Free surface propulsion by electrowetting-assisted ‘Cheerios effect’

This paper presents how “Cheerios effect” is combined with EWOD (electrowetting on dielectric) in order to propel floating objects. Cheerios effect is a common phenomenon in which small floating objects are either attracted or repelled by the sidewall. This behavior is known to be highly dependent on the hydrophobicity of the walls. By electrically controlling the hydrophobicity (contact angle) of the sidewalls in the canal and thus distorting the adjacent interfaces, cheerios effect can be electrically controlled to manipulate floating objects. This work first verifies EWOD-assisted cheerios effect on propulsion. Using this effect, a linear propulsion of millimeter-sized floating objects in a canal is achieved. By sequentially applying a voltage to the arrayed EWOD electrodes on the sidewalls and by carefully controlling the time interval between electrode switching, floating objects can be propelled continuously along the mini canals. In addition, the similar propulsion is achieved in an even smaller canal whose dimension is comparable to the capillary length. Finally, circular rotation of floating objects is realized by arranging EWOD electrodes along a circular path and applying electrical signal sequentially.