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The dynamics of nano-droplets on structured substrates differs qualitatively from the macroscopically expected behavior. The long-range van der Waals interactions acting in such systems induce lateral forces on nano-droplets in the vicinity of chemical and topographical steps even if the three-phase contact line of the droplets does not touch the step. Moreover, macroscopic considerations based on differences or gradients of local equilibrium contact angles are invalid because nano-droplets do not sample the long-range tails of the effective interface potential. As one of the consequences, nano-droplets spanning chemical steps can move towards the less wettable side.
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Rauscher et al. (2009) studied this question.
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