Characterizing wetting with transparent probe

We have developed transparent droplet probe techniques that can directly observe the evolution of the wetting contact line and quantify the contact angle precisely. The probe is composed of a transparent glass cantilever with a droplet probe attached to its end. By precisely controlling the volume and location of the droplet probe and using a normal optical microscope, we can quantify the progression of the apparent contact area and apparent contact line irregularity in different types of superhydrophobic silicon nanograss surfaces. Contact angles near 180˚ can be determined with an uncertainty as low as 0.2˚, which a conventional contact angle goniometer cannot distinguish. We also identify the pinning/depinning sequences of a pillared model surface with excellent repeatability and quantify the progression of the apparent contact interface and contact angle of natural plant leaves with irregular surface topography.

Characterizing the wetting properties of fibers is crucial for many research and industry applications, including textiles for water-oil separation and composite materials. Those fibers are often soft, typically tens of micrometers in diameter but millimeters in length, making manipulation and characterization difficult. Contact angles of single fibers are usually determined by droplet shape analysis or force-based Wilhelmy method. However, these methods are unable to accurately measure contact angles above 60 ° or ensure reliable control of the liquid-fiber interaction process, especially for soft fibers prone to bending. 

We use the transparent droplet probe to measure the advancing and receding contact angles of both soft and rigid single fibers. By analyzing side-view images, we extract key geometrical parameters of the disk-droplet-fiber system, which, when used in detailed simulations, allows for estimating the contact angle of fibers ranging from 20 ° to 140 °.