Accelerated Chemical Reactions for Lab-on-a-Chip Applications Using Electrowetting-Induced Droplet Self-Oscillations
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0915-R06-10
Accelerated Chemical Reactions for Lab-on-a-Chip Applications Using ElectrowettingInduced Droplet Self-Oscillations Joanna Aizenberg, Tom Krupenkin, and Paul Kolodner Materials, Bell Labs/Lucent, 600 Mountain Ave., Murray Hill, 07974 This paper summarizes the results of experimental investigations of the feasibility of applying electrowetting-induced droplet self-oscillations to induce rapid mixing of small quantities of liquids. The concept was tested using video microscopy to monitor the mixing of passive colored dyes, of spatially-separated reactants that change color upon reaction, and of fluorescent DNA oligomers whose light emission vanishes upon hybridization with appropriatelyfunctionalized complementary DNA strands. Droplet self-oscillation was found to increase the rate of mixing by factors ranging from 15 to 100 as compared with the rate of passive diffusion in undisturbed droplets. This demonstrates that self-oscillation-induced mixing is a viable method for substantially enhancing the speed of chemical reactions in general, and biochemical assays in particular, when performed in small volumes of liquids. Introduction Mixing during chemical reactions is often a rate-determining step. This can be a particularly critical problem when rapid assays in small volumes are required (1-3). In droplets of millimeter size, mass diffusion times are typically tens of minutes, so passive mixing by diffusion is very slow. Un-sustained transient motions cannot accomplish mixing, since these die out on the time scale of viscous diffusion, which is typically a few seconds. Furthermore, turbulence, which is commonly used to enhance mixing on large spatial scales, is difficult to sustain at millimeter scales, since Reynolds numbers are proportional to the characteristic length of the system. For speeding up reactions in small volumes, a conceptually new mixing methodology is required. Our approach to this problem is to apply an electrowetting-induced droplet-oscillation technique. This technique employs an electrically-conductive substrate covered with a thin insulating layer and a hydrophobic surface coating. A droplet of a conductive liquid placed on such a surface can be made to oscillate by inserting an electrode into the droplet and applying to the electrode a square-wave voltage with respect to the substrate. This process is illustrated in Fig. 1. During the non-zero-voltage phase of the excitation signal, the droplet charges, which causes it to wet the substrate with a reduced contact angle, as illustrated in the right picture of Fig. 1. During the zero-voltage phase of signal, polarization forces are absent, and the droplet moves back up into the configuration shown in the left picture of Fig. 1.
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Figure 1. Electrowetting-induced self-oscillation of a conductive droplet on a hydrophobic surface. Left picture: zero voltage, high contact angle. Right picture: non-zero voltage, reduced contact angle. With square-wave excitation, the droplet oscillates between these two configurations.
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