Public defence in Engineering Physics, M.Sc. Tommi Huhtamäki

Contact angle measurement is the most common method to characterize surface wetting. This thesis examines the limitations of contact angle measurement. Special emphasis is given to the growing measurement errors when characterizing non-wetting surfaces. Public defence from the Aalto University School of Science, Department of Applied Physics.
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Title of the doctoral thesis: Critical study of contact angle goniometry 

Doctoral student: Tommi Huhtamäki
Opponent: Prof. Glen McHale, The University of Edinburgh, Scotland
Custos: Prof. Robin Ras, Aalto University School of Science, Department of Applied Physics

Wetting is a field of science that studies what happens when comes to contact with a solid surface. Wetting phenomena are involved in all facets of human life, from everyday life to modern technology and industry. Wetting phenomena define in the movement of water droplets on the windshield of a car, fogging of eyeglasses and properties of modern waterproof textiles and self-cleaning windows. 

Contact angle measurement has been the main technique to characterize wetting. Properties of a solid surface define whether a drop of water spreads to form a thin film or remain as a spherical cap. Contact angle is the angle of incidence between a drop of water and the solid it rests on. Contact angle of a thin film is 0°, while a spherical droplet has a contact angle of 180°. Modern contact angle measurement is performed by taking video of a small water droplet placed on a solid surface and finding the shape of the droplet through computer software based image processing and fitting. 

Varying contact angles are often reported in the scientific literature for same materials, even for same surfaces. Real surfaces have a range of stable contact angles, and meaningful contact angles need to be separated from meaningless ones. This thesis presents a measurement protocol that allows reliable, repeatable, and reproducible measurement of contact angles. 

The uncertainties of contact angle measurement are also examined in the thesis. An error of ±2° is often reported for contact angles in the scientific literature. Our research shows that he error from the optical measurement system alone approached ±10° when contact angles approached 180°. The magnitude of the errors demonstrate that for the development of next-generation water-repellent materials, better theoretical understanding of contact angles is needed, together with new characterization techniques are needed.

Key words: Wetting, contact angle, superhydrophobic, silicone nanofilaments

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