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(a) Energy curves for several applied voltages as a function of the height of the liquid protrusion (water). For low voltages the energy curve shows a minimum very close to the sample surface. Above a certain critical voltage the curve shows two minima. A local minimum close to the surface and an absolute minimum where the liquid fills the tip-surface gap. By increasing the voltage the local minima disappears. Average tip-surface separation and . Schematic representation (not to scale) of the geometry of the interface associated with (b) the first local minimum (protrusion) and (c) the absolute minimum (liquid bridge).
Electric field dependence on the liquid shape (bistable regime). (a) Water, and (b) ethanol, . Tip radius . Open symbols are for the liquid protrusion while filled symbols are for a liquid bridge. Fields are 10–100 times higher in the presence of a meniscus because the dielectric constants of liquids are higher than the dielectric constant of air. The fields are calculated at the sample’s solid-liquid interface just under the tip’s apex.
Experiment and theory threshold voltage dependence on tip-surface separation. (a) Water, and (b) ethanol, . Tip radius . The inset shows the dependence of the maximum electric field on tip-surface separation just before (solid line) and after (dotted line) the meniscus formation.
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