Public defence in Automation, Systems and Control Engineering, M.Sc. Arthur Vieira

The title of the thesis: Wetting Characterization of Hydrophobic Opaque Surfaces and Micro Fibers 

Doctoral student: Arthur Vieira
Opponent: Dr. Michael Kappl, Max Planck Institute for Polymer Research, Germany, and Prof. Pasi Kallio, Tampere University, Finland
Custos: Prof. Quan Zhou, Aalto University School of Electrical Engineering, Department of Electrical Engineering and Automation

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, such as bird feathers and the iconic lotus leaf. Characterizing the degree of hydrophobicity is essential for the development of advanced materials. This is typically done by measuring the contact angle between a droplet and a surface it is placed on. However, wetting in real surfaces is often defined by irregularly shaped droplets and multiple contact angles, which are far from the idealized cases that underly most measurement techniques. 

While there have been significant advances in surface wetting characterization techniques, accurately quantifying the liquid-solid interaction is challenging. Contact angle goniometry is the gold standard of wetting characterization. However, its resolution is limited when the surface is extremely water-repellent (a.k.a. superhydrophobic). This has spurred the development of numerous wetting characterization 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 including the development of a novel transparent droplet probe. These allow quantifying contact angles on flat opaque superhydrophobic and hydrophobic surfaces, as well as cylindrical microfibers that are either hydrophobic or hydrophilic (i.e., attract water). The transparent probe allows direct visualization of the interface between the droplet and surface. This enables accurate measurement of the droplet geometry from which the contact angle can be estimated with great accuracy. On the other hand, the transparent probe is also a highly sensitive force-sensor which provides an alternative way to measure contact angles. 

Lastly, a novel method of measuring the wetting properties of single fibers is also presented in the thesis. These can be textile fibers, such as those found on winter jackets, or can be fibers used in special applications such as those used to capture water from moisture in the air, i.e. water-harvesting nets. The method allows measuring contact angles on soft and rigid fibers that can be either hydrophobic or hydrophilic.

Keywords: wetting, surface characterization, contact line, wetting interface, superhydrophobic, droplet

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact:

Doctoral theses in the School of Electrical Engineering: https://aaltodoc.aalto.fi/handle/123456789/53