Influence of wetting conditions on bubble formation at orifice in an inviscid liquid

Prediction of Newtonian Droplet Breaking Time from a Capillary at Low Weber Numbers

Droplet formation and growth processes have numerous scientific and industrial applications. Experimental and numerical studies on the formation, growth, and breaking of droplet are carried out in present work. The numerical results are in good agreement with the experiment. This work focused on the effect of different Weber numbers (We) on the droplet breaking time. The results show when We < 0.05, the length and volume of the droplet increase, and the breaking time decreases rapidly. The resultant force acting on the main droplet suddenly drops around the critical breaking time. The difference rate between the time t _n (when the resultant force is zero) and the breaking time t _b is less than 8.49%. For the dimensional analysis of the numerical results, a prediction formula of breaking time on the Weber number is modeled as aWe ^b + c for We < 0.5.

Asymmetric Bubble Formation at Rectangular Orifices

Bubble formation in liquids is frequently observed in nature and applied in various industrial processes. These include pool and flow boiling for thermal management systems, where bubbles may form asymmetrically at narrow slits and in convective flows. While previous studies have focused on symmetric bubble formation at circular orifices, the dynamics of asymmetric bubble formation remains poorly understood. Here, we experimentally investigate bubble formation at rectangular orifices and examine the effects of the orifice size and aspect ratio and the gas flow rate on the bubble size. The asymmetric bubble shape evolution at the rectangular orifice is analyzed, and we find that the size of the bubble neck is controlled either by the orifice size or by the capillary length. Based on these findings, we develop a static force balance model to predict the bubble size in the quasi-static regime, where the roles of Bond number and aspect ratio are identified. The bubble size evolution in the dynamic regime is further understood by introducing a Weber number that evaluates the effect of the virtual mass force induced by gas flow. Our study provides physical understanding of the dynamics of asymmetric bubble formation and guidance to predict the bubble size at asymmetric orifices.

An Experimental Study on Bubble Growth in Laponite RD as Thixotropic Yield Material

The growth and release of the leading major bubble at the tip of a needle in the thixotropic yield material Laponite RD was different from subsequent minor bubbles. The gas injection experiments combined with high-speed camera were conducted. The results showed that the shape of the major bubbles transformed from an inverted carrot shape to an inverted teardrop shape, while the shape of the minor bubbles tended to be elliptical. In addition, the pressure of bubble emergence consisted of hydrostatic pressure, capillary pressure, and cracking pressure. The major and minor bubbles differed only in the cracking pressure. The pressure when the minor bubble detached could be estimated from the lateral hydrostatic pressure. It can be deduced from dimensionless numbers that buoyancy and viscous forces were, respectively, the main driving force and resistance of bubble growth. The yield stress of Laponite RD and inertial force at the initial moment resulted in distinctive behavior of the major bubble. In addition to the viscosity resistance, surface tension, and hydrostatic pressure had a non-negligible influence on minor bubbles and still accounted for 10-20% of the total resistance in the later stage but less than 5% in major bubble growth.

Impact of the accumulation and adhesion of released oxygen during Scenedesmus obliquus photosynthesis on biofilm formation and growth

Microalgae cells release O₂ during photosynthesis. The gas can accumulate and adhere in form of bubbles, which affect the transport of nutrients in the biofilm and the biofilm microstructure. To investigate the reasons for the adhesion of these oxygen bubbles and their impact on biofilm, polytetrafluoroethylene (PTFE) emulsion was sprayed onto glass surface to change the parameters for gas accumulation and adhesion. The results indicated gas could aggregate into bubbles and adhere to hydrophobic and rough surface. The bubble behaviors caused the biofilm to be porous (with a microporosity of 9.43-20.94%). The biomass concentration of the more porous biofilm increased by 9.26% to 22.42gm^-2 on 1% PTFE-treated surface compared to that on an untreated surface. However, with an increase in PTFE concentration, the amount of adhered bubbles increased. More microalgae cells in biofilms were carried up by bubbles. The biofilm concentration on 5% PTFE-treated surface decreased by 15.30%.

