Electrowetting on silicon single-crystal substrates

A water drop on a silicon substrate takes a nearly hemispherical form because of the surface tension of water and the hydrophobic nature of silicon. In this state, we have observed that a drop spreads over the silicon substrate when a direct voltage is applied between the silicon substrate and the electrode (Pt) immersed in the drop. Our purpose is to clarify the electrowetting induced on silicon substrates using ultrapure water and solutions containing various ions, and to apply the phenomena to Microelectro-Mechanical Systems (MEMS) as an effective driving force for microactuators. First, the applied voltage and the current, which induce the electrowetting on silicon substrates using ultrapure water drops, were undertaken. At the same time, some other substrates were also used for comparison. The experiments were then conducted in the same way using certain ionic solutions instead of the ultrapure water. Based on the experimental results, the electrowetting of ultrapure water on silicon substrates occurs only with the application of negative voltages (the voltage and the current were about -3 V and -7 muA, respectively, for an n-type [111] silicon substrate). The spreading of the drops was not observed with the application of positive voltages. The drops of solutions containing sodium or potassium ions spread on the n-type [111] silicon substrates with the application of either positive or negative voltages. The positive voltage and the current were about 2 V and 5 muA. respectively, for 0.1 mol/l sodium sulfate solution on the n-type [111] silicon substrates.