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TECHNICAL PAPERS

Use of Bacterial Carpets to Enhance Mixing in Microfluidic Systems

[+] Author and Article Information
Min Jun Kim1

Division of Engineering,  Brown University, Providence, RI 02912

Kenneth S. Breuer2

Division of Engineering,  Brown University, Providence, RI 02912kbreuer@brown.edu

1

Present address: Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 19104.

2

Corresponding author.

J. Fluids Eng. 129(3), 319-324 (Sep 07, 2006) (6 pages) doi:10.1115/1.2427083 History: Received February 05, 2006; Revised September 07, 2006

We demonstrate that flagellated bacteria can be utilized in surface arrays (carpets) to achieve mixing in a low-Reynolds number fluidic environment. The mixing performance of the system is quantified by measuring the diffusion of small tracer particles. We show that the mixing performance responds to modifications to the chemical and thermal environment of the system, which affects the metabolic activity of the bacteria. Although the mixing performance can be increased by the addition of glucose (food) to the surrounding buffer or by raising the buffer temperature, the initial augmentation is also accompanied by a faster decay in mixing performance, due to falling pH and oxygen starvation, both induced by the higher metabolic activity of the bacterial system.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the bacterial mixing system and micrograph of the bacterial carpet as it develops inside the microfluidic system. The picture is taken 2000s after the initiation of the flow deposition procedure. The scale bar is 5μm.

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Figure 4

The variation of the maximum of the intensity gradients, from all seven x locations and four different flow rates. Inset shows the gradient of the intensity profiles at different distance from the Y-junction, generated from images for Q=0.468μl∕min. Fluorescence intensity distributions are extracted from photographs shown in Fig. 3, and plotted against τ=x∕U, where U is the average flow velocity. Three sets of data are shown: (i) the clean-walled microchannel, (ii) the channel coated with nonmotile bacterial carpet, and (iii) the channel coated with an active bacterial carpet and the downward shift in each line indicates the increased diffusion due to presence and motion of bacterial flagella. The gradient of the intensity profile decays and spreads as τ=x∕U increases.

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Figure 5

Enhancement over Brownian motion of the tracer particle diffusion coefficient as a function of time. Two measurement series are shown: with the plain motility buffer (circles) and with 20mM glucose added (squares). Error bars represent standard deviations based on five sets of measurement.

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Figure 6

Variation of (a) the initial diffusion enhancement factor and (b) the decay rate for flows above bacterial carpets in response to changes in concentration of glucose (0, 2, 20, 60, and 120mM). Error bars represent standard deviations based on five sets of measurement.

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Figure 7

Enhancement over Brownian diffusion as a function of time at different temperatures. The rising temperatures increase the metabolic activity of the cells resulting in increased diffusion enhancement. However, that increased activity also hastens the carpet’s catabolic poisoning, resulting in a faster decay in the mixing activity. Error bars represent standard deviations based on five sets of measurement.

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Figure 8

Percent variation of (a) the initial diffusion enhancement and (b) the decay rate due to bacterial carpet motility as a function of temperatures. The increase in temperature of the buffer shows an increase in the initial motility and a increase in the decay rate up to 35°C. The decline at T=40°C may be related to changes in tumbling frequency for S. marcescens observed at higher temperatures (16). Error bars represent standard deviations based on five sets of measurement.

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Figure 3

The photographs illustrate typical intensity distributions in the Y-junction microchannel, showing the diffusion profiles (∂I∕∂y) that is established between the labeled and unlabeled streams (x=24mm, Q=0.188μl∕min): (a) the clean-walled microchannel, (b) the channel coated with nonmotile bacterial carpet, and (c) the channel coated with an active bacterial carpet. The enhancement of mixing is apparent.

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Figure 2

Schematic of the test geometries: (a) the Y-junction microchannel and (b) the straight microchannel

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