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Surface tension explained

Figure 1: The high surface tension of water is the reason why a water strider does not sink.

Figure 1: The high surface tension of water is the reason why a water strider does not sink.

The interfacial and surface tension of a liquid allows conclusions about how well the liquid spreads on a solid or mixes with another liquid. It can be measured both with optical and force-based methods. Particularly low interfacial tensions can be determined using the spinning drop method.

What are interfacial and surface tension?

For liquids, the interfacial tension can be equated with the interfacial energy. This is not possible for solids. While the term interfacial tension refers to interfaces between two liquids, the term surface tension refers to the interactions between a liquid and a gaseous phase. An example is the surface tension of water against air. The surface tension has the symbol σ and is given in N/m.

Liquids always strive to reduce their interface. An example is a drop of water in air: it prefers to form a sphere, because a sphere has the smallest possible contact area with the surrounding air. Gravity acts as an external force on this sphere and elongates it, which is why the typical drop-shape of liquids arises.

What are the effects of surface tension?

In practical applications, determining the surface tension allows us to see how liquids wet a solid, how they mix with another liquid, and how they behave against a gas. Generally, the higher the surface tension, the higher the interactions within the phase. This means that liquids with higher surface tension are less likely to mix with another phase.

An example: the surface tension of water (against air) is 72.8 mN/m at 20 °C, and is thus relatively high. Surface tension is the reason why a skin forms on a water surface. This is why, for example, a paper clip floats on the water surface and a water strider can walk on the water. Oil, on the other hand, has a surface tension of only about 35 mN/m - this is why oil spreads easily on a surface or wets it more easily. This property is exploited, for example, in so-called penetrating oils.

Practical applications for the determination of the surface tension of liquids

In chemistry and materials science, the measurement of surface tension gives important insights into the behaviour of liquids on surfaces. This is relevant, for example, in the development of coatings and paints. A low surface tension can cause liquids to spread easily on surfaces and form an even coating, while a high surface tension can lead to uneven wetting.

In the food industry, measuring surface tension is important to ensure the quality of food. If the surface tension is too high, liquids do not penetrate food structures well, resulting in less flavour and aroma absorption. The right surface tension is also important for the formation of emulsions and foams.

In environmental science, measuring surface tension is important for understanding the behaviour of liquids in the environment. This can be important, for example, in the study of oil spills in water bodies or in the development of environmentally friendly cleaning agents.

The measurement of surface tension is also of great importance in pharmacy. Surface tension can influence the adsorption and release of drugs.

Measuring methods for determining the surface tension

Interfacial and surface tensions can be measured using a force-based tensiometer. Common methods for measuring surface and interfacial tensions are the Wilhelmy plate method and the Du-Noüy ring method.

The surface and interfacial tension of liquids can also be determined using an optical contact angle meter. Here, a drop hanging at the end of the dispensing needle is measured. This method is called the pendant drop method.

If very low interfacial tensions between two liquids are to be measured, a spinning drop tensiometer is the measuring instrument of choice. The spinning drop method is based on the optical contour evaluation of a rotating drop.

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Frequently asked questions about surface tension

  • What is surface tension?

    Surface tension is the force per unit length — or equivalently the energy per unit area — acting at the surface of a liquid. It arises because molecules at the liquid–gas interface experience an imbalance of cohesive forces compared with molecules in the bulk, causing the surface to contract to the smallest possible area. Surface tension is denoted σ and expressed in N/m or mN/m.

  • What is the unit of surface tension?

    The SI unit of surface tension is newton per metre (N/m). In practice, millinewton per metre (mN/m) is most commonly used, since typical liquid surface tensions fall in the range of 20–80 mN/m. An equivalent unit is millijoule per square metre (mJ/m²), reflecting the energy-per-area interpretation.

  • What is the surface tension of water?

    The surface tension of water against air is 72.8 mN/m at 20 °C. This relatively high value results from the strong hydrogen bonds between water molecules. Surface tension decreases with increasing temperature and is greatly reduced by surfactants, which adsorb at the water–air interface and disrupt the hydrogen-bond network.

  • What is the difference between surface tension and interfacial tension?

    Surface tension refers to the tension at the boundary between a liquid and a gas (typically air). Interfacial tension is the broader term for the tension at any boundary between two immiscible phases — most commonly two liquids such as water and oil. Both are measured in N/m and reflect excess energy at the interface, but liquid–liquid interfacial tensions are typically lower than the surface tension of either liquid alone.

  • How is surface tension measured?

    The most common techniques are the Wilhelmy plate method (a thin plate is immersed and the wetting force measured), the Du Noüy ring method (a ring is pulled through the surface until detachment), and the pendant drop method (the shape of a hanging droplet is analysed optically). Force-based measurements use a tensiometer such as the DCAT series from DataPhysics Instruments; optical measurements use a contact angle meter such as the OCA series.

  • What factors affect surface tension?

    Surface tension is primarily influenced by temperature (it decreases as temperature rises), chemical composition (polar liquids have higher surface tension than non-polar ones), dissolved substances (surfactants strongly reduce surface tension; electrolytes can slightly increase it), and the nature of the adjacent phase (the value against air differs from the interfacial tension against another liquid).

Products for the Determination of Interfacial and Surface Tension

Contact Angle Meters of the OCA series - Optical measurement of interfacial and surface tension

Contact Angle Meters of the OCA series

Optical measurement of interfacial and surface tension
Tensiometers of the DCAT series - Force-based measurement of interfacial and surface tension

Tensiometers of the DCAT series

Force-based measurement of interfacial and surface tension
SVT 25 Spinning Drop Tensiometer - Measurement of extremely low interfacial tensions

SVT 25 Spinning Drop Tensiometer

Measurement of extremely low interfacial tensions