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The rise of bubbles in liquid is a common phenomenon in chemical engineering applications. Bubble dynamics, however, are not fully understood, particularly at the early stages after bubbles are released from submerged nozzles, or when particles coat the bubble surface. In this work, a detailed investigation of microparticle-laden bubbles rising in water after being released from a nozzle was carried out to determine the influence of bubble surface coverage on the interface dynamics after pinch-off. The use of high-speed photography, at up to 25170 frames per second, allowed two regimes to be systematically investigated for the first time, i.e. an initial bubble shape deformation and shape oscillations. Surface pressure analysis shows that microparticles reduce the apparent surface tension of the interface by generating surface pressure during the initial bubble deformation. In contrast, during shape oscillations, little effect was observed on the period of the dominant harmonic, indicating that surface tension does not change during the oscillations. Harmonic analysis also showed that microparticles at bubble surfaces significantly increase the damping rate of the dominant harmonic, with a dependency on the bubble surface coverage. By quantifying the effect of particles on bubble dynamics, this work contributes to a better understanding of gas–liquid–solid reactors in which particle attachment plays a key role. [Display omitted] • The interface dynamics of particle-laden bubbles were investigated. • Two regimes were observed, an initial bubble shape deformation and shape oscillation. • Particles hinder bubble deformation by decreasing the apparent surface tension. • Particles increase the damping rate of the dominant harmonic significantly. • Particles have a minor influence on the oscillation frequency. [ABSTRACT FROM AUTHOR]

• A simplified model for predicting bubble shape in yield stress fluid is established. • An experiment to observe shape and entrapment condition of bubbles in Yield stress fluid by reducing vessel pressure is carried out. • The yielded region caused by buoyancy depends on liquid yield stress, elasticity and bubble volume. This paper reported a dynamic model of bubble expansion for predicting the geometry of the entrapped bubble by considering the elasto-vicsco-plastic rheology properties of the yield stress fluid and the resistance caused by the ambient yielded region. The yielded region caused by the bubble expansion or collapse is obtained by the Von Mises criterion and the yielded region caused by the buoyancy is obtained by the experiment data. The model can predict the volume and the geometry of the entrapped bubbles in various fluid rheology properties and surface tension. By comparing the bubble geometries obtained from the model and the experiment, the results show that the equivalent bubble radius error is less than 0.89%, the discrepancies of the maximum width and length for the bubble are less than 4.86% and 3.50%. [ABSTRACT FROM AUTHOR]

• First investigation of salt effects on lift coefficients. • Simultaneous bubble and liquid velocity determination. • Contamination effects of NaCl on bubble shape and slip velocity. • Evaluation of hydrophobic tracer particle effects on the bubbles. The lift force is known to strongly influence the lateral bubble distribution in bubbly flows and is therefore an important force that has to be modeled in corresponding CFD simulations. For ellipsoidal bubbles, which are mostly present in industrial cases, the strength as well as the direction in which the lift force acts is thought to be determined by the bubble deformation. The bubble deformation however, can strongly be reduced when surface-active contaminations like salts are present in the liquid bulk, which implies a change of the lift force by contaminations too. In the present work, lift coefficients for single bubbles rising in aqueous NaCl solutions were determined to investigate the influence of such an inorganic surfactant on the lift force. For this purpose, a recently developed method by Ziegenhein et al. (2018) [50] is used, which is capable to measure the lift force in low viscous liquids. Besides the lift force, the bubble shape and slip velocity were studied in detail to connect the results to known contamination effects, which showed different behavior in dependence on the salt concentration. The results reveal that the contamination level plays an important role on changes of the lift force in comparison to clean bubbles. Up to a concentration of 1.0 mol/l the salt has only a weak effect on the lift force of larger bubbles. The lift coefficients of smaller bubbles however, clearly show significant changes, which were also reflected in a change of the bubble shape. However, some findings could only be connected to the slip velocity, which implies a connection of the lift force to more than just the shape. [ABSTRACT FROM AUTHOR]