Acoustofluidics along inclined surfaces based on AlN/Si Rayleigh surface acoustic waves

Abstract

Conventional acoustofluidics are restricted to manipulation of droplets on a flat surface, and there is an increasing demand for acoustofluidic devices to be performed at inclined surfaces to facilitate multilayered microfluidic device design and enhance system compactness. This paper reports theoretical and experimental studies of acoustofluidic behaviors (including transportation/pumping and jetting) along inclined surfaces using AlN/Si Rayleigh surface acoustic waves (SAWs). It has been demonstrated that for droplets with volume smaller than 3 μL, they could be efficiently transported on arbitrary inclined surfaces. The gravity effect would play a more and more important role in uphill climbing with the increased inclination angle. When the inclination angle was increased up to 90º, a higher threshold power was needed to transport the droplet and the maximum droplet volume which can be pumped also reached its minimum value. Effects of surface inclination angle on droplet jetting angles could be neglected for their volumes less than 2 μL. Moreover, microfluidic and acoustic heating performances of AlN/Si SAWs were further studied and compared with those conventional ZnO/Si SAWs with the same electrode configurations.

Mechanowetting drives droplet and fluid transport on traveling surface waves generated by light-responsive liquid crystal polymers

“…Fluid propulsion driven by capillary forces is ubiquitous in nature, 1 and it has inspired many fluid manipulation strategies in the industry, such as functional textiles for self-cleaning fabrics, 2 antiviral surface design, 3 and inkjet printing. 4 Capillary forces are also utilized in microfluidics, 5 enabling the transport of fluids by using, e.g., surface acoustic waves, [6][7][8][9][10] molecular motors, 11 or static wetting gradients. [12][13][14][15][16][17][18][19] Wetting gradients can also be actively controlled by applying an electric potential between the droplet and the (conducting or dielectric) substrate, as in electrowetting, [20][21][22][23] and by using materials that exhibit a chemically induced modification of the contact angle when exposed to light.…”

Nanoscale “Earthquake” Effect Induced by Thin Film Surface Acoustic Waves as a New Strategy for Ice Protection

“…Thin film‐based SAWs are different from the conventional SAWs generated on bulk piezoelectric materials in that they are manufactured by depositing a piezoelectric thin film onto a common substrate. [ 10 ] Thin film based SAWs enable the integration of multiple functions onto different substrates such as silicon, glass, metal film, and polymer with arbitrary geometries, [ 15–17 ] making the technology very advantageous for ice protection applications as they have good structural integrity and system integration capabilities. The propagated waves can be coupled into the water droplets that lead to turbulence and significant streaming, thus preventing ice nucleation.…”

A rapid and controllable acoustothermal microheater using thin film surface acoustic waves

“…However, it will also lead to less acoustic wave generation, thereby weakening the heating effect by the acoustic waves. This is also the reason that a higher threshold microfluidic actuation power is needed for AlN thin film SAWs when compared to those of ZnO thin film SAWs [25].…”

“…In comparison with the above methods, SAW heating technologies have obvious advantages such as low cost, low power consumption, miniaturization and easy implementation [21,22]. In addition, the interaction between the SAW and the liquid can induce the internal streaming within the liquid [23][24][25], thereby enhancing the heat transfer by forced convection and leading to a uniform temperature distribution [26]. At present, certain types of biochemical and biomedical applications including synthetic chemistry, blood coagulation monitoring and polymerase chain reactions (PCRs) have been implemented using SAW heating technologies [27][28][29].…”

“…In comparisons, thin film SAW technologies, such as those based on ZnO or AlN thin films, have significant advantages including suitability for flexible electrode designs, good tolerance to high voltage/power, good thermal stability (e.g., AlN films can maintain good piezoelectric properties in air up to 700℃) and easy integration with microelectronics [30,35]. Another key advantage using piezoelectric thin films is that they can be easily deposited onto various substrates such as silicon, glass, metal and polymer to realize new functions [25]. When piezoelectric films are deposited onto silicon substrate, the fracture or cracking of the SAW devices has seldomly been observed even at a high RF power of 60~70 W for up to a few minutes, mainly due to the high thermal conductivity and good fracture strength of the polished Si substrate [34].…”