The electrochemical conversion of captured carbon dioxide (CO2) at low temperatures holds promise as a sustainable method for producing materials and fuel using renewable energy sources. However, technological hurdles such as mass transfer limitations and operational instability hinder its industrial application. This dissertation aims to address these challenges by exploring the use of gas-liquid Taylor flow (series of confined gaseous CO2 bubbles, which are separated from each other by liquid electrolyte and from the channel walls by a thin liquid film) in electrolysis, which can enhance mass transfer without requiring complex electrode designs, potentially improving long-term operational reliability. Additionally, a multi-scale modelling framework is introduced to evaluate electrolyser designs from an economic standpoint, aiding in the identification of bottlenecks and guiding technology development.
In Chapter 2, we propose a tubular electrolyser design operating under gas-liquid Taylor flow to overcome mass transfer limitations. By developing a numerical model, we investigate the relationship between process conditions, mass transfer, and reactor performance. Insights gained from this model allow us to derive an easy-to-use analytical relation to evaluate the impact of changes in inlet flow rates on Faradaic efficiency and current density. We find that long gaseous CO2 bubbles and low velocities enhance the current density towards CO, outperforming traditional H-cells. However, achieving performance comparable to flow-through electrolysers operated with a gas diffusion electrode (GDE) requires means to increase CO2 solubility in the liquid electrolyte, by for example increasing pressure.
Chapter 3 focuses on experimentally testing how Taylor flow influences the electrolyser performance within the established zero-gap water electrolyser concept adapted for CO2 reduction, by employing a silver gauze as the cathode. Our experimental findings reveal that Taylor flow enhances the Faradaic efficiency towards CO compared to single-phase flow, with minimal influence from gas holdup within the studied velocity range. Contrary to the tubular design, high velocities are desirable to increase the Faradaic efficiency towards CO in the rectangular flow channel. We find that further optimisation of
cathode design and fabrication is needed to fully exploit the potential of this electrolyser concept.
In Chapter 4, techno-economic aspects of electrochemical CO2 conversion are addressed, aiming to optimise operational parameters for industrial applications. A multiscale model capturing mass transfer effects over the channel length of a GDE electrolyser is integrated into an economic framework to analyse the interdependencies of key performance variables on the economic outlook. The analysis indicates that optimal current densities may differ significantly from previously reported benchmarks, emphasising the importance of multi–scale modelling for evaluating electrolyser designs under economic considerations.@en

In recent years, biotechnological processes have gained increased interest due to their potential for high-value compound production and waste recycling. This shift towards biotechnology is driven by global challenges such as food security, climate change, and the transition to renewable resources. To address the limitations of large-scale fermentations, scale-down approaches have been recommended to minimize microbial performance losses during scale-up procedures. Computational fluid dynamics (CFD) coupled with omics-based technologies offer valuable insights into the environmental and intracellular
lifelines of cells. However, current laboratory-scale setups have certain limitations, emphasizing the need for dynamic microfluidic single-cell cultivation (dMSCC) devices. These devices enable the analysis of single-cell behavior in dynamic environments with high temporal resolution.

This thesis focuses on improving the amplitude control while maintaining temporal resolution in dMSCC devices. A new dMSCC device design was analyzed using a 2D model, which was experimentally validated. The results demonstrated that the design mechanism effectively generated concentration profiles resembling discrete and smooth lifelines, albeit with a relatively high response time (30 seconds). A mesh independence study indicated minimal deviations (2 %) in results for different mesh refinements, while complex geometric structures introduced greater variations.

The experimental validation of the 2D COMSOL Multiphysics model highlighted discrepancies between the experimental data and model predictions, both at the outlets of the microfluidic concentration gradient generator (μCGG) and inside the chamber (RMSE=0.1-0.75; >10% of experimental data). However,
the observed trends of the concentration profiles inside the chamber were well-captured. Optimization studies were conducted based on these findings, leading to valuable conclusions. Narrowing the chamber width increased the chip’s response time. Moreover, increasing the space between μCGG outlets
as well as increasing fluid velocity inside the μCGG (while keeping the maximum velocity constant) improved gradient width. The latter approach is preferred to maintain temporal resolution. A comparison between COMSOL Multiphysics (RMSE=0.14) and Ansys Fluent (RMSE=0.15) models revealed that Ansys Fluent better captures experimental trends but has lower prediction accuracy. Further investigations involved a Design of Experiments (DoE), which indicated that the current μCGG design is suitable for fluid velocities preferably lower than 1 · 10−5 m/s and tracers with high diffusion coefficients. These conclusions provide insights into optimizing dMSCC devices and contribute to the broader understanding of mimicking microbial lifelines.

