Due to the advantages of rapid response and the programmability of droplet movements,electrowetting on dielectric (EOWD) has become a major platform for driving droplets in microfluidic systems [1].For applications using EWOD,efficient mixing of samples and reagents is always desired and is still an issue.To achieve an efficient mixing for microfluidic applications using parallel-plate EWOD devices,mixing enhancement using droplet resonance is studied experimentally in this study.The mixing efficiency of a droplet under resonant oscillations and non-resonant oscillations are discussed to verify the influence of droplet resonance on the mixing.Figure 1(a) shows the EWOD device used in our experiment.To excite the droplet resonance,the driving potential is a periodic square wave with offset as shown in Fig.1(b).Figure 2(a) shows the system that is used to derive the resonant frequencies and the oscillation spectrum of a droplet [2].By using this system,the relative amplitudes of droplet oscillation at different driving frequencies can be measured through detecting the corresponding movement of the probing He-Ne laser.Figure 2(b) shows the setup for the droplet mixing experiment.The 7 μL merged droplet consist of two colored droplets both with volume 3.5 μL is placed on the EWOD device.When the driving potential is applied,the images of the mixed droplet are taken by an OM equipped with a CCD camera.We calculate the degree of mixing by analyzing the hue distribution within the droplet.Figure 3(a) shows the amplitude spectrum for the oscillating water droplet.The major peaks in Fig.3(a) correspond to the droplets resonant frequencies at different resonant states,which are 34,66,99,131 and 165 Hz.Figure 3(b) shows the average mixing time,which is defined as the time for 95% of the mixing completed,when the droplet is driven at different driving frequencies and driving voltages.From Fig.3(b),comparing to the case without EWOD actuation (0 Hz),it can be observed that the mixing time is significantly reduced when exciting the droplet resonance.In contrast,when the droplet is driven at its non-resonant frequencies,the mixing times become much longer,though they are still much shorter than the mixing time for a non-oscillated droplet.