Public defence in Engineering Physics, M.Sc. (Tech) Sakari Lepikko

New research helps understanding the mechanisms behind friction at a solid-liquid interface. Public defence from the Aalto University School of Science, Department of Applied Physics.
Friction measurement of a water droplet on a superhydrophobic surface
Friction measurement of a water droplet on a superhydrophobic surface. Photo Sakari Lepikko

Title of the doctoral thesis: Droplet friction on heterogeneous surfaces

Doctoral student: Sakari Lepikko
Opponent: Adjunct Prof. Patrik Hoffmann, Ecole Polytechnique Federale de Lausanne, Switzerland
Custos: Prof. Robin Ras, Aalto University School of Science, Department of Applied Physics

Friction is ubiquitous phenomenon in the world around us. It is often considered to stop or hinder the movement of two solid objects in contact with each other, but friction is present also at the interface of solid and a liquid, for example hindering the movement of a ship or causing sticking of droplets to surfaces. Despite the commonness of the friction between a solid and a liquid, the mechanisms behind its origin are not known accurately. In this thesis, the mechanisms causing friction at the solid-liquid interface are examined and measured. The obtained knowledge can help in designing new surface materials and structures for applications like ship hulls or coatings for various surfaces. 

One of the key findings in the thesis is how surface heterogeneity at the molecular scale affect the friction between a solid and a liquid. The research was performed by coating highly homogeneous silicon substrates with coatings with tunable coverage, resulting in varying levels of heterogeneity on the surface. As a result, the friction was observed to be highest with intermediate coating coverages (high heterogeneity) and lowest with both low- and high-coverage coatings (lowest heterogeneity). This implies that the friction between a solid and a liquid can be tuned by affecting the surface homogeneity for example by the use of coatings. 

Another topic of the thesis was to examine friction of water on so-called superhydrophobic surfaces. These rather uncommon surfaces have the feature of water remaining on top of the surface roughness asperities, meaning that only a fraction of the surface is wetted. It is common for these surfaces that water droplets slide easily off from them. The research shows that the friction of the droplets depends heavily on the contact fraction between the solid and the droplet bottom surface: the larger the contact fraction the larger the friction, and a mathematical relation was determined to describe this. In addition, it was observed that the friction of the droplet is velocity dependent: the faster the water slides, the less roughness asperities it manages to reach, which results in lower contact fraction and friction. If the variations in the asperity heights are small, such dependency of friction on velocity is not observed. These findings help designing superhydrophobic surfaces for various applications such as medical devices or sensor technology. 

Key words: friction, solid-liquid interface, superhydrophobicity, coatings

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