Figure 7. Macroscopic adhesion measurements.

(a) Adhesion force versus spacing ratio for smooth micropillars, nanograssed micropillars and a N. nucifera leaf. (b) Ratio of measured adhesion force per unit length of the contact line of a macroscopic drop to that on a smooth surface versus total pinned fraction for smooth micropillars, nanograss micropillars, flat nanograss and a N. nucifera leaf. Deviation at large pinned fractions is due to interaction of capillary bridges. Dotted line indicates the ratio as predicted by equation (3). Error bars indicate s.e. (c) Adhesion force per unit length of projected contact line of a macroscopic drop normalized to remove the effect of the nanograss hierarchy, plotted against the pinned fraction at the first hierarchy. For the case of nanograssed micropillars, (F/l)₀ was obtained by measuring the adhesion force of a drop on a nanograss surface and normalizing with the projected length of the contact line. For the case of smooth micropillars, (F/l)₀ is simply . Dotted line indicates the normalized force as predicted by equation (4). Error bars indicate s.e.