title: Transport in catalytic microchannels
Creators: name: Antunes, Gonçalo
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: This work focuses on the ways in which the transport of chemically-reactive fluids may be affected by the presence of confinement. Specifically, we study flow through microchannels whose walls are patterned with catalytic material. We explore two setups which differ in their material properties and transport mechanisms. Despite these differences, properties such as the channel corrugation or the distribution of catalytic material play a role in both setups. The first system consists of a gas driven through a catalytic microreactor. On the walls of said reactor the gas undergoes a chemical reaction, which leads to the generation of a second chemical species. The second system comprises a catalytic microchannel/pore filled with a reactive liquid, leading to the formation of a dissolved chemical species. The concentration of this species is spatially-varying near the channel wall. Due to the phenomenon of diffusioosmosis, flows form along the walls. As such, the second system differs fundamentally from the gas-phase reactor, in the sense that motion is driven internally (due to the presence of the chemical reaction) rather than externally (due to the presence of an applied pressure drop). Additionally, the gas-phase reactor explores a regime in which the reactive fluid is compressible. However, it also neglects the inverse chemical reaction that turns product back into reactant. In contrast, said inverse reaction is included in the diffusioosmotic channel. We establish connections between the two above-described systems to the wider topics of microfluidics, synthetic colloidal swimmers, and the design of catalytic reactors. A literature review of these topics is presented, aiming at motivating this work, and to present the state of the art. To perform our studies, we employ both numerical and analytical methods. Our numerical methods consist chiefly of the lattice Boltzmann method, to which we couple an additional finite difference solver. We further present a detailed derivation of this method, and a demonstration of its validity as a solver of the governing equations. To obtain analytical results, we have effectively reduced the dimensionality of the governing equations by employing the Fick-Jacobs and lubrication approximations. By means of such a theory, we are able to obtain closed-form results for the pressure inside the gas-phase reactor, as well as for the flow of product at the reactor outlet. We find an optimum reactor length that maximizes this flow. We show that in the highly-compressible regime, there is also an optimum corrugation height. We apply our theory to a model monolithic reactor and find an optimum channel size. We then perform lattice Boltzmann simulations of diffusioosmosis-driven catalytically-active channels/pores. We find that fore-aft symmetric pores exhibit three different kinds of dynamics: a mixing state with convection currents, a pumping state where a net flow develops, and an oscillating state where the flow rate exhibits sustained oscillations. We discuss the mechanism behind each of these states, and show how they depend on parameters such as the distribution of catalytic material, and the corrugation height of the channel wall. To complement the above-mentioned study, we develop a Fick-Jacobs theory for such active pores. From it, we conclude that the spontaneous symmetry breaking that leads to pumping is governed by three timescales: the timescale for diffusive and advective transport, as well as the average lifetime of the solute. We show how hysteresis and spontaneous jumps in the flow rate are a generic property of fore-aft asymmetric pores. Said effects can be triggered by asymmetry both in the shape of the pore, or in the distribution of catalytic material. Finally, we summarize our main results, and present an outlook for future research.