Cavitation and Bubble Dynamics: Preface

Influence of Bubbles on the Energy Conversion Efficiency of Electrochemical Reactors

Bubbles are known to influence energy and mass transfer in gas evolving electrodes. However, we lack a detailed understanding on the intricate dependencies between bubble evolution processes and electrochemical phenomena. This review discusses our current knowledge on the effects of bubbles on electrochemical systems with the aim to identify opportunities and motivate future research in this area. We first provide a base background on the physics of bubble evolution as it relates to electrochemical processes. Then we outline how bubbles affect energy efficiency of electrode processes, detailing the bubble-induced impacts on activation, ohmic and concentration overpotentials. Lastly, we describe different strategies to mitigate losses and how to exploit bubbles to enhance electrochemical reactions.

Gas bubbles are almost a routine occurrence encountered by researchers working in the field of microfluidics. The spontaneous and unexpected nature of gas bubbles represents a major challenge for experimentalists and a stumbling block for the translation of microfluidic concepts to commercial products. This is a startling example of successful scientific results in the field overshadowing the practical hurdles of day-to-day usage. We however believe such hurdles can be overcome with a sound understanding of the underlying conditions that lead to bubble formation. In this tutorial, we focus on the two main conditions that result in bubble nucleation: surface nuclei and gas supersaturation in liquids. Key theoretical concepts such as Henry's law, Laplace pressure, the role of surface properties, nanobubbles and surfactants are presented along with a view of practical implementations that serve as preventive and curative measures. These considerations include not only microfluidic chip design and bubble traps but also often-overlooked conditions that regulate bubble formation, such as gas saturation under pressure or temperature gradients. Scenarios involving electrolysis, laser and acoustic cavitation or T-junction/co-flow geometries are also explored to provide the reader with a broader understanding on the topic. Interestingly, despite their often-disruptive nature, gas bubbles have also been cleverly utilized for certain practical applications, which we briefly review. We hope this tutorial will provide a reference guide in helping to deal with a familiar foe, the “bubble”.

Nip the bubble in the bud: a guide to avoid gas nucleation in microfluidics

Renewables challenge the management of energy supply and demand due to their intermittency. A promising solution is the direct conversion of the excess electrical energy into valuable chemicals in electrochemical reactors that are inexpensive, scalable, and compatible with irregular availability of electrical power. Membrane-less electrolyzers, deployed on a microfluidic platform, were recently shown to hold great promise for efficient electrolysis and cost-effective operation. The elimination of the membrane increases the reactor lifetime, reduces fabrication costs, and enables the deployment of liquid electrolytes with ionic conductivities that surpass those allowed by solid membranes. Here, we demonstrate a membrane-less architecture that enables unprecedented throughput by 3D printing a device that combines components such as the flow plates and the fluidic ports in a monolithic part, while at the same time, providing tight tolerances and smooth surfaces for precise flow conditioning. We show that inertial fluidic forces are effective even in millifluidic regimes and, therefore, are utilized to control the two-phase flows inside the device and prevent cross-contamination of the products. Simulations provide insight on governing fluid dynamics of coalescing bubbles and their rapid jumps away from the electrodes and help identify three key mechanisms for their fast and intriguing return towards the electrodes. Experiments and simulations are used to demonstrate the efficiency of the inertial separation mechanism in millichannels and at higher flow rates than in microchannels. We analyze the performance of the present device for two reactions: water splitting and the chlor-alkali process, and find product purities of more than 99% and Faradaic efficiencies of more than 90%. The present membrane-less reactor – containing more efficient catalysts – provides close to 40 times higher throughput than its microfluidic counterpart and paves the way for realization of cost-effective and scalable electrochemical stacks that meet the performance and price targets of the renewable energy sector.

