The contact angle is a measure of wettability and is thus often used to evaluate the surface before bonding or coating. In many cases, contact angle measurements are done with water which can give valuable information about hydrophobicity or hydrophilicity of the surface. Contact angle measurements with water are straightforward as drop formation and deposition on the surface is simple. However, in some applications making measurements with coating formulation or surfactant solution directly is advantageous. If the surface tension of the used formulation is low, the dispensing strategies need to be considered.
In optical tensiometers, the droplet is typically formed at the tip of the needle or disposable tip after which the drop is placed on the sample surface. Needles used are typically stainless steel whereas disposable tips are most commonly polypropylene. When water is dispensed, the tip can be selected quite freely. Water's surface tension is high, leading to good drop formation with practically any tip.
However, when the liquid you are dispensing has a low surface tension value (below 30 mN/m), the side of the tip or needle is easily wetted causing problems in dispensing. To be able to dispense low surface tension liquids, the tip material needs to be selected so that the liquid dispensed doesn’t fully wet the tip material. In the case of low surface tension liquids, a fluoropolymer (such as PFA and PTFE) is required to fulfill the requirement.
If you would like to learn more about contact angle measurements, please watch the webinar through the link below.
Susanna is an Application Scientist at Biolin Scientific. In her PhD thesis, she developed fabrication methods for a new type of inorganic-organic polymers. Microfabricated polymer chips were utilized as tool for biomolecule separation in analytical chemistry.
