Flow of Multiple Fluids
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A N A LY T I C A L C H E M I S T R Y / A P R I L 1 , 2 0 0 2
Visualizing thin-layer, D flow and chemical Purnendu K. Dasgupta ¥ Kazimierz Surowiec ¥ Jordan Berg
ike a “T”-junction in a microfluidic sensor, many analytical approaches rely on the simultaneous flow of multiple fluids on a planar surface, in which the vertical dimension of the confined flow surface is small and there is no restriction on the lateral dimension. It can be on the scale of millimeters or even larger. In this article, we will describe devices that can be inexpensively fabricated to demonstrate the principles of two-dimensional (2-D) flow to students, and we offer related information about existing and emerging array approaches for the new chip-based technologies. How the fluid is driven is not of great consequence in the scenarios that follow; pressure- or gravity-driven flow is sufficient. Moreover, these devices can be built without specialized tools or microfabrication facilities. Best of all, we think the experimenter will find these studies have pedagogic and aesthetic merit and are a great deal of fun!
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What’s out there Microfabricated electronics are revolutionizing the way we live.
It is hoped that microfabricated devices—in which liquids flow and analytes interact with biotic and abiotic surfaces and energy fields—will be used to carry out analytical chemistry in the future. As a result, manipulating fluid flows in microdevices is a topic of great interest, primarily centered on driving liquids through narrow (~10,000 beads (1- to 3-µm diam) on a 1-cm2 platform. Imagine the 10 different colors of the beads. Each color represents different fluorogenic chemistries, which are specific for different analytes of interest. For example, red beads may code for glucose, green for cholesterol, etc. After the beads are exposed to a blood plasma sample and the appropriate chemical reactions, color and fluorescence images are taken and superimposed. The beads will glow intensely when large quantities of analyte are present and very little when small quantities are present. In effect, one has performed multiple replicate assays on multiple analytes in parallel. However, chemistries for different analytes often are not mutually compatible. This requires reacting the red beads located in the southwest (SW) corner with the glucose reagent while the green beads in the northeast (NE) corner are exposed to cholesterol reagents. In other words, different areas of the chip must be addressed with different fluids
without mixing. Simultaneous manipulation of different fluids on a small planar surface is thus important in array analysis formats. Other 2-D flow applications are emerging, all of which rely on the following phenomena and factors: mixing at the interfaces between two solutions, which is governed primarily by diffusion and results in sharp boundaries between the fluids at small residence times; the width of the product zone, which is where the reaction product is formed by mixing two fluids at an interface; and the geography of fluid coverage, which is how different fluids occupy the available real estate. The width of the product zone is also related to the concentration of the limiting reagent, and fluid coverage is governed by fluid inlet and outlet geometries, relative fluid velocities, and fluid viscosities. For reactions that form colored compounds, it is possible to study these aspects with small or large transparent 2-D flow cells that can be easily and inexpensively made. Used with a microscope or an overhead projector, these cells show chemical reactions and diffusion at work.
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tility. A reticle for microscopic measurements, a solenoid pump that delivers fluids in short bursts (~$150), and a six-port injection valve are also useful.
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Cells for observing 2-D flow are made with two flat plates, at least one of which is transparent, with a thin spacer or gasket in between. For gasket material, we have used transparencies for a laser printer, which are typically 150-µm thick; these can be used for interesting experiments involving transverse illumination through the spacer. We also used electrician’s tape (polyvinyl chloride), typically ~150– 200-µm thick, which is a particularly good gasket material because it seals well due to its elastic nature. Common Scotch tape (50-µm thick) and inert polyimide tapes, such as Kapton (25–250-µm thick), with or without adhesive on one or both sides, can also be used. Almost any material can be used as the gasket if the bottom of the flow cell is made of a soft material like Teflon. For the transparent observation window, we generally use Plexiglas. Two microscope slides held in a frame separated by a suitable gasket also make a good observation cell, and this is particularly suitable for use with organic solvents. For flow input and output, holes can be made in the glass or in the frame holding the slides with a diamond-tipped drill. As long as the frame is machined to hold the slides snug, without a vertical step, liquid flows with no problems. Consider Figure 1: a flat Plexiglas plate with holes drilled from the bottom at the center of a 10 ⫻ 10-mm area. As shown in Figure 1a, a polyimide tape gasket (orange area) is placed around the 10 ⫻ 10-mm area, and eight more holes are drilled symmetrically around the center. Teflon tubes pushed into these holes introduce and remove liquid. A gasket, cut to the appropriate shape, rests atop the bottom plate containing the holes. When the first Plexiglas plate is covered with a second and the two are
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What you need To do this exercise, you need a homemade planar flow cell with small (
