Imaging Liquid-Liquid interfaces in flat jets
Prof. Andreas Osterwalder
Institute of Chemical Science and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
Background
Professor Andreas Osterwalder is a group leader whose research is mainly focused on novel methods of producing cold neutral molecules and leveraging their usage in fundamental chemical and physical research, as well as targeting interfaces between two liquids, and dynamical processes taking place in such environments.
Liquid microjets which are a powerful tool for spectroscopy, jets with 50 µm diameters are prepared by forcing liquid through a small capillary and form a laminar-flow system. A variant of these are flat jets where a flat leaf-shaped liquid structure a few microns thick and about 1 mm long is formed by crossing two cylindrical jets at a specific angle (Fig.1).
In a recent study [1], Prof. Osterwalder’s team has shown that the two fluids from the original jets do not mix by turbulences in the first leaf, but instead produce a layered structure with a clean interface. This has been demonstrated by monitoring the kinetics of a chemiluminescence reaction (the oxidation of Luminol by hydrogen peroxide), during which blue light is produced wherever the two solutions mix. The resulting image from this study (Fig.2) reveals strong luminescence from the rims of the leaf – indicating turbulent mixing – but only a gradual increase in the centre portion of the first leaf – indicating mixing through diffusion across the interface. Because the layered structure exists only in the first leaf, all further leaves are homogeneously blue.
Figure 1: Example of a liquid flat jet. Two cylindrical jets enter from the left, in a plane perpendicular to the screen. At the crossing point the liquids are pushed outwards and are then pulled back together through viscosity and surface tension, forming the leaf-shaped structure. With the light source behind the leaf, interference fringes are formed by partial reflection at the two air-liquid interfaces.
Figure 2: Luminol chemiluminescence in a liquid flat jet. The two cylindrical jets, each containing one reactant of the luminol oxidation, enter from the left. They are not visible because no reaction takes place. Uniform blue glow in the rims and all leaves after the first one indicate turbulent mixing, while the faint and increasing chemiluminescence in the centre part of the first leaf shows the diffusion dynamics and chemical kinetics of this reaction. Image adapted from [1] Schewe et al. 2022.
Challenge
The preliminary studies in the published paper [1] faced challenges due to the limited sensitivity of CCD camera technology. Prof. Osterwalder required a detector with a high sensitivity to enable quantitative signal measurement, and a camera that would acquire images at high speed to enable a high temporal resolution for this dynamic liquid system. Furthermore, imaging multiple layered liquid interfaces is only feasible with high spatial resolution and a large spatial sampling, requiring a detector with a physically large array of optimally sized pixels across the sensor.
The Kinetix features greater sensitivity, a larger sensor, and higher imaging speed than our previous CCD solution.
Prof. Andreas Osterwalder
Solution
The Kinetix sCMOS camera offers a powerful and flexible solution that meets the necessary specifications for this imaging endeavour. The high sensitivity provided by the 16-bit Dynamic Range mode, combined with the fast readout, enables highly efficient signal capture. The high quantum efficiency of the Kinetix, greater than 95%, even allows for shorter acquisition times without a loss of signal-to-noise ratio.
A particularly advantageous feature of the Kinetix is the possibility to customize the active readout region to a limited portion of the sCMOS sensor. Due to the inherently elongated structure of the flat jet, a large portion of the sensor only records background. This creates unnecessary data and reduces the acquisition speed of the camera. By limiting the readout to an elongated rectangle at the centre of the sensor (more columns than rows), the frame rate can surpass 100 fps while maintaining a 16-bit output of a 3000 pixels wide image. The combined features of sensitivity and field of view are thus paramount to image the flat jet with the necessary spatial and acquisition speed.
Reference
[1] Schewe H.S, Credidio B., Ghrist A.M., Malerz S., Ozga C., Knie A., Haak H., Meijer G., Winter B., and Osterwalder A. (2022) Imaging of Chemical Kinetics at the Water–Water Interface in a Free-Flowing Liquid Flat-Jet, J. Am. Chem. Soc. 144 (17), 7790-7795
