Droplet transportation by adjusting the temporal phase shift of surface acoustic waves in the exciter-exciter mode

Droplet actuation using Surface Acoustic Wave (SAW) technology has been widely employed in 'lab-on-a-chip' applications, such as for on-chip Polymerase Chain Reactions. The current strategy uses the exciter-absorber mode (exciting a single InterDigital Transducer, IDT) to form a pure Travelling Surface Acoustic Wave (TSAW) and to actuate the droplet, where the velocity and direction of the droplet can be adjusted by controlling the on-off and amplitude of the excitation signals applied to a pair of IDTs. Herein, in a way that is different from using the exciter-absorber mode, we propose a method of actuating droplets by using the exciter-exciter mode (exciting a pair of IDTs simultaneously), where the velocity and directional adjustment of the droplet can be realized by changing only one excitation parameter for the signals (the temporal phase shift, θ), and the droplet velocity can also be significantly improved. Specifically, we report for the first time the equation of the vibration of the mixed waves (TSAW and Standing Surface Acoustic Wave (SSAW)) formed on the substrate surface using the exciter-exciter mode. This is analyzed theoretically, where it is shown in this work that the amplitude and direction of the TSAW component of the mixed waves can be adjusted by changing θ. Following that, the velocity and directional adjustment of the droplet has been realized by changing θ and the improvement of the droplet velocity has been verified on a one-dimensional SAW device, using this exciter-exciter mode. Moreover a series of experiments on droplet transportation, along different trajectories in an x-y plane, has been carried out using a two-dimensional SAW device and this has demonstrated the effectiveness of the θ changing-based approach. Here this exciter-exciter mode provides an alternative method for the transportation of droplets in 'lab-on-a-chip' applications.