| Whereas thermodynamics predicts that the contact angle of a liquid droplet at rest on a solid surface is given by the Young-Dupré equation, experiments on real solid surfaces show that this contact angle is not univocal but exhibits a hysteresis which depends on the droplet history. Despite many theoretical and experimental studies devoted to this problem, a quantitative correlation between contact angle hysteresis and surface defects shape or repartition is still missing, mainly due to a lack of experimental techniques allowing to investigate the contact line behavior at nanometer scale. In this work, we studied the pinning of a contact line on nanometric defects by measuring, by atomic force microscopy, the capillary force exerted by a liquid on a non conventional carbon nanotip. This original quasi-1D geometry allowed us to investigate the interaction of the contact line with nanometric defects. We observed localized jumps resulting from the pinning of the contact line on individual defects. A detailed study of the force curves validated the theory of weak and strong defects, postulated by Joanny & de Gennes (J. Chem. Phys., 81 (1984) 552). In particular, we brought the first experimental evidence of weak defects which do not contribute to hysteresis and performed measurement of dissipated energy on strong defects down to values of the order of kT. The measurement of capillary forces on carbon nanotips is therefore a unique method to investigate wetting processes with unprecedented resolution and may open the way to a systematic study of yet unanswered questions on the structure and dynamics of the contact line at the nanometer scale. Preliminary results on the influence of defects on the dissipation at the contact line, investigated by oscillating AFM modes, will be presented. Reference: M. Delmas, M. Monthioux, T. Ondarçuhu*, Phys. Rev. Lett. 106 (2011) 136102. |
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