Study of electrically-induced wetting on silicon single-crystal substrates

Electrically-induced wetting on a silicon substrate shows several stages of droplet formation during the voltage application: (1) intermittent and limited spreading, (2) continuous and unlimited spreading, (3) droplet bursting, (4) generation of microdroplets around the source droplet and (5) further movement of the microdroplets. These stages appeared to be dependent on both the potential and the rate of potential sweep in our experiments. The relationships between these stages of electrically-induced wetting behavior on the silicon substrates and the electrical conditions were experimentally investigated. The positive- and negative-type [111] silicon substrates and a solution of sulfuric acid (0.1 mol/l) for the droplet were used. Experiments were performed using the three-electrode method. When the potential of the substrate was swept from the open circuit potential in the anodic direction (the open circuit potential of the substrates with respect to the reference electrode was about -0.7 to -0.9 V), electrically-induced wetting occurred beyond a threshold of about 0 V. The spreading continued intermittently with the increase in potential, but stopped at 3 V. At this time, the wetted area enlarged about two times of the initial wetted area. Above 3-4 V, droplet spreading re-started, and appeared to be continuous. When the sweep rate was 0.05 V/s or less, the droplet was observed to burst at a potential of 3-4 V. Furthermore, just before the burst of the droplet, numerous microdroplets were generated around the original droplet. Subsequently, the microdroplets combined and moved radially away from the original droplet, and the original droplet burst. These wetting behaviors categorized by applied potential were also observed when the substrate was kept at a constant potential.