Dosing falling drops with a defined volume for contact and roll-off angles

Stefan Benn, Ming Jin, Frank Thomsen, Krüss GmbH

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When it comes to measurements involving falling drops, the drop size and fall height have to be kept constant if comparable results are to be achieved. If the same needle diameter is used, falling drops of the same liquid are indeed always the same size. However, it is almost impossible to specify the volume or to dose different liquids with the same volume. In this Technical Note, we will present a method which can be used to generate drops with an exactly defined volume and purposefully detach them from the needle.
Background information

Although the falling drop measurement is not a standard method for the contact angle, there are some cases where using it makes sense. For highly absorbent materials, for example, the contact angle can often only be measured - in most cases with the help of a high-speed camera - by a drop falling onto the sample. Falling drops can also be used to simulate wetting in the rain. Letting drops fall onto a sloping surface is therefore a common way of measuring the roll-off angle when analyzing self-cleaning surfaces, for example.

It is important to work with defined test parameters when dealing with such issues. Setting the fall height isn't a technical challenge. Measurement with a defined volume causes greater difficulties. If dosing is carried out until the drop falls, then the volume is only constant if both the liquid and the needle diameter are the same. Apparently, the volume can be specifically adjusted only by making tiny changes to the needle diameter, so the time and effort involved is relatively large. Not only does this circumstance make it more difficult to vary the test parameters - for instance, when adding surfactants which reduce the surface tension - but it also reduces the repeatability of wetting tests involving falling drops.

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