Abstract
A capillary interaction between floating objects and adjacent walls, which is known as “Cheerios effect”, is a common phenomenon that generates capillary attraction or repulsion forces between them depending on their wettabilities, densities, geometries, and so on. This paper deals with controlling the capillary forces, specifically, acting on objects floating on a dielectric (non-conductive) fluid. A key control input parameter is the wettability (contact angle) of the sidewall adjacent to the floating object. By introducing dielectrowetting to the sidewall and actively changing the contact angle on the sidewall, the capillary force is controlled and easily reversed between attraction and repulsion. In this reversing process, the tilting angle of the sidewall is another critical parameter. A theoretical relation taking the titling angle into account is compared and in good agreement with experimental results obtained from the trajectory of the floating object. Finally, a continuous motion of the floating object is demonstrated using this control where an array of dielectrowetting electrode pads is sequentially activated.
Highlights
Surface-tension driven phenomena are ubiquitous in everyday life
Small water-dwelling natural creatures, such as water striders, take advantage of surface tension to support their own weight and walk on the water surface. Another interesting example of surface tension is the Cheerios effect, which was coined after observations of cereal flakes floating in milk tending to being attracted to or repelled from the sidewall of the bowl [1,2]
We successfully demonstrated continuous propelling motions of floating objects in a linear path between two vertical walls by sequentially activating arrays of Electrowetting on dielectric (EWOD) electrodes
Summary
Surface-tension driven phenomena are ubiquitous in everyday life. For example, when a glass capillary tube is dipped into water, the water level ascends spontaneously inside the tube due to capillary force. By installing electrowetting electrodes on the floating object or sidewall and applying an electric potential to the electrodes, the air–water interface of the electrodes can be distorted to change the slope angles and control the capillary forces between the object and sidewall By extending this concept, we successfully demonstrated continuous propelling motions of floating objects in a linear path between two vertical walls by sequentially activating arrays of EWOD electrodes. Unlike the previous work with electrowetting where a vertical wall was used, we found that tilting the sidewall is critical in order to control capillary interaction and to reverse motions between attraction and repulsion This is due mainly to the fact that most of the dielectric liquids typically have low gas–liquid interfacial tensions. This control of the Cheerios effect is extended to continuously propel small floating objects along a linear path by using two arrays of electrodes
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