Microfluidic on-demand droplet merging using surface acoustic waves.

Individual droplets can be isolated within microfluidic systems by use of an immiscible carrier layer. This type of two phase systems, often termed "digital microfluidics", find wide ranging applications in chemical synthesis and analysis. To conduct on-chip biochemical analysis, a key step is to be able to merge droplets selectively in order to initiate the required reactions. In this paper, a novel microfluidic chip integrating interdigital transducers is designed to merge multiple droplets on-demand. The approach uses surface acoustic wave induced acoustic radiation forces to immobilize droplets as they pass from a channel into a small expansion chamber, there they can be held until successive droplets arrive. Hence, no requirement is placed on the initial spacing between droplets. When the merged volume reaches a critical size, drag forces exerted by the flowing oil phase act to overcome the retaining acoustic radiation forces, causing the merged volume to exit the chamber. This will occur after a predetermined number of droplets have merged depending on the initial droplet size and selected actuation power.

[1]  Helen Song,et al.  Reactions in droplets in microfluidic channels. , 2006, Angewandte Chemie.

[2]  David A. Weitz,et al.  Electrocoalescence of drops synchronized by size-dependent flow in microfluidic channels , 2006 .

[3]  R. Westervelt,et al.  Dielectrophoretic manipulation of drops for high-speed microfluidic sorting devices , 2006 .

[4]  Anas Alazzam,et al.  Continuous separation of particles in a PDMS microfluidic channel via travelling surface acoustic waves (TSAW). , 2013, Lab on a chip.

[5]  Meng Sun,et al.  Microfluidic static droplet arrays with tuneable gradients in material composition. , 2011, Lab on a chip.

[6]  Helen Song,et al.  A microfluidic system for controlling reaction networks in time. , 2003, Angewandte Chemie.

[7]  Richard M. White,et al.  DIRECT PIEZOELECTRIC COUPLING TO SURFACE ELASTIC WAVES , 1965 .

[8]  G. Whitesides,et al.  Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane). , 1998, Analytical chemistry.

[9]  Nicolas Bremond,et al.  Decompressing emulsion droplets favors coalescence. , 2008, Physical review letters.

[10]  Stephan Herminghaus,et al.  Controlled electrocoalescence in microfluidics: Targeting a single lamella , 2006 .

[11]  D. Weitz,et al.  Fluorescence-activated droplet sorting (FADS): efficient microfluidic cell sorting based on enzymatic activity. , 2009, Lab on a chip.

[12]  A. Wixforth,et al.  Microfluidics for miniaturized laboratories on a chip. , 2008, Chemphyschem : a European journal of chemical physics and physical chemistry.

[13]  Koping Kirk Shung,et al.  Targeted cell immobilization by ultrasound microbeam , 2011, Biotechnology and bioengineering.

[14]  Viktor Skowronek,et al.  Particle deflection in a poly(dimethylsiloxane) microchannel using a propagating surface acoustic wave: size and frequency dependence. , 2013, Analytical chemistry.

[15]  Changyang Lee,et al.  Single beam acoustic trapping. , 2009, Applied physics letters.

[16]  A. Abate,et al.  Ultrahigh-throughput screening in drop-based microfluidics for directed evolution , 2010, Proceedings of the National Academy of Sciences.

[17]  Ximin He,et al.  A double droplet trap system for studying mass transport across a droplet-droplet interface. , 2010, Lab on a chip.

[18]  D. Weitz,et al.  Electric control of droplets in microfluidic devices. , 2006, Angewandte Chemie.

[19]  I-Kao Chiang,et al.  On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves , 2012, Proceedings of the National Academy of Sciences.

[20]  Achim Wixforth,et al.  Acoustic mixing at low Reynold's numbers , 2006 .

[21]  D. Weitz,et al.  Geometrically mediated breakup of drops in microfluidic devices. , 2003, Physical review letters.

[22]  Peng Li,et al.  Surface acoustic wave microfluidics. , 2013, Lab on a chip.

[23]  Malcolm Mackley,et al.  The separation of immiscible liquid slugs within plastic microchannels using a metallic hydrophilic sidestream. , 2011, Lab on a chip.

[24]  David A. Weitz,et al.  Mixing characterization inside microdroplets engineered on a microcoalescer , 2007 .

[25]  Kanglyeol Ha,et al.  A theoretical study of the feasibility of acoustical tweezers: ray acoustics approach. , 2005, The Journal of the Acoustical Society of America.

[26]  Tuncay Alan,et al.  Particle manipulation using an ultrasonic micro-gripper , 2012 .

[27]  D. Weitz,et al.  Dropspots: a picoliter array in a microfluidic device. , 2009, Lab on a chip.

[28]  James Friend,et al.  Particle concentration and mixing in microdrops driven by focused surface acoustic waves , 2008 .

[29]  Tuncay Alan,et al.  Surface acoustic waves for on-demand production of picoliter droplets and particle encapsulation. , 2013, Lab on a chip.

[30]  H. Stone,et al.  Formation of dispersions using “flow focusing” in microchannels , 2003 .

[31]  David N. Adamson,et al.  Production of arrays of chemically distinct nanolitre plugs via repeated splitting in microfluidic devices. , 2006, Lab on a chip.

[32]  T. Franke,et al.  SAW-controlled drop size for flow focusing. , 2013, Lab on a chip.

[33]  Tony Jun Huang,et al.  Surface acoustic wave (SAW) acoustophoresis: now and beyond. , 2012, Lab on a chip.

[34]  T. Pollard,et al.  Annual review of biophysics and biomolecular structure , 1992 .

