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Schematics of the conventional and floating electrode electrowetting. A platinum wire (0.05 mm in diameter with 99.95% purity) is used as the auxiliary electrode immersed in the droplet.
Contact angle measurements on an oxidized Si wafer coated with 300 nm Cytop in conventional and FEE electrowetting systems. The droplet voltage was ramped to positive and negative values in 5 V increments.
(a) Contact angle versus voltage and (b) current versus voltage on three different electrowetting substrates, namely Si/SiO2/Cr/Cytop, Si/SiO2/Al/Cytop, and Si/SiO2/Cytop in the conventional electrowetting system (the electrical connections are made to chromium, aluminum, and silicon layers, respectively). In each test, 15 μl droplet of 0.1M citric acid is placed on the hydrophobic Cytop layer, and then the substrate voltage is ramped up to +70 V in 1 V/70 ms increments with respect to a platinum wire immersed in the droplet. The current passage through the circuit is measured concurrently. To better understand how the droplets contact angle is modulated on the substrates, the contact angle versus voltage curves are also presented in this figure (Figure 3(a) ).
FEE: (a) on a 500 nm SiO2 coated with 300 nm Cytop before voltage application; (b) after voltage application without the conductive layer below the Cytop and (c) with a conductive layer (Cr or Al) between the SiO2 and the Cytop after voltage application. The insets show the corresponding droplet snapshots. The tests with the same magnitude and opposite polarity of the droplet showed the same results, where FEE occurred only on the wafer without the conductive layer. The solid circles on the applied voltage/time curves show the time at which the droplet snapshots were taken.
(a) Memory effect after electrode retrieval in FEE compared with non-charged evaporating droplet. The pictures in the first row show a droplet after applying +80 V in the FEE configuration and removing the electrode from the droplet. The corresponding video of the droplet charging and discharging steps is available online. The pictures in the second row show a droplet placed on the wafer without charging. In both cases droplet volume decreases due to evaporation, but the contact angle stays the same. (b) Contact angle (left and right angles) of the FEE charged and non-charged droplets (enhanced online). [URL: http://dx.doi.org/10.1063/1.4807018.1]doi: 10.1063/1.4807018.1.
Timeline of the floating electrode electrowetting memory effect video in Figure 5(a) .
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