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Dynamic Surface Tension

Figure 1: Spraying a liquid onto a plant surface demonstrates the importance of dynamic surface tension. This is because it influences how quickly the surfactants in the spray solution accumulate at the newly formed liquid-air interface, which affects wetting and coverage.

Figure 1: Spraying a liquid onto a plant surface demonstrates the importance of dynamic surface tension. This is because it influences how quickly the surfactants in the spray solution accumulate at the newly formed liquid-air interface, which affects wetting and coverage.

Surface tension describes the work required to increase the surface area of a liquid. A distinction is made between the static and dynamic surface tension. The dynamic surface tension depends on the age of the surface. Over time, the static surface tension reaches equilibrium.

What is surface tension?

Surface tension describes the work that must be done to increase the surface area of a liquid. It is measured in millinewtons per meter (mN/m). Surface tension is therefore an important parameter for assessing the behavior of a liquid in contact with other phases.

How do surfactants affect surface tension?

Surface tension can be influenced by surface-active substances known as surfactants. Surfactants are amphiphilic: they consist of a polar head group and a non-polar side chain. The polar part of the molecule interacts preferentially with a polar liquid such as water. This part is therefore also called hydrophilic or water-loving. The non-polar part of the molecule interacts preferentially with non-polar liquids, such as oil, or with gases. It is therefore called lipophilic or fat-loving. Due to their two-part structure, surfactants prefer to accumulate on surfaces or interfaces, as they find energetically favorable conditions there.

On a water surface, for example, surfactants orient themselves so that the polar head group points towards the water and the non-polar side chain points towards the gas phase, i.e. the air. This allows surfactants to interact well with both phases. The attachment of surfactants to the surface reduces the surface tension. The addition of surfactants therefore facilitates the mixing of non-polar and polar phases.

The speed at which surfactants accumulate on the surface is a characteristic parameter for each surfactant. It is determined by the diffusion and adsorption speed of the surfactant. For example, if the surface tension of a freshly formed water surface with an added surfactant is measured against air, the measurement initially yields the surface tension value of pure water, i.e. 72.5 mN/m at room temperature. This value decreases over time due to the surfactant.

Static and dynamic surface tension

As the surfactant diffuses to the surface, the surface tension decreases until, after a certain amount of time, a static surface tension is established. The surface tension values measured before equilibrium is reached are referred to as dynamic surface tension. Dynamic surface tension is specified as a function of the surface age. Surface age refers to the time that has elapsed since the surface was formed.

The speed at which surfactants accumulate on a surface is relevant for the investigation of rapid processes. In practice, this includes applications such as wetting surfaces in spray processes in the field of inkjet printing, spray coating or plant protection. Surfactants are also used in other industries, for example in the cleaning agent industry for dishwashing detergents or laundry detergents.

Figure 2: The dynamic surface tension σ decreases with time as the surface-active substances accumulate at the surface. The static surface tension is established after the equilibrium is reached.

Figure 2: The dynamic surface tension σ decreases with time as the surface-active substances accumulate at the surface. The static surface tension is established after the equilibrium is reached.

Method of maximum bubble pressure for determining dynamic surface tension

The dynamic surface tension can be determined using the maximum bubble pressure method. This involves creating gas bubbles in a liquid or liquid mixture, such as water with added surfactant.

In practice, a bubble pressure tensiometer such as the MBP 200 from DataPhysics Instruments is used for such a determination. The aim is to measure the surface tension as a function of the surface age of the bubble. With a constant volume flow, the surface tension value for a certain surface age is obtained. To determine values for different surface ages of the bubble, the volume flow must be varied. This allows dynamic processes, such as the speed at which surfactants accumulate on the bubble surface, to be investigated.

Dynamic surface tension depends on other external variables. Measurements should therefore be carried out at well-defined temperatures and surfactant concentrations.

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Products for Determining the Dynamic Surface Tension

MBP 200 Bubble Pressure Tensiometer

MBP 200 Bubble Pressure Tensiometer

Measurement of the dynamic surface tension of liquid solutions
LDU 25 Liquid Dosing System

Accessories: LDU 25 Liquid Dosing System

Automatic concentration-dependent measurement series
MBPSoftware

Accessories: dpiMAX Software

Performing various measurements with associated software