Photoisomeric molecular tagging velocimetry with CCVJ

Various physical principles can be utilized for molecular tagging velocimetry (MTV). In this article, the capabilities and limitations of photoisomerization in MTV are studied. The molecular rotor 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) exists in two isomers. Although the $E$ isomer, which is present in the absence of light, yields a fluorescent behavior, the $Z$ isomer is a photoproduct with no detectable luminescence. This regenerative behavior is utilized for MTV. The two time constants define the experimental limitations of the method for tagging and recovery. It is shown that, whereas the tagging time constant is in the range of milliseconds to seconds, the recovery time is in the range of minutes to hours, enabling measurements of slow flow phenomena. A flow in a rectangular channel with a Reynolds number of $9\times {10}^{-3}$ and a Péclet number of 1009 is investigated. Furthermore, the addition of methyl-$\beta$-cyclodextrin (methyl-$\beta$-CD) to the solution increases the fluorescent quantum yield and the isomeric rate constants, supporting the hypothesis of a complex formation with CCVJ. It is shown that photoisomeric MTV is suitable for analyzing slow flow regimes in the liquid phase. The addition of methyl-$\beta$-CD increases the dynamic range of the results. In comparison with existing MTV principles, photoisomerization has the advantage of an intrinsic regenerative behavior. The cost-efficient and simple setup make it an alternative in biological and environmental flow studies.

Figure 1

Experimental setup.

Figure 2

Diffusion of a tagging front in a cuvette after 100, 200, and 250 s. The absolute intensity values are normalized with a reference image without tag and averaged in the beam direction. The blue dots denote the data with their respective sample standard errors whereas the line represents a fit of Eq. (4).

Figure 3

Determination of the recovery time constant after complete tagging of the cuvette. Data represent the average of three experiments and are normalized with the last acquired data point. The results of the fit lie within the standard error.

Figure 4

Relative intensity decay data (circles) and standard error (bars) for different transmittances of the ND filters. The solid lines represent the respective fits. Note that the measurement with transmittance 0.621 is acquired with a different tagging volume, resulting in a different relative intensity decay.

Figure 5

Dependency of the tagging time constant on laser power density and the respective $1\sigma$ confidence interval.

Figure 6

Relative intensity difference with $\mathrm{Re}=9\times {10}^{-3}$ and $\mathrm{Pe}=1009$ at 10, 50, 100, and 200 s (left to right) after a tag of 0.05 s for a single acquisition. The standard deviation of the relative signal is 0.0047.

Figure 7

Relative intensity difference with $\mathrm{Re}=9\times {10}^{-3}$ and $\mathrm{Pe}=1009$ at 10, 50, 100, and 200 s (left to right) after a tag of 0.05 s for three averaged acquisitions. The standard deviation of the relative signal is 0.0036.

Figure 8

Relative intensity difference with $\mathrm{Re}=9\times {10}^{-3}$ and $\mathrm{Pe}=1009$ at 120 s, the concentration of methyl-$\beta$-CD of 0 mmol/L (left) and 3 mmol/L (center) for ten averaged acquisitions. The right subfigure is the difference of the two. Note the different color-bar limits of the right subfigure.

Figure 9

Relative intensity difference with $\mathrm{Re}=9\times {10}^{-3}$ and $\mathrm{Pe}=1009$ at 360, 460, 600, and 720 s (left to right) after a tag of 4 s for a single acquisition. The standard deviation of the relative signal is 0.0057.

Figure 10

Relative intensity difference with $\mathrm{Re}=9\times {10}^{-3}$ and $\mathrm{Pe}=1009$ at 360, 460, 600, and 720 s (left to right) after a tag of 4 s for three averaged acquisitions. The standard deviation of the relative signal is 0.0047.

Figure 11

Comparison of the analytical result of the Poiseuille flow with experimental data from Fig. 7. The single points are the result of a fit for each pixel column to Eq. (8) and the computation of the effective velocity. The results collapse to the Poiseuille profile for all observed times.