  Spilling tea or coffee leads to a tell-tale circular stain after the drying
of the droplet. This phenomenon was termed after the latter example as the
"coffee ring effect". The evaporation of suspension droplets is a complex
physical process, and prediction and control over particle deposit patterns
obtained from sessile droplet evaporation are essential for many industrial
processes such as ink-jet printing or crop-care applications. In this article,
we present a systematic investigation of the effect of surface wettability on
the evaporation dynamics of a particle-laden droplet, including the effect on
the contact line stick-slip, the hydrodynamic flow of the suspended particles
and the resulting particle deposit after evaporation. We tune the wettability
of glass slides using silanisation and quantify the internal flow during the
evaporation by tracking fluorescent tracer particles. We find that the internal
flow shifts from a predominantly outward flow towards the contact line for low
contact angles to an inward flow for large contact angles. Additionally, the
corresponding deposit gradually changes from the typical coffee-ring to a
central stain upon increasing the hydrophobicity of the substrate. Last, we
corroborate these experimental findings with dynamic density functional theory,
modelling the droplet evaporation process and stick-slip behaviour of the
contact line. Our investigation suggests that the wettability of the substrate
can substantially alter hydrodynamic flow within drying droplets and therefore
the resulting particle deposit.
