Recent advances of molecular dynamics simulation on bubble nucleation and boiling heat transfer: A state-of-the-art review.

Boiling heat transfer has become increasingly importance in a variety of industrial fields, but it involves chaotic nature phenomena that remain experimentally challenging. From the perspective of nucleation, bubble embryos emerge at the early stage on extremely small time and length scales. Therefore, molecular dynamics (MD) simulation is a popular and useful tool to uncover the distinctive boiling mechanisms at microscale. Recently, such method has yielded meaningful achievements, but there is still elusive on the current status and bottlenecks behind complex boiling processes. In this work, the state-of-the-art studies on bubble nucleation and boiling heat transfer that covers 129 papers up to 2024 have been comprehensively reviewed. Meanwhile, fundamental concepts of MD are briefly introduced, including MD principles, force fields, and determination of nucleation-related parameters. In contrast to microscopic boiling, bubble nucleation stems from the competition between potential and kinetic energies on micro/nano scale. Then, the key factors such as interfacial wettability and mixture component are thoroughly elucidated for bubble nucleation. In addition, both passive and active techniques are systematically discussed to unveil the underlaying mechanisms for boiling heat transfer enhancement. Finally, the ongoing trials needed for MD simulation are identified, together with an outlook for how to address these challenges. This review aims to offer an up-to-date summary of boiling mechanisms and draw more attention to the development of advanced MD techniques.

Mechanisms controlling the transport and evaporation of human exhaled respiratory droplets containing the severe acute respiratory syndrome coronavirus: a review.

Transmission of the coronavirus disease 2019 is still ongoing despite mass vaccination, lockdowns, and other drastic measures to control the pandemic. This is due partly to our lack of understanding on the multiphase flow mechanics that control droplet transport and viral transmission dynamics. Various models of droplet evaporation have been reported, yet there is still limited knowledge about the influence of physicochemical parameters on the transport of respiratory droplets carrying the severe acute respiratory syndrome coronavirus 2. Here we review the effects of initial droplet size, environmental conditions, virus mutation, and non-volatile components on droplet evaporation and dispersion, and on virus stability. We present experimental and computational methods to analyze droplet transport, and factors controlling transport and evaporation. Methods include thermal manikins, flow techniques, aerosol-generating techniques, nucleic acid-based assays, antibody-based assays, polymerase chain reaction, loop-mediated isothermal amplification, field-effect transistor-based assay, and discrete and gas-phase modeling. Controlling factors include environmental conditions, turbulence, ventilation, ambient temperature, relative humidity, droplet size distribution, non-volatile components, evaporation and mutation. Current results show that medium-sized droplets, e.g., 50 µm, are sensitive to relative humidity. Medium-sized droplets experience delayed evaporation at high relative humidity, and increase airborne lifetime and travel distance. By contrast, at low relative humidity, medium-sized droplets quickly shrink to droplet nuclei and follow the cough jet. Virus inactivation within a few hours generally occurs at temperatures above 40 °C, and the presence of viral particles in aerosols impedes droplet evaporation.

New protocol for the R134a cryogen spray cooling assisted 1064-nm laser lipolysis.

Laser lipolysis is a promising body contouring technology. However, the skin tissue could be thermally damaged owing to the laser energy absorption by water, which limits the lipolysis efficiency. To protect skin tissue and improve the lipolysis effect, cryogen spray cooling is introduced and synergized with the pulsed laser irradiation aiming to propose a new therapy protocol. By simulating heat conduction in the skin after spray cooling assisted laser lipolysis, the temperature distribution in the skin tissue was obtained to analyze the tissue damage by the Arrhenius integral. After parameter optimization according to the damage threshold of skin and adipose tissue, a new protocol with high laser intensity and short time was proposed including 150-ms R134a spray cooling with spray distance of 30 mm, and 100 ms 1064 nm laser irradiation with energy density of 20 J/cm2, with a relaxation for 9.75 s. This cycle of 10 s can be repeated 90 to 150 times for a total of 15 to 25 min. Compared with previous treatment procedure, new protocol can increase the fat dissolution depth from 2 to 4.5 mm beneath the dermis with same order laser fluence.