The interactions between liquid droplets and solid surfaces
during
wetting and phase change are important to many applications and are
related to the physicochemical properties of the substrate and the
fluid. In this work, we investigate experimentally the evaporation
of pure water, pure ethanol, and their binary mixture droplets, accessing
a wide range of surface tensions, on hydrophobic micro-pillared surfaces
varying the spacing between the pillars. Results show that on structured
surfaces, droplets evaporate following three classical evaporative
behaviors: constant contact radius/pinning, stick–slip, or
mixed mode. In addition, we report two further droplet evaporation
modes, which are a mixed stick–slip mode where the contact
angle increases while the contact radius decreases in a stick–slip
fashion and a mixed stick–slip mode where both the contact
angle and the contact radius decrease in a stick–slip fashion.
We name these evaporation modes not yet reported in the literature
as the increasing and decreasing contact angle mixed stick–slip
modes, respectively. The former ensues because the fluid surface tension
increases as the most volatile fluid evaporates coupled to the presence
of structures, whereas the latter is due to the presence of structures
for either fluid. The duration of each evaporation mode is dissimilar
and depends on the surface tension and on the spacing between structures.
Pure water yields longer initial pinning times on all surfaces before
stick–slip ensues, whereas for binary mixtures and pure ethanol,
initial pinning ensues mainly on short spacing structures due to the
different wetting regimes displayed. Meanwhile, mixed stick–slip
modes ensue mainly for high ethanol concentrations and/or pure ethanol
independent of the solid fraction and for low ethanol concentrations
on large spacing. Contact line jumps, changes in contact angle and
pinning forces are also presented and discussed. This investigation
provides guidelines for tailoring the evaporation of a wide range
of surface tension fluids on structured surfaces for inkjet printing,
DNA patterning, or microfluidics applications.