Influence of Surface Wettability on Microbubble Formation

The production and utilization of microbubbles are rapidly becoming of major importance in a number of global applications, from biofuel production to medical imaging contrast agents. Many aspects of bubble formation have been studied, with diffuser characteristics (such as pore size, pore orientation) and gas flow rate all being shown to influence the bubble formation process. However, very little attention has been paid to the influence of surface wettability of the diffuser and the detailed role it plays at the triple interface of gas-liquid-diffuser. Here, we investigate how the wettability of the diffuser surface impacts upon the dynamics of the bubble formation process and examine the effect both at the orifice and upon the bubble cloud produced as a result of the engineered wetting variations. Experimental data shown here indicate the presence of a switching point at a contact angle of θ = 90°, where bubble size vastly changes. When a surface exhibits a contact angle below 90°, bubbles emitted from it are considerably smaller than those emitted from a surface with an angle in excess of 90°. This effect is observable over flow rates ranging from 2.5 to 60 mL min(-1) from a single pore, an array of controlled pores, and the industrially relevant and commercially available sintered metals and sintered ceramic diffusers. It is also observed for both thiol and silane modified surfaces, encompassing a range of contact angles from 10° to 110°. In addition, the importance of the diffuser plate's surface topography is discussed, with elevated roughness acting to reduce the effect of surface chemistry in some instances.

Commercial Applications of Metal Foams: Their Properties and Production

This work gives an overview of the production, properties and industrial applications of metal foams. First, it classifies the most relevant manufacturing routes and methods. Then, it reviews the most important properties, with special interest in the mechanical and functional aspects, but also taking into account costs and feasibility considerations. These properties are the motivation and basis of related applications. Finally, a summary of the most relevant applications showing a large number of actual examples is presented. Concluding, we can forecast a slow, but continuous growth of this industrial sector.

Modification of the Young-Laplace equation and prediction of bubble interface in the presence of nanoparticles

Bubbles are fundamental to our daily life and have wide applications such as in the chemical and petrochemical industry, pharmaceutical engineering, mineral processing and colloids engineering. This paper reviews the existing theoretical and experimental bubble studies, with a special focus on the dynamics of triple line and the influence of nanoparticles on the bubble growth and departure process. Nanoparticles are found to influence significantly the effective interfacial properties and the dynamics of triple line, whose effects are dependent on the particle morphology and their interaction with the substrate. While the Young-Laplace equation is widely applied to predict the bubble shape, its application is limited under highly non-equilibrium conditions. Using gold nanoparticle as an example, new experimental study is conducted to reveal the particle concentration influence on the behaviour of triple line and bubble dynamics. A new method is developed to predict the bubble shape when the interfacial equilibrium conditions cannot be met, such as during the oscillation period. The method is used to calculate the pressure difference between the gas and liquid phases, which is shown to oscillate across the liquid-gas interface and is responsible for the interface fluctuation. The comparison of the theoretical study with the experimental data shows a very good agreement, which suggests its potential application to predict bubble shape during non-equilibrium conditions.

Bubble formation in a quiescent pool of gold nanoparticle suspension

This paper begins with an extensive review of the formation of gas bubbles, with a particular focus on the dynamics of triple lines, in a pure liquid and progresses into an experimental study of bubble formation on a micrometer-sized nozzle immersed in a quiescent pool of aqueous gold nanofluid. Unlike previous studies of triple line dynamics in a nanofluid under evaporation or boiling conditions, which are mainly caused by the solid surface modification due to particle sedimentation, this work focuses on the roles of nanoparticles suspended in the liquid phase. The experiments are conducted under a wide range of flow rates and nanoparticle concentrations, and many interesting phenomena are revealed. It is observed that nanofluids prevent the spreading of the triple line during bubble formation, i.e. the triple line is pinned somewhere around the middle of the tube wall during the rapid bubble formation stage whereas it spreads to the outer edge of the tube for pure water. A unique 'stick-slip' movement of the triple line is also observed for bubbles forming in nanofluids. At a given bubble volume, the radius of the contact line is found to be smaller for higher particle concentrations, but a reverse trend is found for the dynamic bubble contact angle. With the increase of particle concentration, the bubble frequency is raised and the bubble departure volume is decreased. The bubble shape is found to be in a good agreement with the prediction from Young-Laplace equation for given flow rates. The influence of nanoparticles on other detailed characteristics related to bubble growth inside, including the variation of bubble volume expansion rate, the radius of the curvature at the apex, the bubble height and bubble volume, is revealed. It is suggested that the variation of surface tensions and the resultant force balance at the triple line might be responsible for the modified dynamics of the triple line.