1 Direct observation of transitions between surface- dominated and bulk diffusion regimes in nanochannels Nicolas F.Y. Durand, *†‡ Claudio Dellagiacoma, §‡ Raphaël Goetschmann, † Arnaud Bertsch, † Iwan Märki, § Theo Lasser, § and Philippe Renaud † Microsystems Laboratory and Biomedical Optics Laboratory, Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland * To whom correspondence should be addressed. E-mail: nicolas.durand@epfl.ch † Microsystems Laboratory § Biomedical Optics Laboratory ‡ These authors contributed equally to this work.