Surface acoustic wave induced streaming and pumping in 128° Y-cut LiNbO₃for microfluidic applications

Abstract

This paper provides a detailed study on surface acoustic wave (SAW) induced acoustic streaming and pumping, focusing on the effects of the wave mode and surface modification. SAW devices with wavelengths of 32 and 64 µm were fabricated on 128° Y-cut lithium niobate substrates with aluminium interdigitated transducers. A higher order harmonic mode wave appears in addition to the fundamental Rayleigh wave for samples with metallization ratios less than 0.6. Both waves have demonstrated the ability to induce acoustic streaming and to pump liquid. A high streaming velocity, and hence a high mixing efficiency and a higher acoustic force, can be obtained using the fundamental Rayleigh wave as the high harmonic waves have large propagation losses. A linear relationship between the streaming velocity and RF signal voltage has been obtained, and effective mixing can be achieved. An acoustic wave has also been utilized to manipulate and pump droplets with sizes up to 5 µl, and a moving speed of ∼1.4 cm s−1 has been obtained on an octadecyltrichlorosilane-treated SAW device using a signal voltage of 40 V.

Slippery Liquid-Infused Porous Surfaces and Droplet Transportation by Surface Acoustic Waves

“…For microfluidic applications Rayleigh SAWs, which propagate along a solid surface with their energy confined to within one wavelength of the surface, are often used [13,15,27,28]. These are generated by applying a radio frequency (RF) signal to interdigital transducers (IDTs) which are patterned on top of a piezoelectric substrate ( Fig.…”

“…The energy and momentum of the longitudinal wave dissipated into the droplet provide a pressure or a body force on the droplet in the direction of propagation of the SAW. This provides the basis for droplet movement, pumping and mixing, streaming, manipulation and jetting/nebulisation [13,15,26,27,29].…”

“…This reduces the length of the droplet-solid surface contact line, but also reduces the solid-liquid contact area critical to the acoustic wave-droplet interaction. Various hydrophobic polymer coatings, such as octadecyltrichlorosilane (OTS), Teflon AF ® and CYTOP TM have previously been used to increase the contact angle of the droplet on the surface [13,[27][28][29]. Most of the hydrophobic polymers coated onto SAW device surfaces have contact angles of around 90-120° [13,[29][30].…”

Streaming by leaky surface acoustic waves

“…The potential of these methods has been demonstrated in a number of experiments (Moroney et al 1991;Suri et al 2002;Wixforth 2003;Guttenberg et al 2004;Sritharan et al 2006;Frommelt et al 2008a;Du et al 2009;Tan et al 2009;). Most of these share the same method of excitation of acoustic waves in the fluid, based on leaky surface acoustic waves (LSAWs).…”

Acoustic Wave Based Microfluidics and Lab-on-a-Chip

“…This is several orders of magnitude larger than other microfluidics, which are typically in the range of hundreds of micrometres to several millimetres per second [1,2,13]. Figure 16 shows the streaming velocity as a function of the amplitude of RF signal for LiNbO3 SAW devices with the IDT structure as a parameter [70]. It shows that the SAW device with a shorter wavelength has more power to induce streaming with higher velocity, and more IDT fingers are beneficial for coupling more RF power into the liquid.…”