Electric Control of Droplets in Microfluidic Devices

Abstract

The precision manipulation of streams of fluids with microfluidic devices is revolutionizing many fluid-based technologies and enabling the development of high-throughput reactors that use minute quantities of reagents. However, as the scale of these reactors shrinks, contamination effects due to surface adsorption and diffusion limit the smallest quantities that can be used. The confinement of reagents in droplets in an immiscible carrier fluid overcomes these limitations, but demands new fluid-handling technology. We present a platform technology based on charged droplets and electric fields that enables electrically addressable droplet generation, highly efficient droplet coalescence, precision droplet breaking and recharging, and controllable droplet sorting. This is an essential enabling technology for a high-throughput droplet microfluidic reactor.Networks of small channels are a flexible platform for the precision manipulation of small amounts of fluids. [1,2] The utility of such microfluidic devices depends critically on enabling technologies such as the microfluidic peristaltic pump, [3] electrokinetic pumping, [4,5] and dielectrophoreticpump or electrowetting-driven [6] flow; these technologies can form the essential building blocks for the assembly of fluidhandling modules.[7] These modules can be used to perform a variety of key tasks including the measurement of precise aliquots of fluids, the combination of fluid streams, and the mixing of multiple fluid components. The assembly of such modules into complete systems provides a convenient and robust way to construct microfluidic devices. These have myriad uses; for example, high-throughput screening, [8] the exploration of chemical phase diagrams, assays of biological molecules, [9][10][11] single-cell analysis, [12][13][14][15][16][17] and combinatorial approaches to protein crystallization [18] can all be performed with only minimal consumption of reagents. However, virtually all microfluidic devices are based on flows of streams of fluids; this sets a limit on the smallest volume of reagent that can be used effectively because of the contaminating effects of diffusion and surface adsorption. As the dimensions of small volumes are decreased, diffusion becomes the dominant mechanism for mixing leading to dispersion of reactants. Moreover, surface adsorption of reactants, although small, can be highly detrimental at low concentrations and small volumes. As a result current microfluidic technologies cannot be reliably used for applications involving minute quantities of reagent-for example, bioassays at levels down to the single molecule are not easily performed. An approach that overcomes these limitations is the use of aqueous droplets in an immiscible carrier fluid; [19] these droplets provide a well-defined, encapsulated microenvironment that eliminates cross-contamination or changes in concentration caused by diffusion or surface interactions. Droplets provide the ideal microcapsule that can isolate reactive materials, cells, or small particles f...

Oscillatory motion of water droplets in kerosene above co-planar electrodes in microfluidic chips

“…For example, charged water droplets dispersed in an oil phase can be addressed by means of DC electric field into specific parts of a microfluidic structure. 8 The droplets can be used as containers for chemical and biological molecules or cells. 9 For example, precise control of droplet motion/manipulation via external electric fields has been employed in works.…”

“…: +420 2 2044 3168, Fax: +420 2 2044 4320. Fundamental work on an advanced manipulation of leaky dielectric droplets (water electrolytes) dispersed in a dielectric fluid (nonconductive oil) was published by Link et al in 2006. 8 A pair of microelectrodes was used to electrically charge water droplets.…”

Coalescence and splitting of confined droplets at microfluidic junctions

“…8,9 Additionally, surfactants are often used to stabilize droplets, hindering coalescence. 6,10 In microfluidics, flow-induced coalescence in droplet streams has been demonstrated in long slowly diverging channels.…”

“…Active control of droplet coalescence in microfluidics has been achieved using electric fields to bring together oppositely charged droplets in T-junctions 8 and straight channels. 9,13,14 Magnetic fields have also been used with success.…”

Microbridge structures for uniform interval control of flowing droplets in microfluidic networks

“…Spontaneous and combinatorial pairing of droplets requires precise synchronization of approaching droplets in time and space by adjusting the droplet interval. However, in order to prevent irregularities in the spacing control of droplets, it is required to integrate with additional microfluidic components such as electrode for applying electric field 12,13 or multi-layered chambers 14 and valves 15 for temporal stopping droplets. These integration issues have limitations in further incorporation of other microfluidic components for extra droplet manipulation.…”