Droplet friction on heterogeneous surfaces

The ability to control friction between a solid and a liquid is becoming more and more important in various existing applications as well as in novel ones. However, the mechanisms behind this liquid-solid friction are not yet sufficiently understood. This thesis compiles research performed in three publications where origin of the liquid-solid friction of water droplets is examined for various surfaces with properties ranging from hydrophilic to superhydrophobic and from a regular surface structure to a stochastic one. Publication I examines how molecular level heterogeneity of a surface affects the contact line friction between the surface and water. This is performed by preparing and characterizing self assembled monolayers with varying level of molecular coverages of hydrophobic alkyl tails on hydrophilic silicon wafer substrates. The results show that the low- and high-coverage surfaces with least chemical heterogeneity have the lowest friction while the intermediate-coverage surfaces with most heterogeneity have the highest friction. Publication II focuses on the relation of the contact line friction and the liquid-solid contact fraction of superhydrophobic surfaces. The friction is shown to scale with the contact fraction, being lowest with the lowest contact fraction, and a mathematical model is provided to describe this relation. The model works over wide ranges of friction and contact fraction values, both extending almost over three orders of magnitude. Another important message is that conical microstructures can be used to create surfaces with an extremely low liquid-solid contact fraction that results in an extremely low contact line friction. Publication III explores how superhydrophobic surfaces with stochastic roughness in the nanoand micrometre scales affect the liquid-solid friction. The wetting characterization shows that the friction is time dependent such that static droplets have time to adapt to the surface roughness while moving droplets do not have such time. This adaptation increases the liquid-solid contact fraction, which causes the increased contact line friction. Effectively, this creates a static friction barrier that pins static droplets to the surface but does not restrict the movement of already mobiledroplets. The obtained results of Publications I-III help determining critical surface parameters when designing functional surfaces for applications where low friction between a solid and a liquid is needed.