Wettability gradient on surface by Withdrawing Electrochemical Desorption

"Surface gradient materials, of which the surface physical and chemical properties change . continuously along the materials, have generated much interest for many practical . applications in recent years [1 and ref. therein]. . All sorts of gradient, such as chemical composition, morphology, density, and nanowire . length, have been experimentally realized though different techniques [2, 3 and ref. therein]. . Here we report a method for producing a wettability gradient on gold surfaces, starting from . an alkanethiols self-assembling monolayer. . The new methodology combine the reductive electrochemical desorption of thiols with the . withdrawal of the gold surface from the liquid, by using a linear motor (realized for the . experiment). . The controlled desorption of the alkanethiols led to a linear gradient in the monolayer . coverage, then saturated with a complementary thiol with a different functionality on the tail, . an hydroxyl group. . Chemical Gradient Characterization by Contact Angle . The wettability gradient was measured by static contact angle, showed in the figure, where it . is shown the contact angle along the gradient after the controlled desorption and after the . mixed SAM formation. The wettability gradients on the surfaces induce the self-propelled . motion of a drop along the surface, without external power [4-6]. . [1] Liu, K. and Jiang, L. 2011, Nanoscale, 3, 825-838. . [2] Morgenthaler, S.; Zink, C. and Spencer, N.D. 2008, Soft Matter, 4, 419-434. . [3] Genzer, J. and Bhat, R. R. 2008, Langmuir, 24, 2294-2317. . [4] Daniel, S. et al. 2001, Science, 291, 633-636. . [5] Liao, Q. et al. 2006, Series E: Tech. Sci., 49(6), 733-741. . [6] Lai Y.H. et al. 2010, Lab On A Chip, 10, 499-504. "