The FYTRONIX Contact Angle tester is used to measure contact angles and surface tensions of liquid droplets.  A contact angle tester is an instrument that measures the contact angle of a droplet on a surface. This is a useful, indirect measurement of surface wetting. When a droplet starts to spread out over a surface, its contact angle becomes <90°. Complete wetting occurs when the droplet is flat, or in other words, when the droplet has a contact angle of 0°. By suspending a droplet from the tip of a needle and comparing droplet models to the image captured by the system, contact angle goniometers can also be used to perform optical tensiometry measurements.

An important application of the contact angle measurement is to determine a solid-surface’s characteristic surface free energy (SFE). Picture a single molecule in a drop of liquid. The molecule is surrounded by a homogeneous environment and will experience cohesive forces from adjacent molecules, causing the molecule to tend to stay in the bulk. As we move toward the surface of the liquid where it is in contact with another phase, the molecule will experience cohesive forces toward the bulk but also some weaker adhesive forces toward the adjacent phase. The result is a net attraction into the bulk that tends to reduce the number of molecules at the surface and increase the intermolecular space between surface molecules. The increased separation requires energy, just like when stretching a spring, and this excess energy gives rise to surface tension and surface free energy (SFE).

Surface tension and SFE are mathematically equivalent but subtly different in their interpretation of the surface. Surface tension is a tension force per length acting in all directions along the surface, and its common units are mN/m or dyn/cm. SFE is interpreted as the energy required to create new surface area, and its common units are mJ/m2 (equivalent to mN/m). The SFE of a solid-vapor surface is a critical property dictating how a liquid will wet the surface. Understanding SFE is important in many applications including coatings, water repellent fabrics, enhanced oil recovery, and much more.