In this paper, we examined mixing of various two-fluid flows in a silicon/glass microchannel based on the competition of dominant forces in a flow field, namely viscous/elastic, viscous/viscous and viscous/inertial. Experiments were performed over a range of Deborah and Reynolds numbers (0.36 < De < 278, 0.005 < Re < 24.2). Fluorescent dye and microshperes were used to characterize the flow kinematics. Employing abrupt convergent/divergent channel geometry, we achieved efficient mixing of twodissimilar viscoelastic fluids at very low Reynolds number. Enhanced mixing was achieved through elastically induced flow instability at negligible diffusion and inertial effects (i.e. enormous Peclet and Elasticity numbers). This viscoelastic mixing was achieved over a short effective mixing length and relatively fast flow velocities.
Inkjet printing has proven to be a promising and flexible process methodology for low cost and drop-on-demand pattern formation in small-scale production of micro-electro-mechanical systems. To optimize the micro-patterns formed by inkjet printing, an accurate control of droplet volume is essential and critical. In this study, an inkjet system with a nozzle driven by a circular piezoelectric element was used to explore the impact of different waveforms on droplet volume. The investigation into this study included the impact of unipolar, bipolar, M-shaped and W-shaped waveforms as well as the effects of their amplitudes and pulse durations. The inkjetting behavior of Newtonian and non-Newtonian fluids under different actuating waveforms was studied in order to obtain a maximum reduction in ejected droplet sizes. An effective reduction of droplet volume in the range of 50–80% was demonstrated. The results of inkjetting PEDOT ink on a polished silicon surface showed that a 50% reduction in line width was achieved.
We exploited the viscoelasticity of biocompatible dilute polymeric solutions, namely, dilute poly͑ethylene oxide͒ solutions, to significantly enhance mixing in microfluidic devices at a very small Reynolds number, i.e., ReϷ 0.023, but large Peclet and elasticity numbers. With an abrupt contraction microgeometry ͑8:1 contraction ratio͒, two different dilute poly͑ethylene oxide͒ solutions were successfully mixed with a short flow length at a relatively fast mixing time of Ͻ10 s. Microparticle image velocimetry was employed in our investigations to characterize the flow fields. The increase in velocity fluctuation with an increase in flow rate and Deborah number indicates the increase in viscoelastic flow instability. Mixing efficiency was characterized by fluorescent concentration measurements. Our results showed that enhanced mixing can be achieved through viscoelastic flow instability under situations where molecular-diffusion and inertia effects are negligible. This approach bypasses the laminar flow limitation, usually associated with a low Reynolds number, which is not conducive to mixing.
We demonstrated rapid mixing of viscoelastic fluids in microchannels constructed based on polymethyl methacrylate. Viscoelastic mixing without diffusion was achieved with an effective mixing length of less than 5mm and a relatively fast flow rate. With an abrupt contraction microgeometry (8:1 contraction ratio), we mixed two different viscoelastic fluids experimentally at very low Reynolds numbers, but enormous Peclet and elasticity numbers. This special geometrical configuration triggers flow instability, leading to turbulent and efficient mixing. This flow regime has negligible inertia effects but significant elastic effects.
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