Underground Hydrogen storage (UHS) is an attractive technology for large-scale energy storage. The UHS safety and efficiency depends highly on accurate characterization of H2 interactions with reservoir fluids, specially wettability analyses for H2/brine/rock systems. This thesis reports experimental measurements of advancing and receding contact angles of H2/water, N2/water and CO2/water systems at P = 10 bar and T = 20 °C using a microfluidic chip (channel widths: 50 - 130 μm). The results indicate strong water-wet conditions with H2/water advancing and receding contact angles of respectively 13 - 39°, and 6 - 23°. It was found that the contact angles decrease with increasing channel widths. Little hysteresis was measured, and consequently, the results are not in line with Morrow’s curve. The receding contact angle measured in the smallest channel agrees well with the literature coreflood tests. The N2/water and CO2/water systems showed similar behaviours as the H2/water system.

A crucial challenge during the initial stages of bioprocess development is that tools used to screen microorganisms and optimize cultivation conditions do not represent the environment imposed at industrial scale. Inside an industrial-scale bioreactor, microorganisms are often cultivated under fed-batch conditions, where nutrients are supplied during the culture. Additionally, microorganisms continuously keep crossing zones with low and high concentrations of substrate and dissolved oxygen. However, during initial bioprocess development, growth and productivity of microorganisms are evaluated under batch conditions due to the difficulty of dynamically controlling nutrient and dissolved oxygen concentrations in screening equipment such as micotiter plates. This inconsistency in cultivation conditions often leads to selection of strains that fail to perform at industrial scale. The difficulty in continuously supplying minute amounts of nutrients to microorganisms in microtiter plates and imposing dynamic dissolved oxygen levels throughout the cultivation experiment necessitates an alternative approach. Microfluidic technology holds the potential to address this inconsistency with fidelity by offering high-throughput experimentation and excellent control over the culture microenvironment. The central theme of this Ph.D. project is the design and development of droplet-based microfluidic technology, that enable studying microorganisms under such dynamically controlled cultivation conditions. As such, the outcomes from this Ph.D. project form a foundation step towards narrowing the gap between screening and industrial-scale use, with an eye to keeping the technology sufficiently simple to be adopted by the biotechnology and bioengineering community. @en

The cellular environment is characterized by confinement and macro-molecular crowding: both concepts that have been studied separately. To understand kinetics of enzymatic reactions, there is a need to understand how the diffusional encounters of enzyme and substrate proceed in an environment
that is confined and crowded simultaneously. The project carried out in this thesis is the first step towards achieving the ultimate goal of studying biochemical reactions in native cellular environment - Understanding diffusion in confinement. Despite multiple investigations of diffusion of analytes in confinement, there exists a research gap. There is inconsistency in the interpretation of the results and in the dependence of diffusion properties of analytes of different sizes in channels of different dimensions. Hence, to bridge the research gap, the main goal of this thesis project was to find the diffusion coefficient of 100 nm polystyrene beads in microchannel (200 µm wide and 4.5 µm high) and nanochannel (5 or 10 µm wide and 300 nm high). The Brownian motion of particles was observed using Confocal Laser Scanning Microscope. A preliminary study first confirmed the reliability and optimization of the particle tracking method of finding diffusion coefficient. Diffusion coefficient of the particles determined experimentally in microchannel (bulk system) was in agreement with the theoretical estimate and statistically significant. Experiments in the nanochannel revealed a reduction in the particle diffusion coefficient of about ∼52% compared to bulk, due to interactions with the confining depth
(300 nm) of nanochannel. An interesting behaviour was also exhibited by particles diffusing close to the side wall along the width of nanochannels, which was not confining (5 or 10 µm). The diffusion coefficient in such a case reduced by around ∼90% relative to bulk. The reduction in both cases can be mainly attributed to hydrodynamic interactions. The experimental investigations of diffusion coefficient carried out in this study were in agreement with long standing theoretical predictions. However, the research gap could not be fully expelled.