Tensiometers like the KRUSS tensiometers measure the surface tension of fluids to give you a better understanding of their interfacial behaviour.

How exactly does a tensiometer measure surface tension? There are a number of different tensiometer types, each with their own unique method of calculating surface tension.

What is a tensiometer?

At its most basic, a tensiometer is an instrument used to measure the surface tension of liquids. This can include:

• liquid coatings
• liquid adhesives
• liquid lacquers

In practical application, they help scientists predict and develop the behaviour of a whole host of products, including cleaning agents industrial parts cleaning, electroplating processes, measure soil moisture, and the wettability, absorption and density of powders.

What is surface tension?

Surface tension is the tendency for fluid surfaces to display an elastic behaviour, acquiring the smallest surface area possible. Think about a pool of liquid on a bench.

• Why does it tend to bunch together?
• Why does the exterior of the mass look like it is bonded by an elastic band?

To answer these questions we must first understand how fluid molecules behave.

The relationship between water and air

At the interface of two molecular masses – fluids, solids or gasses – a lot can happen. The molecules at the interface tend to be more attracted towards their own molecules. This is what gives fluids “shape”. It’s also why they tend to be the smallest size possible, as the molecules at the interface are pulled ‘inwards’.

This is easy to demonstrate with water, a liquid with a higher surface tension than most other fluids. Where water meets air, water molecules have a higher attraction to each other than the air molecules. We see water molecules at the interface of air and water resistant to air and attracted to other water molecules.

Cohesion and adhesion

The two principles at play in all interfacial tensions are cohesion and adhesion:

• Cohesion: Referring to the attraction of molecules of the same kind, this force is responsible for giving fluids their shape. In a mass of water molecules, all molecules are attracted equally to neighbouring water molecules. At the edge or surface, where there are no fluid molecules on one side, there is more ‘pull’ towards the centre, resulting in an inward pressure.
• Adhesion: Adhesion is a similar principle, but defines how molecules from different substances attract to one another, such as the adhesion of water molecules to a glass test tube leading to an upward turning meniscus and contributing to capillary action .

The end result is that there is an inward force on the fluid surface that causes the water to behave as if their was an invisible elastic membrane around the water. This imbalance of tensions is where we get the name “interfacial tension”.

What is interfacial tension?

Interfacial tension means practically the same thing as surface tension. The main difference is that surface tension refers to a single liquid surface, where interfacial tension refers to the interface of two immiscible liquids, solids or gases.

Tip: Surface tension is a subset of interfacial tension where the force exerted by the second surface is zero.

Types of tensiometers

Now that you understand surface tension, let’s look at how you can measure it using a tensiometer.

There are several different types of tensiometer, each measuring the surface tension of a fluid in a distinct way. The best method to use depends on the volume, viscosity, and availability of the fluid.

Contact Angle Goniometer/ tensiometer

The Goniometer is most commonly used to measure the surface tension and interfacial tension of expensive liquids, as it only requires a few drops to get a reading.

Using either the pendant or sessile drop methods, drops of fluid are captured by the Charge-Coupled Device camera, a device used to capture the movement of electrical charge.

The digital image of the charge is processed through computer software using the Young-Laplace equation, which describes the capillary pressure difference at the interface between two fluids.

Wilhelmy Plate tensiometer

The simplest and most accurate form of tensiometer, the Wilhelmy Plate only requires a plate to make contact with a liquid to provide a result. The plate is usually made from filter paper, glass or platinum. In truth, it doesn’t matter what the plate is made from as long as the material can be ‘wetted’ by the fluid. The measurement is of the force exerted on the plate. Because the plate doesn’t move, it is generally a highly accurate measurement.

Du Noüy-Padday method tensiometer

The Du Noüy-Padday method works by dipping a rod in and out of a pool of liquid and measuring the precise force used to extract the rod from the fluid. The main advantages of this method are that it is repeatable and accurate even on fluids with high viscosity. The main drawback is that it requires a pool of fluid so it might not be realistic for expensive liquids.

Du Noüy Ring tensiometer

Similar to the above method, only the Du Noüy Ring tensiometer uses a platinum ring which is submersed and held horizontally in liquid and then pulled out. The ring has to be almost exactly perfect. Any scratches or damage to the ring material can alter the ring significantly.

Bubble pressure tensiometer

By pumping a gas stream into a capillary that is immersed in fluid, this tensiometer is able to measure surface tension through the behaviour expected by bubbles forming at the tip of the capillary.

Recommended tensiometers

The KRUSS tensiometers are among the most accurate devices for measuring static surface tension and interfacial tension of fluids.

Using the tensiometry principal, the Attension range tensiometers measure:

• surface and interfacial tension
• critical micelle concentration
• dynamic contact angle
• surface free energy
• powder wettability
• density

Take your research and development to the next level with accurate measuring tools at cost-effective price points. Contact ATA Scientific to find out more about our preferred scientific instruments today.