1Mechanical Engineering, Monash University, Clayton, VIC, Australia
2Melbourne Centre for Nanofabrication, Clayton, VIC, Australia
Microfluidics is a burgeoning new field of research with considerable promise in biomedical applications, yet suffers from a lack of methods for fluid transport, mixing, and manipulation as a consequence of its viscosity and surface tension-dominated behavior at such small scales. The transmission of acoustic waves can be used to solve the problems inherent in microfluidics through acoustic streaming and direct acoustic forces upon particles within. Our objective is to pump fluids in a microchannel containing such particles while demonstrating an ability to mix, concentrate and separate them. In this study we have combined several techniques for transmitting surface acoustic waves along the long axis of a microchannel to either pump fluid within with speeds of ~2 cm/s or efficiently mix it on demand. This is accomplished by choosing either the fundamental or a harmonic resonance of the SAW as formed in the lithium niobate 127.68Y-X propagating substrate: the acoustic wavelength can be changed in the fluid in the channel. We cut rectangular and asymmetrically-cut anechoic cross-sectioned channels along the X-axis of lithium niobate with a variety of widths and a depth of 100 micrometers. We generated SAW on the substrate at 30 MHz using a single-phase unidirectional transducer with an elliptically-shaped focus to form a line of intense SAW aligned with the cut channel. For the anechoic cross-sectioned channel, a split interdigital electrode was used with a ground aligned along the X-axis and a “left” and “right” side of the electrode to permit SAW to be generated on one side of the channel and subsequently be transmitted across it. We find that the transition from weak flow sufficient to manipulate particles in the suspension to strong flow for mixing and pumping is at about 2 nm vibration amplitude for SAW in our system. Using strong acoustic radiation above 1 nm amplitude, uniform (nearly plug) flow is generated if the acoustic wavelength is larger than the width of the channel. Switching to a harmonic so the acoustic wavelength is much less than the width of the channel, the flow changes to a mixing vortex flow in a few milliseconds. Using SAW at less than 1 nm amplitude, suspended particles within can be caused to be organized along lines parallel to the channel’s long axis from direct acoustic forces upon them. Using an anechoic asymmetric cross-sectioned channel allows suspended particles to be driven across the channel using a split transducer design to generate traveling waves across it. A complete method for pumping and mixing fluids in a channel and manipulating the particles within will be demonstrated with ample videos and physical explanation of the phenomena. The work demonstrates for the first time a micro-sized device that can perform previously difficult microfluidics operations on a fixed geometry all enabled by SAW.