A MILLISECONDS MICROFLUIDIC MIXER BASED ON SINGLE BUBBLE STREAMING
Daniel Ahmed, Xiaole Mao, Bala Krishna Juluri, Jinjie Shi and Tony Jun Huang
The Pennsylvania State University, USA ABSTRACT
Rapid mixing and homogenization of chemical and biological species in micro-flows is of great importance for the development of microfluidic platforms for a wide variety of biotechnological and chemical kinetics studies. We have achieved homogeneous, milliseconds mixing of two chemical species in a microfluidic cham-ber by utilizing acoustic streaming phenomenon of a single bubble that is initiated in a microscale horseshoe-shaped structure fabricated inside the micro channel. KEYWORDS: Acoustic Streaming, Horseshoe-structure, Bubble. INTRODUCTION
Microfluidic devices provide unique opportunities for studies on drug delivery, DNA hybridization, protein folding, enzyme reactions and chemical kinetics. Such devices must homogeneously mix chemical and biological species before said spe-cies can react, in order to probe the transient events during rapid chemical and bio-logical processes [1]. Rapid, homogenous mixing is difficult at the microscale due to the diffusion-controlled mass transport at low Reynolds numbers (Re) of microscale flow. In this work, we demonstrate homogeneous and milliseconds mixing of two side-by-side laminar micro flows inside a microfluidic channel. A microscale horse-shoe-shaped structure is fabricated inside a channel such that when two fluids con-taining the to-be-mixed chemical species are pumped, the liquid passes through the horseshoe structure and induce a single bubble due to surface tension. The mem-brane of the bubble is acoustically driven at the resonant frequency that eventually perturbs the innate laminar flow interface of the channel thus exhibiting excellent homogenized mixing in only few milliseconds. THEORY
The membrane of the an air-filled bubble in liquid oscillates when excited by acoustic waves. This oscillation is maximized when the bubble is driven at the reso-nant frequency. The natural frequency f of a spherical bubble can be estimated by the Minnaert equation: ργπ /32/1 oo PRf = (1) where oR is the bubble’s equilibrium radius, γ the ratio of specific heats of gas, oP is the hydrostatic liquid pressure, and ρ is the density of the liquid [2].
This oscillation of the bubble generates fluctuation in velocity and pressure in the surrounding fluid [2] thus resulting in a strong recirculating force in the liquid, the phenomenon is known as acoustic micro-streaming. The prominent streaming pattern induced by the bubble oscillation perturbs the laminar flow
Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA
interface inside the chamber thus exhibiting excellent homogenized mixing in milliseconds. EXPERIMENTAL
Figure 1 illustrates the experimental setup for fast microfludic mixing. A micro-fluidic channel is placed adjacent to the piezo transducer. Inset is the demonstration of trapped bubble and the streaming phenomenon when the bubble is acoustically driven at the resonant frequency.
Figure 1. Schematic of fast mixing experimental setup
A simple polydimethylsiloxane (PDMS) microchannel is fabricated using
standard soft-lithography and mold replica technique [3]. The channel is activated with oxygen plasma and bonded to a plastic substrate. The PDMS channel has width, depth of 240 µm, 150 µm respectively and is 800 µm in length, while the dimensions of the horse-shoe structure is shown in Figure 1. A Piezo transducer (Model No. 273-073, Radioshack) is bonded using epoxy on the same plastic substrate and is placed adjacent to the PDMS channel. RESULTS AND DISCUSION
DI water and fluorescene dye (Fluorescein) solutions are injected into the channel at 8µl/min for each flow as shown in Figure 2.
Figure 2. (a) Laminar flow of fluorescene and DI water in absence of acoustic wave.
(b) Homogenized mixing of chemical species in presence of acoustic waves.
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Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA
Figure 2a shows the laminar flow of two chemical species before and after the horseshoe-shaped structure. A single bubble is trapped inside the horseshoe structure and the bubble is excited at its natural frequency by the transducer. Streaming phenomenon is attained in the surrounding liquid. As a result, rapid interchanging of liquid between water and fluorescein solution is developed that eventually breaks the laminar flows enabling fast and homogenized mixing as shown in Figure 2b.
Quantitation of mixing results are evaluated by measuring the gray-scale value of the pictures obtained. Normalized concentration of the mixing result after passing through the horseshoe-shaped structure is plotted across the width (240 µm) of the channel. Figure 3a. shows the uniform normalized gray scale values acorss the width, demonstrating homogenized mixing of the two chemical species. The average mixing time is estimated by Vdt /= , where d = 50 µm as shown in Figure 3b is the distance from unmixed to mixed regions and V is the average fluid velocity. Estimated mixing time is ~ 7 milliseconds.
Figure 3. (a) Plot of normalized dye concentration across the channel width, and (b)
Plot of normalized concentration along the length of unmixed to mixed region
CONCLUSIONS In summary, we have introduced an active microfuidic mixer based on the
principle of acoustic streaming phenomenon around a single bubble. The mixing result is analyzed and quantified. An excellent homogenized mixing across the entire width of the channel was achieved in only a few miliseconds. REFERENCES [1] K. Jensen. Chemical kinetics: Smaller, faster chemistry, Nature, 393, pp. 735-
737, (1998) . [2] P. Tho, R. Manasseh and A. OOI, Cavitation microstrreaming patterns in sin-
gle and multiple bubble system, J. Fluid Mech, 576, pp. 191-233, (2007). [3] J. Shi, X. Mao, D. Ahmed, A. Colletti and T. J. Huang, Focusing microparti-
cles in a microfludic channel with standing surface acoustic waves, Lab Chip, 8, pp. 221 – 223, (2008).
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