Frequency dependence of surface acoustic wave swimming.

C Pouya,H R Peter,S H Gossage,K Hoggard,T Poole,G R Nash

doi:10.1098/rsif.2019.0113

C Pouya, H R Peter + Show 4 more

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https://doi.org/10.1098/rsif.2019.0113

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Abstract

Surface acoustic waves (SAWs) are elastic waves that can be excited directly on the surface of piezoelectric crystals using a transducer, leading to their exploitation for numerous technological applications, including for example microfluidics. Recently, the concept of SAW streaming, which underpins SAW microfluidics, was extended to make the first experimental demonstration of ‘SAW swimming’, where instead of moving water droplets on the surface of a device, SAWs are used as a propulsion mechanism. Using theoretical analysis and experiments, we show that the SAW swimming force can be controlled directly by changing the SAW frequency, due to attenuation and changing force distributions within each SAW streaming jet. Additionally, an optimum frequency exists which generates a maximum SAW swimming force. The SAW frequency can therefore be used to control the efficiency and forward force of these SAW swimming devices. The SAW swimming propulsion mechanism also mimics that used by many microorganisms, where propulsion is produced by a cyclic distortion of the body shape. This improved understanding of SAW swimming provides a test-bed for exploring the science of microorganism swimming, and could bring new insight to the evolutionary significance for the length and beating frequency of swimming microbial flagella.

Highlights

The properties of surface acoustic waves (SAWs) have been investigated since Lord Rayleigh delivered the first mathematical discussion on the propagation of waves on the free surface of an elastic solid in an address to the London Mathematical Society in 1855 [1]. It was the invention of the interdigital transducer (IDT) in 1965 by White & Voltmer [2], allowing SAWs to be directly excited on the surface of piezoelectric crystals, that enabled SAW devices to be developed for applications such as signal processing
To validate the experimental set-up, which consisted of a SAW device mounted on a polystyrene vessel, the net forward force resulting from the SAW force was first measured as a function of SAW power at a SAW frequency of 11 MHz, as shown in electronic supplementary material, figure S2
The size of the measured SAW swimming force is consistent with the maximum value, 8 mN, obtained by Bourquin & Cooper [14] at a SAW frequency of approximately 11 MHz, transducer aperture of 15 mm, and an acoustic power of 1.7 W

Summary

Introduction

The properties of surface acoustic waves (SAWs) have been investigated since Lord Rayleigh delivered the first mathematical discussion on the propagation of waves on the free surface of an elastic solid in an address to the London Mathematical Society in 1855 [1]. It was the invention of the interdigital transducer (IDT) in 1965 by White & Voltmer [2], allowing SAWs to be directly excited on the surface of piezoelectric crystals, that enabled SAW devices to be developed for applications such as signal processing. Applications of acoustic streaming include micro-manipulators for small particles or cells [8], microchannel transport [9], atomization [10,11], microfluidic-mixing [12], among many others [13]

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