Measuring the contact angle of fibers is a crucial process in understanding their surface properties and wettability. This measurement provides insights into the adhesion characteristics and surface energy of fibers, which are essential for applications in textiles, composites, and other materials. There are several different ways to measure the contact angles of fibers. In this blog post, the methods are reviewed and compared.
The contact angle is the angle formed at the junction where a liquid interface meets a solid surface. It is a measure of the wettability of a surface by a liquid. A smaller contact angle indicates better wettability, while a larger angle suggests poor wettability. Contact angle measurement is straightforward, especially on a flat surface where the droplet is easily placed. However, when fibers or other thin objects are measured, more consideration is needed to select the best method.
Optical Tensiometer provides two means for contact angle measurements on fibers: sessile drop and meniscus method. 
The sessile method involves placing a droplet of liquid onto the fiber and capturing the image of the droplet to measure the contact angle. When the sessile drop method is used, the drop diameter needs to be small enough so that the drop will fit on the fiber. In principle, this means that a picoliter dispenser needs to be used. With picoliter-sized drops, the drop diameter can be down to 100 um. This method is thus suitable for fibers that can support a droplet without significant deformation.
The Meniscus method is another contact angle measurement method where an optical tensiometer is used. In this method, the fiber is immersed in the liquid and the meniscus formed is measured. The method is limited to contact angles below 90 degrees as higher than that would lead to an inward meniscus impossible to measure.
Wilhelmy plate is a contact angle measurement technique that uses a force tensiometer. In this technique, a single fiber is immersed in a liquid, and the force exerted by the liquid on the fiber is measured. This force is used to calculate the contact angle. The Wilhelmy method is particularly useful for fibers that are too small or flexible to support a droplet. One of the challenges of the technique is that fiber diameter needs to be known.
Each of the above-mentioned methods has its benefits and challenges. What should be especially noted is that the results obtained with the different methods cannot be directly compared. This is because each method measures slightly different contact angles. In picoliter measurement, the contact angle is static although when small drops are used, the fast evaporation of the drop can also lead to closer to receding contact angles. In the meniscus method, the fiber is immersed in the liquid after which it’s pulled upwards. Thus, the measured contact angle is closer to the receding one even though the contact line is not moving during the measurement. With Wilhelmy plate measurement, very thin fibers can be measured but the diameter of the fiber needs to be known for reliable results.
Understanding the contact angle of fibers is essential for optimizing processes such as dyeing, coating, and adhesion in composite materials. It helps in tailoring the surface properties of fibers to meet specific application requirements, enhancing product performance and durability.
If you would like to learn more about contact angle measurement techniques for fibers, please register for 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.
