  We derive a model for voltage-induced wetting, so-called electrowetting, from
the principle of virtual displacement. Our model includes the possibility that
charge is trapped in or on the wetted surface. Experimentally, we show
reversible electrowetting for an aqueous droplet on an insulating layer of 10
micrometer thickness. The insulator is coated with a highly fluorinated layer
impregnated with oil, providing a contact-angle hysteresis lower than 2
degrees. Analyzing the data with our model, we find that until a threshold
voltage of 240 V, the induced charge remains in the liquid and is not trapped.
For potentials beyond the threshold, the wetting force and the contact angle
saturate, in line with the occurrence of trapping of charge in or on the
insulating layer. The data are independent of the polarity of the applied
electric field, and of the ion type and molarity. We suggest possible
microscopic origins for charge trapping.
