Oscillation, coalescence and shape deformation of confined bubbles in an oscillating depth microchannel: application to the fragmentation of electrocrystallized ramified branches for an alternative synthesis of colloidal metallic nanoparticles
Résumé
This work focuses on the behavior of bubbles confined in an oscillating depth microchannel. This situation is encountered in a recently developed device, an "electrochemical and vibrating Hele-Shaw cell", designed to synthesize colloidal metallic nanoparticles (cMnP) [1]. The principle is to make grow metallic ramified branches by galvanostatic electrolysis of a metal salt aqueous solution inside the cell. These fragile branches are composed of nanocrystals (the desired cMnP), whose the dissociation is achieved by the mechanical action resulting from the activation of a piezoelectric diaphragm (PZT) integrated into the cell as one of its largest side; the variations of the channel depth are induced by the bending of the PZT surface. In the case of the production of iron cMnP, the branches growth is accompanied by the formation of H2 bubbles (co-reduction of H+). High speed visualizations of the fragmentation process highlight the key role of these bubbles whose the oscillations induce both microstreaming and branches fragmentation in close vicinity of their surface. An efficient fragmentation is obtained only when the PZT is driven with a square signal. The observed dependence of the bubbles behavior, to the waveform used, is analyzed with the support of specifically derived theoretical models for both breath oscillations (Rayleigh-Plesset like equation) and shape oscillations (stability analysis) of a single bubble in an oscillating depth microchannel. The specificity of the oscillating depth (the driving force), acting simultaneously on both the bubbles size and the liquid flow, is taken into account using a modified Darcy's law adapted to small depth variations.
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