Tensiometer

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Definition:

Force measuring instrument for determining surface / interfacial tensions of liquids, and dynamic contact angles of (regular shaped) solids, fibers, powders and porous materials.

Summary/Descritpion:

The measurement of surface / interfacial tension of liquids and contact angles of liquids against solids as performed by a tensiometer is based on force measurements of the interaction of a solid probe with the interface of a liquid or between 2 immiscible liquids.

In these experiments a probe is hung on a balance and brought into contact with the liquid interface. The forces experienced by the balance as the probe interacts with the surface of the liquid can be used to calculate surface / interfacial tension or contact angles. The forces present in this situation depend on the following factors; size and shape of the probe, contact angle of the liquid/solid interaction and surface tension of the liquid. The size and shape of the probe are easily controlled.

For surface / interfacial tension measurements the contact angle of the liquid towards the probe is controlled to be zero (complete wetting). This is achieved by using probes with high energy surfaces. For example, probes made of a platinum/iridium alloy insure complete wetting and they are very easily cleaned in a reliable way. The mathematical interpretation of the force measurements depends on the shape of the probe used. Two types of probes are commonly used for surface / interfacial tension measurements, the Wilhelmy plate and DuNouy Ring.

The Wilhelmy plate method utilizes the interaction of a platinum plate with the liquid interface being tested. The calculations for this technique are based on the geometry of a fully wetted plate in contact with, but not submerged in, the liquid. In this method the position of the probe relative to the surface is significant. Therefore the measurement is made at the so called zero depth of immersion of the probe, and the force acting on the probe at this position is registered and can directly be used to calculate the surface tension of the liquid when the perimeter of the plate is accurately known.

If only limited quantity of the liquid to be tested is available one may consider using a thin round platinum rod as the probe. In such a case the measurement is exactly the same as with the Wilhelmy plate, but the probe dimensions of the probe will be smaller which affects the accuracy of the measurement and hence also might affect the reproducibility of the end results.

The Du Nouy ring method utilizes the interaction of a platinum ring with the liquid interface being tested. The ring is submerged below the interface and subsequently raised upwards. As the ring moves upwards it raises a meniscus of the liquid. Eventually this meniscus tears from the ring and returns to it�s original position. Prior to this event, the volume, and thus the force exerted, of the meniscus passes through a maximum value and begins to diminish prior to the actually tearing event. The calculation of surface or interfacial tension by this technique is based on the measurement of this maximum force. As an additional volume of liquid which is raised due to the proximity of one side of the ring to the other mathematical corrections are needed in order to obtain the correct surface / interfacial tension values.

The tensiometric method for measuring dynamic contact angles (Advancing and Receding contact angles) measures the forces that are present when a sample of solid is brought into contact with a test liquid. If the forces of interaction, geometry of the solid and surface tension of the liquid are known the contact angle may be calculated. The solid sample has to have a symmetrical and regular shape (rod, cube, round rod, rectangle, wire etc.) in order to use this method. The advantages of this method are that the actual measurement is very simple to perform, one measurement gives an average for the whole sample and that wetting characteristics of single fibers can easily be obtained.

The Tensiometer instrument also enables to determine the wettability characteristics i.e. contact angles of powders and porous solids by using the Washburn method (Powder Wettability Measurement).