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Visualizing Flow in Paper Networks

The Need for Flow Visualization Methods

In conventional transparent channels, the movement of small particles can be tracked in a process known as particle imaging velocimetry (PIV). However, paper scatters light strongly, so except in unusual circumstances, conventional PIV does not work. Flow visualization and measurement tools in highly scattering porous matrices, such as nitrocellulose, are needed for multiple reasons: (1) understanding the basics of transport in paper networks, (2) characterization of materials, (3) characterization (and optimization) of the fabrication process, and (4) characterization of paper device operation. We have developed several simple methods to visualize flow in paper. Here we describe two of the most practical visualization methods.

Electrochemical Marking

In the electrochemical marking method, we create a high pH band in a low pH background of indicator dye. The schematic of Figure 1 shows the set-up for this method. Two wire electrodes are attached to the paper device. The positive electrode is located at the source and the negative electrode is placed at the location of initial marking. The electrode triggers a local pH change in an indicator like phenol red, which is yellow at pH 7 and undergoes a color change to red near pH 8. Thus, a reaction at the negative electrode to create a locally high pH band results in a red band that can be tracked with time in the paper network. For this method to succeed, the tracer species must have a known Rf  value, ideally 1.0 (i.e., zero affinity for the stationary phase). 

Figure 1. Schematic of the set-up for the electrochemical marking method. A zone of high pH is electrochemically generated in a lower pH background such that a pH indicator dye acts as a tracer species.

Shown in Figure 2 is an image demonstrating the electrochemical marking method. A series of red bands have been created at the negative electrode and have propagated upwards. The inset shows a close-up of one of the bands. This method has several advantages. First, it is easily automated for high-resolution timing. For example, the bands on the right were automatically generated from user-defined pulse widths and pulse intervals. A second advantage is the ability to mark multiple locations of the network. This is straightforward with the addition of electrodes. A third advantage is that either spot or band markings are possible with this method. Finally, it is easy to implement and inexpensive, with a materials cost of less than $100. For the truly impoverished, it can be performed with no circuitry at all other than the experimenter’s ability to touch a wire to a battery for a fixed period.

Figure 2. Electrochemical marking method. In the image, the time between pulses is two minutes, and the pulse width is 10 seconds bracketed on either side with 1-second reverse polarity pulses.

Fluorescence-Based Marking

A second method demonstrated is based on the localized uncaging of an initially caged fluorophore species.  Caged fluorophores are compounds that greatly increase in fluorescence quantum efficiency after a photolysis reaction. A schematic of this method is shown in Figure 3. We used a commercial photo flash unit as an inexpensive ultraviolet light source. [WARNING: photoflash units produce a high-voltage discharge, so if you take a can opener to a commercial camera (as we did) you must be very careful not to connect yourself or others to the high-voltage leads that run to the lamp.] The caged fluorescein species serves as the background solution and is uncaged by exposure to the 355-nm light in a pattern defined by a opaque mask. The pattern in the demonstration shown below is a narrow band that can be tracked with time and was used to determine the flow rate in a strip of constant width. Because this method depends on fluorescence detection, it is not as simple to visualize as the electrochemical marking system, but it can still be easy and inexpensive to implement.

Figure 3. Schematic of the set-up for the fluorescence-based marking method. A tracer species that is initially caged, is subsequently uncaged when selectively exposed to 355 nm light to form fluorescent markings that can be tracked with time.
Figure 4. Fluorescence-based marking method. A fluorescent band was created in the leftmost image. The image series shows the motion of a fluorescent band within a paper strip over 1 minute.

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