Redox control of capillary filling speed in poly(ferrocenylsilane)-modified microfluidic channels for switchable delay valves

We present a method to reversibly change the wetting of gold-coated microchannel walls, and, as a result, alter the capillary filling speed of water inside such modified microchannels. To this end, we employ the redox-response of surface-anchored poly(ferrocenylsilane) (PFS) films, which allows for a controlled and repeatable switching of the surface energy under low voltages (<1 V). Cyclic voltammetry and chronoamperometry were used to confirm the presence of PFS coatings, to determine surface coverage and to study the redox reaction kinetics of the films inside of microchannels where a 0.1 M sodium perchlorate aqueous solution served as electrolyte. SEM images attested thin, uniform and selective surface modification of the gold electrode by PFS films. Oxidation and reduction of the surface-immobilized PFS were both reached after a characteristic time below 20 s. Capillary filling experiments revealed that the meniscus speed clearly depends on the redox state of the PFS film. The velocity of the meniscus was reversibly switched between 1.8 ± 0.1 mm/s (reduced state) and 3.4 ± 0.1 mm/s (oxidized state). Model calculations, based on the Lucas-Washburn equation and adapted to our specific channel setup and partial surface modification, showed a linear dependency of the meniscus position on our observation time interval, in agreement with our experimental results. Meniscus velocities predicted by the model agreed well with the experimentally determined capillary meniscus velocity.