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.

What is Surface Tension?

Surface tension is a physical characteristic of liquids that appears as a result of cohesion forces between molecules at the points where the liquid-air interface happens. It is the measure to which the liquid surface attempts to reach a compromise or smaller area among the molecules of liquid interface, this ends up in the formation of an incredibly thin film or membrane at the surface. This phenomenon is related to the disturbed distribution of interactions between surface molecules and the rest of the liquid. Surface tension results in various effects, like, capillary action, droplet formation, and giving buoyancy to certain things on the surface of a liquid.

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[P_inside – P_outside]/ r

where:

• P_inside is the pressure inside the bubble,
• P_outside 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.

FAQs on Methods to Determine Surface Tension

What factors affect the surface tension of a liquid?

The surface tension is impacted in term of intermolecular forces, temperature, impurities, and pressure. Intermolecular force may increase in strength which raise the level of surface tension. Temperature and impurities normally will cause the reduction of surface tension.

Can surface tension be measured directly using instrumentation?

Yes, the quantities of surface tension can be measured the direct way by using devices like tensiometers, pendant drop analyzers, and bubble pressure tensiometers. These tools work on the principles, like capillarity and stress measurement, which is why their results are reliable.

How does temperature influence the surface tension of a liquid?

Normally, higher temperature corresponds to a shorter surface tension of the liquid caused by the increased thermal kinetic energy of the molecules, which cause a gradual weakening of the cohesive forces at the surface.

What are some common techniques for reducing surface tension?

Some of the strategies for reducing surface tension include the usage of surfactants or agents that have a surface activity which destroy the interactions of the molecules on the liquid surface. The mechanics include mechanical agitation or modifying the contents of the liquid to change its properties at the interface.

What is the importance of surface tension in biological systems?

Surface tension is one of the key mechanisms in the biological systems that helps in processes like cell formation and cell membrane, lungs during the process of breathing, and formation of biological interfaces such as tears and mucus.

What is the significance of surface tension in the formation of soap bubbles?

А foam formation is an outcome of the molecule properties of a surfactant to reduce surface tension and enlarge the area of existing stable thin movies. Such unique features are highly useful for activities such as bubble blowing extension or foam stabilization.