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Sessile drop technique

The sessile drop technique is a test performed to determine the chemical affinity that a liquid has to a solid. The test is usually done to either examine the physical properties of the liquid against different solid surfaces or the properties of a solid surface against different liquids.

Experimental method

A drop of liquid is placed (or allowed to fall from a certain distance) onto a solid surface. When the liquid has settled (has become sessile) the drop will retain its surface tension and become ovate against the solid surface. The contact angle at which the oval of the drop contacts the surface determines the affinity between the two substances. That is, a flat drop indicates a high affinity, in which case the liquid is said to wet the substrate. A more rounded drop (by height) on top of the surface indicates lower affinity because the angle at which the drop is attached to the solid surface is more acute. In this case the liquid is said not to wet the substrate.

Potential problems

The presence of surface active elements such as oxygen and sulphur will have a large impact on the measurements obtained with this technique. Surface active elements will exist in larger concentrations at the surface than in the bulk of the liquid, meaning that the total levels of these elements must be carefully controlled to a very low level. For example, the presence of only 50 ppm sulphur in liquid iron will reduce the surface tension by approximately 20%.

Practical applications

This technique is very useful in determining the surface tension and density of different liquids. It is also useful to determine the effectiveness of waterproofing, for example, as water droplets will have higher affinity for untreated wood, and lower affinity for treated wood. It can also be applied in the testing of oil spill recovery techniques using different recovery materials in different conditions.

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