Numerical investigation of acoustic cavitation and viscoelastic tissue deformation

Acoustic cavitation and tissue deformation are studied by modifying a level-set method for compressible two-phase flows to consider viscoelastic tissue deformation. The numerical simulations performed using different shear moduli and bubble-tissue distances demonstrate various interactions between bubble and viscoelastic tissue, including inverted cone-shape bubbles, bubble migration, liquid jet formation, compressive and expansive tissue deformation, and tissue perforation. The bubble is observed to grow larger with increasing tissue bulk modulus and density. The maximum tissue deformation generally increases with decreasing initial bubble-tissue distance and with increasing tissue bulk modulus and density. The tissue shear modulus conditions that maximize tissue deformation are in the range of 1-10 MPa, unless the tissue density is very large.

Flat bread baking: Single to double layer transition

Double-layered flat bread features an impressive oven rise and delamination during baking, leading to the formation of an internal pocket capable of holding various solid foods - a key quality criterion for consumers. These breads are unique in their baking method, which requires specialized ovens and high temperatures between 350 and 550 °C. Use of high baking temperatures to achieve the double layering development (called delamination) during baking has raised concerns over excessive energy consumption. In this study, whether or not the application of fermentation (affecting the accumulation of dissolved CO2) and baking temperature (which affects gas generation) were varied and the impacts on delamination were evaluated. To complement this evaluation, dough water content and temperature were monitored during baking. In a novel manner, this study characterized water distribution in flat bread, explaining the surprising decrease of water loss with increasing baking temperature. Our study has clearly shown water vapor to be the prime cause of delamination. The role of water pressure in dough inflation and in causing the edges to detach from the deck and the heterogeneity of flat bread water content were highlighted and a concept map proposed.

Inline imaging reveals evolution of the size distribution and the concentration of microbubbles in dissolved air flotation

Dissolved air flotation (DAF) is an efficient process to remove impurities from fresh or salt water. As the removal is based on the agglomeration of impurities on the generated microbubbles, the size distribution and concentration of air bubbles are key parameters in dissolved air flotation. However, the development of microbubbles in the whole flotation process remains unexplored. In this study, we show that state-of-the-art inline microscopy enables the image acquisition of bubbles in DAF. Based on image analysis, thousands of microbubbles (10-200 µm) were analyzed within 6-12 min experiments. Consequently, bubble size distributions and bubble concentrations can be determined with moderate effort. Bubble size distributions were measured in a lab-scale DAF comprising a saturation unit, a decompression valve in/after which the bubbles are formed, and the actual flotation tank. The state of the microbubbles is not only determined at different positions within the tank but also in the supply pipe from the decompression valve to the tank. All bubble size distributions were unimodal and can be described well with Burr XII distributions. For fresh water, bubble size increased while bubble concentration decreased along the supply pipe between the decompression valve and the inlet of the flotation tank, indicating bubble coalescence. Compared to freshwater, saltwater inhibited this bubble coalescence in the pipe. Within the flotation tank, the bubble size did not change drastically for neither salt- nor freshwater. However, the bubble concentration decreased for both waters, which could be explained by dilution effects. Our results demonstrate that the developed inline method is a promising tool to study the evolution of microbubbles in flotation systems. Further, it might also be applied to investigate microbubbles in other processes such as fermentation, decomposition of organic compounds, and fouling mitigation in membranes.

Bubble growth analysis during subcooled boiling experiments on-board the international space station: Benchmark image analysis

