  The wetting properties of multi-component liquids are crucial to numerous
industrial applications. The mechanisms that determine the contact angles for
such liquids remain poorly understood, with many intricacies arising due to
complex physical phenomena, for example due to the presence of surfactants.
Here, we consider two-component drops that consist of mixtures of vicinal
alkane diols and water. These diols behave surfactant-like in water. However,
the contact angles of such mixtures on solid substrates are surprisingly large.
We experimentally reveal that the contact angle is determined by two separate
mechanisms of completely different nature, namely Marangoni contraction
(hydrodynamic) and autophobing (molecular). It turns out that the length of the
alkyl tail of the alkane diol determines which mechanism is dominant,
highlighting the intricate coupling between molecular physics and the
macroscopic wetting of complex fluids.
