  The growing concerns over desertification have spurred research into
technologies aimed at acquiring water from non-traditional sources such as dew,
fog, and water vapor. Some of the most promising developments have focused on
improving designs to collect water from fog. However, the absence of a shared
framework to predict, measure and compare the water collection efficiencies of
new prototypes is becoming a major obstacle to progress in the field. We
address this problem by providing a general theory to design efficient fog
collectors as well as a concrete experimental protocol to furnish our theory
with all the necessary parameters to quantify the effective water collection
efficiency. We show in particular that multi-layer collectors are required for
high fog collection efficiency and that all efficient designs are found within
a narrow range of mesh porosity. We support our conclusions with measurements
on simple multi-layer harp collectors.
