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

Abstract

Droplet actuation using Surface Acoustic Wave (SAW) technology has recently been widely employed in 'lab on a chip' applications. In this paper, SAWs generated by Inter-Digital Transducers (IDTs) were used to actuate micro-droplets, where their velocity and direction could be adjusted by changing only the excitation phase shift, θ, of the voltage applied to the two IDTs. Specifically, an analytical expression for the acoustic Standing Wave Ratio (SWR), operating in the Exciter-Exciter mode, has been derived by the authors, and given as a function of θ and the spatial phase difference. It can be seen from the expression that the directions of the Traveling Surface Acoustic Waves (TSAWs) will be opposite to each other, in the range of θ given by (0, ) and (, 2), and the component of the TSAWs can be adjusted by changing θ in each direction. Following the theoretical analysis discussed here, the IDTs fabricated on a LiNbO3 substrate have been excited to generate a mixture of TSAWs and Standing Surface Acoustic Waves (SSAWs). A series of experiments was carried out to control the velocity and direction of the actuated droplets, by changing only θ. 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 routine Exciter-Absorber mode.

Acoustic-propelled micro/nanomotors and nanoparticles for biomedical research, diagnosis, and therapeutic applications

“…The “excitation-absorption” mode can also be used to generate travelling surface acoustic waves (TSAW), so that liquid-based transportation of micro/nano robots can be achieved. TSAW could be formed either by the use of sound-absorbing gels ( Renaudin et al, 2006 ; Li et al, 2007 ; Collignon et al, 2015 ) or by setting a spatial phase difference between the pairs of the IDTs ( Sui et al, 2022 ; Dong et al, 2023 ), as shown in Figure 2B . Another method for forming TSAW is to use single curved-shape transducer that operates at high excitation frequency from 100 MHz to 1 GHz ( Destgeer et al, 2017 ).…”