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A versatile and membrane-less electrochemical reactor for the electrolysis of water and brine

Recent studies indicate that the transition from sheet to cloud cavitation depends on both cavitation number and Reynolds number. In the present paper this transition is investigated analytically and a physical model is introduced. In order to include the entire process, the model consists of two parts, a model for the growth of the sheet cavity and a viscous film flow model for the so-called re-entrant jet. The models allow the calculation of the length of the sheet cavity for given nucleation rates and initial nuclei radii and the spreading history of the viscous film. By definition, the transition occurs when the re-entrant jet reaches the point of origin of the sheet cavity, implying that the cavity length and the penetration length of the re-entrant jet are equal. Following this criterion, a stability map is derived showing that the transition depends on a critical Reynolds number which is a function of cavitation number and relative surface roughness. A good agreement was found between the model-based calculations and the experimental measurements. In conclusion, the presented research shows the evidence of nucleation and bubble collapse for the growth of the sheet cavity and underlines the role of wall friction for the evolution of the re-entrant jet.

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The transition from sheet to cloud cavitation

Investigations about the role of nuclei and nucleation for the inception and formation of cavitation have been part of cavitation research since Harvey et al (J Cell Physiol, vol 24 (1), 1944, pp 1–22) postulated the existence of gas filled crevices on surfaces and particles in liquids In a supersaturated liquid, surface nuclei produce small gas bubbles due to mass transfer of gas or themselves work as weak spots in the liquid that are necessary for a phase change under technically relevant static pressures Although various theories and models about nuclei and nucleation have found their way into standard literature, there is a lack of experimentally validated theories that describe the process of diffusion-driven nucleation in hydrodynamic cavitation In order to close this gap we give new theoretical insights into the physics of this nucleation mechanism at technically relevant low supersaturations validated with extensive experimental results The nucleation rate, the number of produced bubbles per second, is proportional to the supersaturation of the liquid and shows a nonlinear dependence on the shear rate at the surface nucleus A model for the Strouhal number as dimensionless nucleation rate is derived allowing the estimation of nucleation rates from surface nuclei in hydrodynamic cavitation The model provides three asymptotes, being a function of Peclet number, Weber number, the supersaturation of the liquid and gas solubility for three different detachment mechanisms, with  The theoretical findings are in good agreement with experimental results, leading to a new assessment of the role of diffusion in cavitating flows

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Diffusion-driven nucleation from surface nuclei in hydrodynamic cavitation

The formation of gas bubbles at gas cavities located in walls bounding the flow occurs in many technical applications, but is usually hard to observe. Even though, the presence of a fluid flow undoubtedly affects the formation of bubbles, there are very few studies that take this fact into account. In the present paper new experimental results on bubble formation (diffusion-driven nucleation) from surface nuclei in a shear flow are presented. The observed gas-filled cavities are micrometre-sized blind holes etched in silicon substrates. We measure the frequency of bubble generation (nucleation rate), the size of the detaching bubbles and analyse the growth of the surface nuclei. The experimental findings support an extended understanding of bubble formation as a self-excited cyclic process and can serve as validation data for analytical and numerical models.

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Bubble nucleation from micro-crevices in a shear flow

Booster systems are used to ensure the water supply in multistory buildings, office buildings, and public institutions. The question is how to operate a booster system to achieve optimal po...

Analytical Approach for the Optimal Operation of Pumps in Booster Systems

The importance of nucleation from wall-bounded nuclei for cavitation and especially cavitation inception is undisputed. Although various theories and models found their way to standard literature, there is a lack of experiments allowing a closer look onto the process of nucleation. In the present paper we present a new experimental set-up that allows the investigation and analysis of nucleation from wall-bounded nuclei. The experimental findings support an extended understanding of nucleation as a self excited cyclic process. Impressive high-speed visualisations can be found in the supplementary material.

T. F. Groß

Papers

The transition from sheet to cloud cavitation

Diffusion-driven nucleation from surface nuclei in hydrodynamic cavitation

Bubble nucleation from micro-crevices in a shear flow

Analytical Approach for the Optimal Operation of Pumps in Booster Systems

Experimental evidence of nucleation from wall-bounded nuclei in a laminar flow