  Surface wettability has a huge influence on its functional properties. For
example, to minimize smudging, surfaces should be able to repel oil droplets.
To quantify surface wettability, the most common approach is to measure the
contact angles of a liquid droplet on the surface. While well-established and
relatively easy to perform, contact angle measurements are crude and imprecise;
moreover, they cannot spatially resolve surface heterogeneities that can
contribute to surface fouling. To address these shortcomings, we report on
using an Atomic Force Microscopy (AFM) technique to quantitatively measure the
interaction forces between a micro-droplet and a surface with piconewton force
resolution. We show how our technique can be used to spatially map
topographical and chemical heterogeneities with micron resolution.
