This paper is concerned with the dynamics of large bubbles subject to various strengths of buoyancy effects, which are associated with applications for underwater explosion. The bubble is produced by electric discharge in a low-pressure tank to enhance the buoyancy effects. Experiments are carried out for a bubble in an infinite field, below a free surface and above a rigid boundary. The effects of buoyancy are reflected by the dimensionless parameter δ = √ ρgR m /(p amb − p v ), where R m , p amb , p v , ρ and g are the maximum bubble radius, ambient pressure, saturated vapour pressure, density of water and the acceleration of gravity respectively. A systematic study of buoyancy effects is carried out for a wide range of δ from 0.034 to 0.95. A series of new phenomena and new features is observed. The bubbles recorded are transparent, and thus we are able to display and study the jet formation, development and impact on the opposite bubble surface as well as the subsequent collapsing and rebounding of the ring bubble. Qualitative analyses are carried out for the bubble migration, jet velocity and jet initiation time, etc. for different values of δ. When a bubble oscillates below a free surface or above a rigid boundary, the Bjerknes force due to the free surface (or rigid boundary) and the buoyancy are in opposite directions. Three situations are studied for each of the two configurations: (i) the Bjerknes force being dominant, (ii) the buoyancy force being dominant and (iii) the two forces being approximately balanced. For case (iii), we further consider two subcases, where both the balanced Bjerknes and buoyancy forces are weak or strong. When the Bjerknes and buoyancy forces are approximately balanced over the pulsation, some representative bubble behaviours are observed: the bubble near free surface is found to split into two parts jetting away from each other for small δ, or involutes from both top and bottom for large δ. A bubble above a rigid wall is found to be subject to contraction from the lateral part leading to bubble splitting. New criteria are established based on experimental results for neutral collapses where there is no dominant jetting along one direction, which correlate well with the criteria of Blake et al. (J. but agree better with the experimental and computational results.

Small-charge underwater explosion experiments were performed to investigate bubbles subjected to gravity and various boundary conditions, including single boundary (free surface and rigid wall boundary), combined boundaries of free surface and solid wall, solid wall boundaries with a circular opening, and resilient wall boundaries. With high speed camera and pressure sensors, the behavior of explosion bubbles was studied and features of associated pressure pulses were analyzed. Detailed image analysis on the final stages of bubble collapse was carried out and revealed a possible explanation for the weakening of pressure waves at bubble rebound as the bubble approaches a wall boundary. Certain features also indicate that the magnitude of the pressure peaks induced by bubble rebound is related to the shape of the bubble shape during collapse. Pressure pulses arising from the two types of bubble behavior, specifically the collision of an annular jet and the impact of a jet with the wall boundary, were measured. Other curious types of bubble behavior were found, including jetting induced by suction when a bubble collapses covering a circular opening on a solid wall, and bubble splitting in interaction with a resilient wall boundary.

This work focuses on using the power of a collapsing bubble in ice breaking. We experimentally validated the possibility and investigated the mechanism of ice breaking with a single collapsing bubble, where the bubble was generated by underwater electric discharge and collapsed at various distances under ice plates with different thicknesses. Characteristics of the ice fracturing, bubble jets and shock waves emitted during the collapse of the bubble were captured. The pattern of the ice fracturing is related to the ice thickness and the bubble–ice distance. Fractures develop from the top of the ice plate, i.e. the ice–air interface, and this is attributed to the tension caused by the reflection of the shock waves at the interface. Such fracturing is lessened when the thickness of the ice plate or the bubble–ice distance increases. Fractures may also form from the bottom of the ice plate upon the shock wave incidence when the bubble–ice distance is sufficiently small. The ice plate motion and its effect on the bubble behaviour were analysed. The ice plate motion results in higher jet speed and greater elongation of the bubble shape along the vertical direction. It also causes the bubble initiated close to the ice plate to split and emit multiple shock waves at the end of the collapse. The findings suggest that collapsing bubbles can be used as a brand new way of ice breaking.

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Experiments are carried out on the interaction and coalescence of two, three and four bubbles with approximately the same sizes, distributed evenly and symmetrically. The bubbles are generated simultaneously by electric discharges, using an in-house designed series circuit, and their interaction is captured using a high-speed camera. Particular attentions are paid to if/when coalescence of bubbles happens, and the motion of the joined bubbles. Some new features are observed, which depend mainly on the dimensionless distance γ bb = d bb /R max , where d bb is the inter-bubble distance and R max is the maximum bubble radius. For γ bb > 2, a jet forms and penetrates each side bubble, directed to the center of the configuration, resulting in a protrusion. Towards the end of collapse, a large portion of bubble gases is compressed into the protrusion from the main part of the toroidal bubble. For γ bb < 2, the bubbles coalesce during expansion, and the part of the joined bubble's surface distal from the center of the configuration collapses faster than elsewhere. The experiments show that the oscillation period of multi-bubbles does not change appreciably without coalescence but increases significantly with coalescence. For three bubbles initiated at collinear positions with γ bb > 2, the jets that form from the side bubbles are towards the middle, and the middle bubble splits into two parts, moving towards the two side bubbles. For γ bb < 2, the side bubbles merge with the middle bubble during expansion, forming an ellipsoid bubble; the joined bubble collapses predominantly from two sides, where two inward jets form towards the end of collapse.

Dry tea matrix contains an abundance of caffeine and polyphenols which are different from the food matrix (e.g., protein, lipid, and carbohydrates), and only a few studies have tried aflatoxins determination with tea samples. Here, a specific, accurate, and sensitive method was developed and validated for the simultaneous determination of aflatoxin B1, B2, G1, and G2 in dark teas. Aflatoxins were extracted by acetonitrile/water, press-filtered, and cleaned by multifunctional purification column (MFC) and immunoaffinity column (IAC) in tandem. The cleaned extract was analyzed by liquid chromatography tandem mass spectrometry. The matrix interference was effectively reduced by MFC-IAC cleaning method. Recoveries at the spiking concentrations of 5–60 μg/kg ranged from 77.5 to 93%, with relative standard deviations <11.0%. The correlation coefficients of aflatoxins standard were >0.998. The limits of detection were 0.024–0.21 μg/kg and the limits of quantification were 0.08–0.74 μg/kg. The intra- and interday accuracy ranged from 74 to 87%, and the intra- and interday precisions ranged from 0.4 to 3.1%. After the method validation, the aflatoxins contaminations in 100 collected dark teas were detected, and the results were compared with those of other methods.

In this work, we established a novel theory for the dynamics of oscillating bubbles such as cavitation bubbles, underwater explosion bubbles, and air bubbles. For the first time, we proposed bubble dynamics equations that can simultaneously take into consideration the effects of boundaries, bubble interaction, ambient flow field, gravity, bubble migration, fluid compressibility, viscosity, and surface tension while maintaining a unified and elegant mathematical form. The present theory unifies different classical bubble equations such as the Rayleigh-Plesset equation, the Gilmore equation, and the Keller-Miksis equation. Furthermore, we validated the theory with experimental data of bubbles with a variety in scales, sources, boundaries, and ambient conditions and showed the advantages of our theory over the classical theoretical models, followed by a discussion on the applicability of the present theory based on a comparison to simulation results with different numerical methods. Finally, as a demonstration of the potential of our theory, we modeled the complex multi-cycle bubble interaction with wide ranges of energy and phase differences and gained new physical insights into inter-bubble energy transfer and coupling of bubble-induced pressure waves.