Micromixing and flow manipulation with polymer microactuators


Polymer actuators based on Gold/PolyPyrrole bilayers were microfabricated and their properties tested for flow promoting in the microdomain. When implemented in microchannels these actuators behaved as efficient micromixers for both, flow-through and stagnant conditions. Particle tracking experiments and numerical simulations of cross-sectional domains verified the capacity of these devices to promote complex, high velocity flows with chaotic advection properties in microscopic environments. Thinner devices could be actuated at higher frequencies than thicker devices, up to 10 Hz for 10 nm thick Gold layers with voltages not over 0.6 V (vs. Ag/AgCl), which led to enhanced flow generation properties. The results herein demonstrate that these actuators are practical candidates for fluid manipulation in the microdomain (for applications such as micromixing and pumping, and possibly even for propelling of swimming microdevices).

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  1. 1.

    Since the flows generated by the flaps were dynamic (depending on their position and direction of actuation over time) methods for cross-sectional mixing evaluation, as reported in (Stroock et al. 2002), were not feasible in our setup (the speed at which the confocal microscope could “slice” the volume under study was slower than that of the flaps). Therefore, to estimate mixing, planar, top-view images had to be used. The pixels of these images were evaluated (using Matlab image processing tools) according to their brightness. Three different pixels where initially identified for the section of interest: unmixed bright (brightest pixel, bp), unmixed dark (darkest pixel, dp) and perfectly mixed (pm, brightness after allowing the two solutions, seeded and unseeded, to mix in the microchannel until no further changes in brightness distribution were observed). Each pixel’s brightness, b, was then associated with a ratio of mixing, r, depending on its proximity to one of the extreme cases, bp or dp:

    \( r = {{\left( {bp - b} \right)} \mathord{\left/ {\vphantom {{\left( {bp - b} \right)} {\left( {bp - pm} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {bp - pm} \right)}} \) or \( r = {{\left( {b - dp} \right)} \mathord{\left/ {\vphantom {{\left( {b - dp} \right)} {\left( {pm - dp} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {pm - dp} \right)}} \)

    By averaging the r value of all the pixels across the section under study, a ratio of mixing along that section, R, was obtained. Since the brightness of the pixels that were being analyzed on the planar images could come from several fluorescent particles positioned in the same plane coordinates but at different depths, this treatment was necessary in order to relate the brightness distribution of the planar images to the volumetric degree of mixing in the section of interest of the microchannel.

  2. 2.

    A video of this experiment can be found as Electronic Supplementary Information (video 1).

  3. 3.

    Diffusion was expected to be faster right after stopping the flow, since concentration gradients were the highest, but to decay rapidly after the first seconds in stagnant conditions. It was evaluated that by only diffusive means, the whole width of the channel was lit up after 300 s (M = 1). Since diffusive mass transport was the sole reason for mixing in the upper and lower regions of the channel, the averaged M for those regions at each time were directly subtracted from the M values of the central region (containing the flaps). Diffusion effects for the upper and lower regions of the channel, where no flaps were present, were responsible for an average M of 0.13 and 0.15, between ab and ac respectively. The final 50 and 75% mixing performance after 4 and 7.5 s of actuation, respectively, was thus obtained by direct subtraction of the diffusive mixing coefficient.

    Also, the presence of the actuators behaved as a “black body” (when the flaps were positioned between the fluorescent molecules and the objective, no fluorescent signal could be obtained). When this effect was clearly noticeable, the area where the flaps were suspected to be present was not taken into consideration for the calculation of the section’s mixing ratio, R.

  4. 4.

    A movie of this experiment (video 2) can be found as Electronic Supplementary Information.

  5. 5.

    In the chaotic mixer presented by (Stroock et al. 2002), for Pe numbers of 9 × 105 a 90% of mixing is achieved after a length of 1.7 cm. In our system the Pe (Pe = Ul/D) was estimated using Einstein’s relation to determine the Diffusivity of the microparticle tracers (D = 2 × 10−13 m2/s). This yields a Pe = 3.5 × 106. Hence, at even higher Pe numbers (or in conditions where diffusion plays a less relevant role in mixing) our devices were capable of attaining a 75% after a length of only 700 μm.

  6. 6.

    A movie of this experiment is available as Electronic Supplementary Information (video 3). The effect of natural convection due to evaporation of the liquid in the open cuvette was assessed to be negligible for the timescale of the experiment.

  7. 7.

    In order to obtain reliable data on particle orbits and speeds, tracking was only preformed for particles whose trajectory remained in the plane of focus for at least 1.5 s. A movie with the tracked particle trajectories can be found as Electronic Supplementary Information (video 4).

  8. 8.

    The device used for this calculation can be found in video 5 as Electronic Supplementary Information. The speed of the tip of the flaps, u, was estimated to be 5 mm/s during the downwards stroke, the tip displacement, l, was of 350 μm and an estimated kinematic viscosity, υ, of the medium of 9.4 × 10−7 m2/s. With these parameter the Re number was calculated to be: \( \text{Re} = {\frac{u \cdot l}{\upsilon }} \approx 2 \).

  9. 9.

    Video 6 in the Electronic Supplementary Information displays a single device (of dimensions 350 × 35 μm, and Au and PPy thicknesses of 10 and 500 nm, respectively) actuated with a square function of 1.2 V vs. Ag/AgCl at different frequencies: beginning at 1 Hz (visualized initially in bright-field mode, and afterwards in fluorescent mode), it then switched to 10 Hz, and finally back again to 1 Hz. The device was placed at the bottom of a channel of 1 mm width and 500 μm height. The channel was filled with medium seeded with 5 μm fluorescent spheres. Given the particular motion of the flap, which was very asymmetric, and its high flapping amplitudes and frequencies a vigorous vortex effect was observed.


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The author would like to thank Prof. Ali Mohraz and Mr. Bharath Rajaram for lending their confocal microscope setup to execute the experiments presented and for their helpful insights and advice on the implementation of the particle tracking procedure. Dr. Lawrence Kulinsky and Prof. Marc Madou would like to acknowledge the support of WCU program as well as funding support of National Science Foundation (ECCS 0801792, CBET 0709085) and UC-LANL Lab Fees Research Program.

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Correspondence to Xavier Casadevall i Solvas.

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Supplementary material 1 (MPG 1792 kb)

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Supplementary material 5 (MPG 1892 kb)

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Casadevall i Solvas, X., Lambert, R.A., Kulinsky, L. et al. Micromixing and flow manipulation with polymer microactuators. Microfluid Nanofluid 11, 405–416 (2011).

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  • Polymer actuators
  • Microfluidics
  • Micromixing
  • Artificial cilia