Dynamic measurement of low contact angles by optical microscopy.

Precise measurement of contact angles is an important challenge in surface science, in the design and characterisation of materials and in many crystallisation experiments. Here we present a novel technique for measuring the contact angles of droplets between about 2° and 30°, with lowest experimental uncertainty at the lower end of this range, typically ±0.1°. The lensing effect of a droplet interface roduces the appearance of bright circles in low-aperture light, whose diameter is related to the contact angle. The technique requires no specialised equipment beyond an ordinary optical microscope, and may be used to study the dynamic evolution of the contact angle in-situ during an experiment.