[35]  A. Lee,et al.  1-Million droplet array with wide-field fluorescence imaging for digital PCR. , 2011, Lab on a chip.

[36]  Daniel Ahmed,et al.  Focusing microparticles in a microfluidic channel with standing surface acoustic waves (SSAW). , 2008, Lab on a chip.

[37]  A. deMello,et al.  Pillar-induced droplet merging in microfluidic circuits. , 2008, Lab on a chip.

[38]  Sehyun Shin,et al.  Size-dependent microparticles separation through standing surface acoustic waves , 2011 .

[39]  K. Franke,et al.  The highly sensitive optical measurement of absolute SAW amplitudes for power flow analysis , 2003, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[40]  Daniel Ahmed,et al.  Acoustic tweezers: patterning cells and microparticles using standing surface acoustic waves (SSAW). , 2009, Lab on a chip.

[41]  Slawomir Jakiela,et al.  Automated generation of libraries of nL droplets. , 2012, Lab on a chip.

[42]  K. Svoboda,et al.  Biological applications of optical forces. , 1994, Annual review of biophysics and biomolecular structure.

[43]  Leslie Y Yeo,et al.  Interfacial jetting phenomena induced by focused surface vibrations. , 2009, Physical review letters.

[44]  Nicolas Vandewalle,et al.  Colloids and Surfaces A: Physicochemical and Engineering Aspects , 2009 .

[45]  G. Whitesides.  The origins and the future of microfluidics , 2006, Nature.

[46]  Lin Wang,et al.  Standing surface acoustic wave (SSAW) based multichannel cell sorting. , 2012, Lab on a chip.

[47]  Adel O. Sharif,et al.  Electrostatic enhancement of coalescence of water droplets in oil: a review of the current understanding , 2001 .

[48]  Erich J. Windhab,et al.  Drop formation in a co-flowing ambient fluid , 2004 .

[49]  Adrian Neild,et al.  The mechanics of microparticle collection in an open fluid volume undergoing low frequency horizontal vibration , 2013 .

[50]  L. R. Fisher,et al.  Interfacial Tensions of Commercial Vegetable Oils with Water , 1985 .

[51]  Nam-Trung Nguyen,et al.  High-throughput micromixers based on acoustic streaming induced by surface acoustic wave , 2011 .

[52]  G. Whitesides,et al.  Formation of droplets and bubbles in a microfluidic T-junction-scaling and mechanism of break-up. , 2006, Lab on a chip.

[53]  K. Yosioka,et al.  Acoustic radiation pressure on a compressible sphere , 1955 .

[54]  D. Grier.  A revolution in optical manipulation , 2003, Nature.

[55]  Gunnar Berg,et al.  A simplified model of electrocoalescence of two close water droplets in oil , 2006 .

[56]  Dan Bratton,et al.  Static microdroplet arrays: a microfluidic device for droplet trapping, incubation and release for enzymatic and cell-based assays. , 2009, Lab on a chip.

[57]  Adel O. Sharif,et al.  Experimental studies of deformation and break-up of aqueous drops in high electric fields , 2003 .

[58]  A. Abate,et al.  Surface acoustic wave (SAW) directed droplet flow in microfluidics for PDMS devices. , 2009, Lab on a chip.

[59]  T. Huang,et al.  Continuous particle separation in a microfluidic channel via standing surface acoustic waves (SSAW). , 2009, Lab on a chip.

[60]  A. Lee,et al.  Alternating droplet generation and controlled dynamic droplet fusion in microfluidic device for CdS nanoparticle synthesis. , 2006, Lab on a chip.

[61]  Tuncay Alan,et al.  Particle separation using virtual deterministic lateral displacement (vDLD). , 2014, Lab on a chip.

[62]  Charles N. Baroud,et al.  Rails and anchors: guiding and trapping droplet microreactors in two dimensions. , 2011, Lab on a chip.

[63]  Mojtaba Ghadiri,et al.  Drop–drop coalescence in an electric field: the effects of applied electric field and electrode geometry , 2003 .

[64]  A. Lee,et al.  Droplet coalescence by geometrically mediated flow in microfluidic channels , 2007 .

[65]  Adrian Neild,et al.  Selective particle trapping using an oscillating microbubble. , 2011, Lab on a chip.

[66]  Wei Wang,et al.  On-demand microfluidic droplet trapping and fusion for on-chip static droplet assays. , 2009, Lab on a chip.

[67]  A. Ashkin,et al.  Optical trapping and manipulation of neutral particles using lasers. , 1997, Proceedings of the National Academy of Sciences of the United States of America.

[68]  K Kirk Shung,et al.  Radiation forces exerted on arbitrarily located sphere by acoustic tweezer. , 2006, The Journal of the Acoustical Society of America.

[69]  Vittorio Cristini,et al.  Design of microfluidic channel geometries for the control of droplet volume, chemical concentration, and sorting. , 2004, Lab on a chip.

[70]  A. deMello,et al.  A microdroplet dilutor for high-throughput screening. , 2011, Nature chemistry.

[71]  L. Rayleigh.  On Waves Propagated along the Plane Surface of an Elastic Solid , 1885 .

[72]  Adrian Neild,et al.  Selective particle and cell clustering at air–liquid interfaces within ultrasonic microfluidic systems , 2013 .

[73]  Thomas Laurell,et al.  Decomplexing biofluids using microchip based acoustophoresis. , 2009, Lab on a chip.

[74]  S. Quake,et al.  Dynamic pattern formation in a vesicle-generating microfluidic device. , 2001, Physical review letters.