Velocity and Direction Adjustment of Actuated Droplets Using the Standing Wave Ratio of Surface Acoustic Waves (SAW)

Droplet actuation using surface acoustic wave (SAW) technology has recently been widely employed in lab-on-a-chip applications. In this article, SAWs generated by interdigital transducers (IDTs) were used to actuate microdroplets, where their velocity and direction could be adjusted by changing only the excitation phase shift, theta, of the voltage applied to the two IDTs. Specifically, the equation of the vibration of the mixed traveling surface acoustic waves (TSAWs) and standing surface acoustic waves (SSAWs) formed on the substrate surface operating in the exciter-exciter mode has been reported by the authors. An analytical expression for the acoustic standing wave ratio has been derived and given as a function of theta and the spatial phase difference. It can be seen from the expression that the directions of the TSAWs will be opposite to each other, in the range of theta given by (0, pi) and (pi, 2 pi), and the component of the TSAWs can be adjusted by changing theta in each direction. Following the theoretical analysis discussed here, two IDTs have been excited to generate a mixture of TSAWs and SSAWs. A series of experiments was carried out to control the velocity and direction of the actuated droplets, by changing only theta. In addition, an experiment performed to compare the techniques shows that the upper limit of the velocity of the actuated droplets can be significantly increased using the exciter-exciter mode, showing that it has the potential to be an alternative method to the traditional exciter-absorber mode.

Automated summary

This article discusses the use of surface acoustic wave (SAW) technology in lab-on-a-chip applications for droplet actuation. By adjusting the excitation phase shift, theta, of interdigital transducers (IDTs), the velocity and direction of microdroplets can be controlled. The authors present the equation for the vibration of mixed traveling surface acoustic waves (TSAWs) and standing surface acoustic waves (SSAWs) formed on the substrate surface operating in the exciter-exciter mode. They derive an analytical expression for the acoustic standing wave ratio as a function of theta and the spatial phase difference. Experimental results demonstrate the potential of the exciter-exciter mode in increasing the velocity of actuated droplets compared to the traditional exciter-absorber mode.