°, gives the

maximum weight of a pendant drop for a liquid with a

given surface tension, γ.

This relationship is the basis of a convenient method of

measuring surface tension, commonly used in the petroleum

industry. More sophisticated methods are available when the

surface tension is unknown that consider the developing

shape of the pendant as the drop grows.

The pendant drop test illustrated

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5. Spinning drop method

Spinning drop method (rotating drop method) is one of the methods that is used to measure

interfacial tension. Basically, measurements are carried out in a rotating horizontal tube which

contains a dense fluid. One drop of a less dense liquid is placed inside the fluid. Since the rotation of

the horizontal tube creates a centrifugal force towards the tube walls, the liquid drop starts to be

elongated and this elongation stops when the interfacial tension and centrifugal forces are balanced.

Values obtained at this equilibrium point are used to estimate surface tension of the particular liquid

by using appropriate correlations. A device called “spinning drop tensiometer” is generally utilized

for this purpose.

Spinning drop method is usually preferred for the accurate measurements of surface tensions below

10−2 mN/m. This is because of the limitations in the correlations where a sufficient elongation of the

liquid is required. It refers to either using the fluids with low interfacial tension or working at very

high angular velocities. This method is widely used in many different applications such as measuring

the interfacial tension of polymer blends and copolymers.

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6. Maximum bubble pressure method

In physics, the maximum bubble pressure method, or in short bubble measure method, is a

technique to measure the surface tension of a liquid, with surfactants.

7. Stalagmometric method

The stalagmometric method is one of the most common methods for measuring surface tension.

The principle is to measure the weight of the drops of the fluid falling from the capillary glass

tube, and then calculate the surface tension of the specific fluid which we are interested in. We

know the weight of each drop of the liquid by counting the number of the drops falling out.

From this we can determine the surface tension.

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8. Sessile drop technique

The Sessile Drop Technique is a method used for the characterization of solid surface energies,

and in some cases, aspects of liquid surface energies. The main premise of the method is that by

placing a droplet of liquid with a known surface energy, the shape of the drop, specifically the

contact angle, and the known surface energy of the liquid are the parameters which can be used

to calculate the surface energy of the solid sample. The liquid used for such experiments is

referred to as the probe liquid, and the use of several different probe liquids is required.

An illustration of the sessile drop technique with a liquid droplet partially wetting a solid substrate . θC is the contact angle, and , , represent the solid–gas, gas–liquid, and liquid–solid interfaces,

respectively.

SHAHO ABDUL QADER