Using a bio-inspired copper complex to investigate reactive mass transfer around an oxygen bubble rising freely in a thin-gap cell

Abstract

The present study describes an original colorimetric method to visualize and quantify the local oxygen mass transfer around a rising bubble in reactive media. This method is based on the use of a colorless bio-inspired copper complex, Cu(btmgp)I, specially tailored for the study, which, dissolved in acetonitrile, oxidizes into an orange copper-complex [Cu2O2(btmgp)2]I2. The latter complex, unstable at ambient temperature, decays quite fast into two Cu(II) complexes, leaving a permanent pale-green color as final products. The flow investigated consists in a pure oxygen single bubble rising freely in a confined thin-gap cell (400 × 200 × 1 mm). A wide range of motion regimes for the bubbles are observed as the Archimedes number ranges from 860<Ar<18,800 and the Reynolds number from 580<Re<8200. A high-resolution 16-bit sCMOS camera, combined with specific filters, is used to capture images from a region-of-interest of the cell, illuminated by a white LED backlight panel. An ad hoc calibration protocol is developed to correlate the grey-levels from the colored signal to the equivalent oxygen concentrations. This procedure then allows to measure indirectly the amount of oxygen transferred to the liquid phase. The series of images are also treated to identify the bubble motion and properties. Thanks to this method, equivalent oxygen concentration fields, gap-averaged and time-averaged, can be reached with high precision in the far-field wake of the bubbles, enabling thus to deeply characterize the mass transfer mechanisms under reactive conditions in such confined configuration, and to establish a dimensionless representation in terms of Sherwood number versus Peclet number. At last, thanks to the knowledge of the kinetic rate of the reaction and of the diffusion coefficients, the Hatta number and the enhancement factor are estimated, and thus the intrinsic Sherwood numbers; these results demonstrate that the enhancement of the mass transfer by the reactions involved with the copper-complexes is not negligible (almost 12–15%).