Cheerio effect

Most micro self-assembly processes are performed in a fluid medium and use gravity and/or surface tension for assembly. A prime example is self-assembly of small (submillimeter) flat parts that can be formed into a crystallike structure on a fluid surface caused by minimization of interfacial surface tension (otherwise known in layperson s terms as the Cheerios effect). Another prime example is the utilization of gravitational forces to flow parts over a surface, where shape-recognition is used to place parts and surface tension and van der Walls forces hold the parts in place during subsequent processing. 

Cheerios Effect and its Control by Contact Angle Modulation... 

Keywords Lateral capillary force, wettability, electrowetting-on-dielectric (EWOD), Cheerios effect... 

Figure 3.1 Examples of Cheerios effect, (a) Bubbles migrate to the sidewall [1] and (b) A waterlily leaf beetle climbs the inclined air-water interface by arching its back [4].

The Cheerios effect is due to the horizontal component of the capillary force generated by the interface distortion. In order to find the horizontal component, the shape of the air-liquid interface deformed by objects or walls needs to be determined. Let be a function describing the shape (vertical position) of the interface ... 

The last configuration for Cheerios effect in this article is a spherical particle with capillary charge Q floating in vicinity of an infinite vertical wall. In order to find the horizontal capillary force between the wall and particle, one may consider a simple case where the contact angle at the wall is fixed at 0 = 90°, i.e., the boxmdary condition is... 

Figure 3.15 shows a conceptual sketch how to implement EWOD to control Cheerios effect. In the configuration of the wall and floating object, the EWOD electrode is installed on the wall. Initially, the wall surface of EWOD is hydrophobic, so the slope angle is negative (Figures 3.15(a) and (c)). In this case, as already discussed in the previous section, the... 

Figure 3.15 Concept of EWOD-controUed Cheerios effect, (a) and (b) Low-density object with positive slope angle iff. (a) EWOD is OFF (b) EWOD is ON. (c) and (d) High-density object with negative slope angle y/. (c) EWOD is OFF. (d) EWOD is ON.

In order to control the Cheerios effect, we implemented electrowetting-on-dielectric (EWOD) on the vertical plate. By simply turning on/off the EWOD electrode on the plate, we were able to change the contact angle on the plate and reverse the capillary force between the plate and the floating object. This results in reversing of attraction and repulsion behaviors... 

J.Q. Yuan and S.K. Cho, Free surface propulsion by electrowetting-assisted Cheerios effect , in Proc. IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS), pp. 991-994, San Francisco, CA. (2014). 

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