  The evaporation of droplets occurs in a large variety of natural and
technological processes such as medical diagnostics, agriculture, food
industry, printing, and catalytic reactions. We study the different droplet
morphologies adopted by an evaporating droplet on a surface with an elliptical
patch with a different contact angle. We perform experiments to observe these
morphologies and use numerical calculations to predict the effects of the
patched surfaces. We observe that tuning the geometry of the patches offers
control over the shape of the droplet. In the experiments, the drops of various
volumes are placed on elliptical chemical patches of different aspect ratios
and imaged in 3D using laser scanning confocal microscopy, extracting the
droplets shape. In the corresponding numerical simulations, we minimize the
interfacial free energy of the droplet, by employing Surface Evolver. The
numerical results are in good qualitative agreement with our experimental data
and can be used for the design of micropatterned structures, potentially
suggesting or excluding certain morphologies for particular applications.
However, the experimental results show the effects of pinning and contact angle
hysteresis, which are obviously absent in the numerical energy minimization.
The work culminates with a morphology diagram in the aspect ratio vs relative
volume parameter space, comparing the predictions with the measurements.
