When contact angles are discussed, the word sessile drop is often used. But what is meant by the sessile drop, and when is it useful?
The word sessile comes from the Latin word sessilis which could be translated as “sitting”. Or in botany, it means attached directly by the base. Both words describe the sessile drop as it is “sitting” on the substrate and attached to it by the base of the drop.
Among all the different methods used to measure the contact angle, the sessile drop is by far the most utilized. It provides a quick method for contact angle measurement as the drop can be easily placed on the sample surface, and analysis is done automatically by the software.
One of the main purposes of the sessile drop method is to define the surface free energy of the solid surfaces. As the surface energy of the solid cannot be directly measured, the sessile drop method is utilized to measure the contact angles of different probe liquids on the surface. The contact angle values are then used to model the surface free energy of the surface.
Due to simple and quick measurement, the sessile drop method is also used to quickly check the wettability of the surface. Typically, the measurement is done with water as the water contact angle will determine the hydrophilicity of the surface. Surface hydrophilicity is often needed for example when coatings are applied as hydrophilicity is a prerequisite for good wettability of the surface. Sessile drop measurements are thus also often used as a quality control tool prior to coating to ensure optimum surface properties.
In principle, the sessile drop measurement requires a camera and a light source as well as a mean to dispense the droplet on the surface. In practice, these are integrated into one instrument called an optical tensiometer. Optical tensiometers vary from manual to completely automated systems.
The modern instruments utilize high-resolution cameras and LED light for optimum image quality. The dispensing can be done manually but much more common is to use automated disposable tip dispensers to diminish the contamination risks. The droplet placement on the sample surface is also often done automatically. Sofware is naturally a big part of the sessile drop method as the fitting algorithms are typically utilized to receive the contact angle data immediately after droplet deposition.
The addition of the automated sample stages makes it possible to do multiple sessile drops in a row with one click of a button. This enables the mapping of large areas without user interference in between the measurement points.
If you would like to read more about sessile drop or other contact angle measurement methods, please download the white paper through the link below.
HMDS is used to render the surface of silicon more hydrophobic and thus to match better with the chemistry of the photoresist.
Quality control is needed to detect any possible problems early on and contract angle measurement provides an excellent tool for that.
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