However, current tensiometers are not sensitive enough to detect droplet adhesion forces below 1вЂ‰ВµN. Smaller forces correspond to larger contact angles. It has been experimentally verified that snap-in (first droplet contact) and pull-off (droplet separation) adhesion forces on hydrophobic surfaces are related to, respectively, the advancing contact angle ( Оё adv) and the receding contact angle ( Оё rec) 19. Wetting properties have also been characterized by droplet friction forces, i.e., resistance to lateral motion 20, 21, 22 and by droplet adhesion forces, i.e., resistance to detaching a droplet in the normal direction 8, 19, 23, 24. Moreover, as an optical method, contact angle measurements become increasingly inaccurate for contact angles beyond 150В° 17, 18 due to resolution limit of the optical system, and often suffer from obscured view of the contact line on curvy surfaces 9. As the measurement is based on observing a moving contact line, it is inherently unsuitable for precise spatial mapping. The contact angle method, describing a surface by a single pair of apparent advancing and receding contact angle values, is still viewed as the gold standard in hydrophobic surface characterization. Even though these variations have been considered in theory 1, so far they have not been probed experimentally, partly because existing contact angle and force-based methods lack sensitivity and spatial resolution 17, 18, 19. Such irregularities in surface texture and chemical composition lead to spot-to-spot variation of wetting properties 11, 12, which may affect or even govern droplet mobility 13, 14, icing 15, and condensation 16. Biological superhydrophobic surfaces often contain irregular surface texture and details 9, 10, such as creases or veins, and synthetic surfaces are prone to fabrication defects. Understanding how water-repellency emerges from the microscale and nanoscale features 1 is critical to advance the development of these surfaces 6, 7, 8. Superhydrophobic surfaces enable exceptional functions in biology and technology 1, 2, 3, 4, 5. Furthermore, the technique reveals wetting heterogeneity of micropillared model surfaces previously assumed to be uniform. The microscope allows characterization of challenging non-flat surfaces, like the butterfly wing, previously difficult to characterize by contact angle method due to obscured view. We develop scanning droplet adhesion microscopy, a technique to obtain wetting maps with spatial resolution down to 10вЂ‰Вµm and three orders of magnitude better force sensitivity than current tensiometers. Here, we introduce wetting maps that visualize local variations in wetting through droplet adhesion forces, which correlate with wettability. However, effective ways to quantify and map microscopic variations of wettability are still missing, because existing contact angle and force-based methods lack sensitivity and spatial resolution. These surfaces often contain microscopic irregularities in surface texture and chemical composition, which may affect or even govern macroscopic wetting phenomena. Droplets slip and bounce on superhydrophobic surfaces, enabling remarkable functions in biology and technology.

Plasmalab system 100