Wetting Characterization of Hydrophobic Opaque Surfaces and Micro Fibers

Abstract

Wetting plays a key role in everyday phenomena, from the adhesion of sand particles in sandcastles to the visibility of windshields of cars under the rain. Of particular interest are the surfaces that repel water, a.k.a. hydrophobic. Characterizing the degree of hydrophobicity is essential for the development of advanced materials, which is typically done by measuring advancing and receding contact angles. However, wetting in real surfaces is often defined by irregularly shaped solid-liquid interfaces and multiple contact angles, which are far from the idealized cases that underly most measurement techniques.

While there have been significant advances in wetting characterization, techniques that accurately quantify the liquid-solid interface are lacking. Contact angle goniometry is the gold standard. However, its resolution falls short in the superhydrophobic regime, which has spurred the development of numerous alternative techniques. Despite their innovations, these are either limited to transparent surfaces, demand specialized equipment, or involve complex experimental procedures.

This thesis presents new wetting characterization techniques based on a transparent droplet probe, with a focus on superhydrophobic surfaces. The methods allow quantifying the advancing and receding contact angles on flat opaque superhydrophobic and hydrophobic surfaces, as well as cylindrical soft micro fibers that are either hydrophobic or hydrophilic.

Firstly, the transparent probe allows direct visualization of the contact line from which the mean advancing and receding contact angles can be measured with an experimental uncertainty as low as 0.2 °, near 180 °. Secondly, a technique based on the finite element method is described that allows measuring the contact angle along irregularly shaped contact lines. The capability is also used to map advancing and receding contact angles on micro-patterned surfaces, with an unprecedented spatial resolution of 3 μm. The technique can distinguish contact angles that vary only 1 ° between zones of the same micro-patterned surface, near 180 °.

Thirdly, a force-based method is presented that allows distinguishing the two key adhesion force components acting on the wetting interface: force due to internal droplet pressure, i.e. Laplace pressure; and the force due to surface tension. The method combines the force information with top-view and side-view images to provide a full description of the droplet-sample interaction, including an alternative way to accurately estimate the contact angle near 180 °.

Lastly, finite-element-method analysis is combined with side-view imaging to characterize the wetting properties of single-fibers. The method is used to estimate advancing and receding contact angles of both soft and rigid fibers, and in both hydrophilic and hydrophobic regimes.