Surface Acoustic Waves: a New Paradigm for Driving Ultrafast Biomicrofluidics

Surface acoustic waves (SAWs), which are 10 MHz order surface waves roughly 10 nm in amplitude propagating on the surface of a piezoelectric substrate, can offer a powerful method for driving fast microfluidic actuation and microparticle or biomolecule manipulation. We demonstrate that sessile drops can be linearly translated on planar substrates or fluid can be pumped through microchannels at typically one to two orders of magnitude faster than that achievable through current microfluidic technologies. Micromixing can be induced in the same microchannel in which fluid is pumped using the SAW simply by changing the SAW frequency to superimpose a chaotic oscillatory flow onto the uniform through flow. Strong inertial microcentrifugation for micromixing and particle concentration or separation can also be induced via symmetry-breaking. At low SAW amplitudes below that at which flow commences, the transverse standing wave that arises across the microchannel afford particle aggregation and hence sorting on nodal lines. Other microfluidic manipulations are also possible with the SAW. For example, capillary waves excited on a sessile drop by the SAW can be exploited for microparticle or nanoparticle collection and sorting. At higher amplitudes, the large substrate accelerations drives rapid destabilization of the drop interface giving rise to inertial liquid jets or atomization to produce 1–10 μm monodispersed aerosol droplets. These have significant im∗Address all correspondence to this author. plications for microfluidic chip mass spectrometry interfacing or pulmonary drug delivery. The atomization also provides a convenient means for the synthesis of 150–200 nm polymer or protein particles or to encapsulate proteins, peptides and other therapeutic molecules within biodegradable polymeric shells for controlled release drug delivery. The atomization of thin films containing polymer solutions, in addition, gives produces a unique regular, long-range spatial polymer spot patterning effect whose size and spacing are dependent on the SAW frequency, thus offering a simple and powerful method for surface patterning without requiring physical or chemical templating. INTRODUCTION Surface acoustic waves are 10 nm order amplitude elastic waves that propagate along the surface of a piezoelectric substrate. The SAW devices here predominantly consist of a double port interdigitated transducer (IDT) with several aluminum electrode finger pairs sputter-deposited onto a single crystal 127.68◦ yx-cut lithium niobate (LiNbO3) piezoelectric crystal substrate. By applying an oscillating electrical signal to the IDT using a RF signal generator and power amplifier, a SAW can be generated, which propagates across the substrate as a Rayleigh wave with a wavelength λ that correlates with the IDT finger width and spacing; specifically, the finger width and spacing are both λ/4. The corresponding SAW resonant frequency is then f = cs/λ, where cs ≈ 3965 m/s is the speed of the SAW in the substrate. 1 Copyright c © 2009 by ASME Proceedings of the ASME 2009 2nd Micro/Nanoscale Heat & Mass Transfer International Conference MNHMT2009 December 18-21, 2009, Shanghai, China