Fig. 5.
Active equilibration of sessile water droplets. (A) Optical micrograph of a ∼2.4-fL water drop, which has been formed and stabilized on an ITO substrate at 367 V. Owing to the small contact angle of 4.6° the thin wedge of the drop is not fully visible. Based on optical interferometry (Materials and Methods) the droplet radius is estimated at ∼3.35 µm. The full droplet extent is indicated by a dashed green circle. (B) Sessile water droplet with rD ∼7.5 µm, sustained at 345 V. (C) The equality of measured optical intensity at the background and the positive interference ring, along the profile indicated in B, demonstrates that the droplet is stable. If the droplet fluctuated at a rate invisible to the camera (25 Hz), the interference rings would be smeared. It is also noteworthy that independent of the substantial substrate roughness in B the drop attains an almost perfectly circular shape.