title: Ultrafast microfluidics using surface acoustic waves.
abstract: We demonstrate that surface acoustic waves (SAWs), nanometer amplitude Rayleigh waves driven at megahertz order frequencies propagating on the surface of a piezoelectric substrate, offer a powerful method for driving a host of extremely fast microfluidic actuation and microbioparticle manipulation schemes. We show that sessile drops can be translated rapidly on planar substrates or fluid can be pumped through microchannels at 1-10 cms velocities, which are typically one to two orders quicker than that afforded by current microfluidic technologies. Through symmetry-breaking, azimuthal recirculation can be induced within the drop to drive strong inertial microcentrifugation for micromixing and particle concentration or separation. Similar micromixing strategies can be induced in the same microchannel in which fluid is pumped with the SAW by merely changing the SAW frequency to rapidly switch the uniform through-flow into a chaotic oscillatory flow by exploiting superpositioning of the irradiated sound waves from the sidewalls of the microchannel. If the flow is sufficiently quiescent, the nodes of the transverse standing wave that arises across the microchannel also allow for particle aggregation, and hence, sorting on nodal lines. In addition, the SAW also facilitates other microfluidic capabilities. For example, capillary waves excited at the free surface of a sessile drop by the SAW underneath it can be exploited for micronanoparticle collection and sorting at nodal points or lines at low powers. At higher powers, the large accelerations off the substrate surface as the SAW propagates across drives rapid destabilization of the drop free surface giving rise to inertial liquid jets that persist over 1-2 cm in length or atomization of the entire drop to produce 1-10 mum monodispersed aerosol droplets, which can be exploited for ink-jet printing, mass spectrometry interfacing, or pulmonary drug delivery. The atomization of polymerprotein solutions can also be used for the rapid synthesis of 150-200 nm polymerprotein particles or biodegradable polymeric shells in which proteins, peptides, and other therapeutic molecules are encapsulated within for controlled release drug delivery. The atomization of thin films behind a translating drop containing polymer solutions also gives rise to long-range spatial ordering of regular polymer spots whose size and spacing are dependent on the SAW frequency, thus offering a simple and powerful method for polymer patterning without requiring surface treatment or physicalchemical templating.
authors.affiliation: MicroNanophysics Research Laboratory, Monash University, Clayton, VIC 3800, Australia.
authors.forename: Leslie Y
authors.initials: LY
authors.lastname: Yeo
authors.forename: James R
authors.initials: JR
authors.lastname: Friend
references.citation: Lab Chip. 2008 Feb;8(2):221-3
references.pmid: 18231658
references.citation: Lab Chip. 2006 Jun;6(6):776-81
references.pmid: 16738730
references.citation: Biotechnol Bioeng. 2009 Jun 1;103(2):387-401
references.pmid: 19160380
references.citation: Lab Chip. 2007 May;7(5):618-25
references.pmid: 17476381
references.citation: Nanotechnology. 2008 Apr 9;19(14):145301
references.pmid: 21817755
references.citation: Lab Chip. 2008 Sep;8(9):1557-63
references.pmid: 18818813
references.citation: Anal Bioanal Chem. 2008 Jul;391(5):1509-19
references.pmid: 18265962
references.citation: Biomed Microdevices. 2007 Oct;9(5):647-56
references.pmid: 17530412
references.citation: Nature. 2005 Jun 23;435(7045):1045-6
references.pmid: 15973398
references.citation: Nanotechnology. 2008 Nov 12;19(45):455103
references.pmid: 21832762
references.citation: Langmuir. 2008 Oct 7;24(19):10629-32
references.pmid: 18795809
references.citation: Lab Chip. 2004 Aug;4(4):265-77
references.pmid: 15269791
references.citation: Phys Rev Lett. 2008 Aug 22;101(8):084502
references.pmid: 18764621
references.citation: Biomaterials. 2007 Oct;28(28):4098-104
references.pmid: 17588654
references.citation: Ultrasound Med Biol. 1997;23(6):953-9
references.pmid: 9300999
references.citation: Lab Chip. 2005 Mar;5(3):308-17
references.pmid: 15726207
references.citation: Anal Chem. 2003 Oct 1;75(19):5097-5102
references.pmid: 27669630
references.citation: Biomicrofluidics. 2007 Jan 01;1(1):14103
references.pmid: 19693352