Ophthalmic solutions and liquids (eye drops) are used to apply active pharmaceutical ingredients (API) directly to the eye. Surface tension measurement plays a key role for these solutions, as the surface tension is the only parameter manufacturers can control over a wide range that determines the droplet size. According to Tate, drop volume decreases with decreasing surface tension. The accessibility and significance of the surface tension makes it an important parameter for quality control (QC) measurements.