Mini-/Micro-Scale Free Surface Propulsion

This work reports theoretical studies and experimental proofs of the propulsion of mini-/micro-scale floating objects that propel on air-liquid interface by using two different principles. The devices are extremely simple and do not include any moving parts. The first principle takes advantage of three-phase contact line oscillation that is activated by AC electrowetting on dielectric (EWOD) to propel the floating object. The capillary wave that is generated by the free surface oscillation is visualized by using the Free-Surface Synthetic Schlieren (FS-SS) method. A 3-D flow field sketch is constructed based on the flow visualizations and PIV measurements. The flow field and trajectories of seeded particles suggest that Stokes drift is the responsible mechanism for the propulsion. The propulsion speed of the floating object highly depends on the amplitude, frequency, and shape of the EWOD signal. These phenomena are also explained by the measured oscillation amplitudes and Stokes drift relations. Additionally, it is shown that a wider EWOD electrode generates a faster propelling speed. Finally, with stacked planar receiver coils and an amplitude modulated signal, a wirelessly powered AC EWOD propulsion is realized. The second principle of floating object propulsion is the Cheerios effect, which is also generally known as lateral capillary force. Four common physical configurations (interactions between two infinite vertical walls, two vertical circular cylinders, two spheres, and a sphere and a vertical wall) are reviewed. Through theoretical analysis, it has been revealed that not the wettability of the surface but the slope angle of the object is the most important parameter for the Cheerios effect. A general rule for this effect is that the lateral capillary force is attractive if the slope angles of the interacting objects have the same sign, otherwise the force is repulsive. In addition to the surface wettability, the size and the density of floating spheres are also important for the slope angle. Active control of the Cheerios effect is achieved by implementing EWOD and dielectrowetting methods to adjust the surface wettability. By sequentially activating micro-fabricated EWOD/dielectrowetting electrodes, linear translations of floating objects in the small scale channel are accomplished. A continuous rotational motion of the floating rod is achieved in a circular container by the EWOD method