This work compares four different image processing algorithms for the analysis of image data obtained during the Multiscale Boiling Experiment of ESA, executed on-board the International Space Station. Two separate experimental campaigns have been performed in 2019 and 2020, aiming to investigate boiling phenomena in microgravity, with and without the presence of shear flow and electric field. A heated substrate, at the bottom of the test cell, creates a temperature profile across the liquid bulk above it. A laser beam hits a designated microcavity at the middle of the substrate, to initiate nucleation of a single, isolated bubble. In the presence of shear flow or electric field forces, the bubble slides or detaches respectively, leaving the cavity free for the nucleation and growth of a new bubble. The growth of such a bubble within the prescribed temperature profile is studied for varying experimental conditions (i.e. pressure, heat flux, subcooling temperature) by capturing high speed, black and white video images. The presence of light reflections at random locations around the bubble contour vary with bubble size and population. This, combined with the refraction induced optical distortion of vertical image dimension close to the heater, make the accurate detection of bubbles contour a real challenge. Four research teams, namely the University of Pisa (UNIPI), the Institute of Fluid Mechanics of Toulouse (IMFT), the joint group of Aix Marseille University (AMU) and Kutateladze Institute of Thermophysics (IT), and the joined group of Aristotle University of Thessaloniki (AUTH), Technical University of Darmstadt (TUD) and Foundation of Research and Technology in Crete (FORTH), developed separate specialized algorithms to: a) detect bubble edges and b) use these edges to calculate basic bubble geometrical features, such as contact line diameter, bubble diameter and contact angles. These four different approaches diverge in complexity and concept. In the absence of reference measurements at microgravity conditions, measurements efficiency is evaluated based on the comparison of the estimated bubble geometrical features along with pertinent physical arguments. Results show that the efficiency of each approach varies with the nature of measurement. The studied benchmark dataset is published allowing other research groups to test further their own image processing algorithms.

Numerical study of the effect of liquid compressibility on acoustic droplet vaporization

In acoustic droplet vaporization (ADV), a cavitated bubble grows and collapses depending on the pressure amplitude of the acoustic pulse. During the bubble collapse, the surrounding liquid is compressed to high pressure, and liquid compressibility can have a significant impact on bubble behavior and ADV threshold. In this work, a one-dimensional numerical model considering liquid compressibility is presented for ADV of a volatile microdroplet, extending our previous Rayleigh-Plesset based model [Ultrason. Chem. 71 (2021) 105361]. The numerical results for bubble motion and liquid energy change in ADV show that the liquid compressibility highly inhibits bubble growth during bubble collapse and rebound, especially under high acoustic frequency conditions. The liquid compressibility effect on the ADV threshold is quantified with varying acoustic frequencies and amplitudes.

Effects of beverage carbonation on lubrication mechanisms and mouthfeel

The perception of carbonation is an important factor in beverage consumption which must be understood in order to develop healthier products. Herein, we study the effects of carbonated water on oral lubrication mechanisms involved in beverage mouthfeel and hence taste perception. Friction was measured in a compliant PDMS-glass contact simulating the tongue-palate interface (under representative speeds and loads), while fluorescence microscopy was used to visualise both the flow of liquid and oral mucosal pellicle coverage. When carbonated water is entrained into the contact, CO₂ cavities form at the inlet, which limit flow and thus reduce the hydrodynamic pressure. Under mixed lubrication conditions, when the fluid film thickness is comparable to the surface roughness, this pressure reduction results in significant increases in friction (>300% greater than under non-carbonated water conditions). Carbonated water is also shown to be more effective than non-carbonated water at debonding the highly lubricious, oral mucosal pellicle, which again results in a significant increase in friction. Both these transient mechanisms of starvation and salivary pellicle removal will modulate the flow of tastants to taste buds and are suggested to be important in the experience of taste and refreshment. For example this may be one reason why flat colas taste sweeter.

Review-Physicochemical hydrodynamics of gas bubbles in two phase electrochemical systems

Electrochemical systems suffer from poor management of evolving gas bubbles. Improved understanding of bubbles behavior helps to reduce overpotential, save energy and enhance the mass transfer during chemical reactions. This work investigates and reviews the gas bubbles hydrodynamics, behavior, and management in electrochemical cells. Although the rate of bubble growth over the electrode surface is well understood, there is no reliable prediction of bubbles break-off diameter from the electrode surface because of the complexity of bubbles motion near the electrode surface. Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) are the most common experimental techniques to measure bubble dynamics. Although the PIV is faster than LDA, both techniques are considered expensive and time-consuming. This encourages adapting Computational Fluid Dynamics (CFD) methods as an alternative to study bubbles behavior. However, further development of CFD methods is required to include coalescence and break-up of bubbles for better understanding and accuracy. The disadvantages of CFD methods can be overcome by using hybrid methods. The behavior of bubbles in electrochemical systems is still a complex challenging topic which requires a better understanding of the gas bubbles hydrodynamics and their interactions with the electrode surface and bulk liquid, as well as between the bubbles itself.