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Sketch of the nanoscale dispensing (NADIS) principle. A hollow AFM tip with an aperture at its apex is loaded with liquid. By bringing the tip briefly into contact with the sample, some liquid is transferred by capillarity onto the surface (no external pressure is applied to the liquid). A SPM drives the probe, so that high accuracy is provided in the positioning of the droplets.
(a) Scanning electron micrograph . Tip with a aperture at its apex. (b) Optical micrograph . Upper face of a NADIS probe with the loading area partially filled with liquid. (c) Optical micrograph . Poorly designed loading area. The surface tension of the liquid prevents the liquid flowing towards the acute angle of the triangular-shaped loading area and reaching the hollow tip.
Topographic tapping AFM micrographs. (a) Array of dots made of the nonvolatile residue formed after the evaporation of glycerol droplets deposited by NADIS. (b) Array of polystyrene nanoparticles dispensed by NADIS. The dots consist of a few beads (upper inset). On the hydrophilic surface, the smallest dot spacing achieved is , whereas on the hydrophobic substrate, it is about . The misalignment of the dots is due to the hysteresis of the SPM piezo-scanner, and to pinning effects of the three-phase contact line (droplet border) during evaporation.
Theoretical axial symmetric shapes of the liquid meniscus between sample and tip in hydrostatic equilibrium (graph unit: micrometer). The thick line represents the tip, and the horizontal axis the sample surface. The mean curvature radius is identical in all three cases. (a) With hydrophilic tip and sample surfaces, hydrostatic equilibrium results in a huge meniscus. (b) With a tip having a hydrophobic outer wall, the liquid will spread less over the hydrophilic sample. (c) A small meniscus is obtained with a hydrophobic sample.
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