Breaking Bubbles: The Isothermal Illusion in Nanobubble Dynamics

Significance 

Nanobubbles, unlike their macro counterparts, exhibit unique properties due to their size and the effects of surface tension. Nanobubbles are tiny gas-filled cavities in liquids, exhibiting unique properties due to their nanoscale size. They play a pivotal role in various applications like wastewater treatment, cancer therapy, diagnostics, and microfluidic cleaning. Previously, it was believed that nanobubble oscillations (expansion and contraction) were isothermal, meaning the temperature remains constant during these processes.

A new study published in Nano Letters by Dr. Duncan Dockar, Livio Gibelli, and Dr. Matthew Borg from the University of Edinburgh focused on the thermal and oscillatory behaviors of nanobubbles, offering new insights that challenge existing understanding in this area. The researchers employed the vdW equation of state to account for the nonideal nature of the gas inside the nanobubbles. They introduced a temperature jump term at the liquid-gas interface, considering nonequilibrium effects that were previously overlooked. The team conducted MD simulations, analyzing a nitrogen nanobubble in water subjected to oscillations. They monitored the radius, pressure, and temperature of the nanobubble over time under various oscillation frequencies.

Contrary to the traditional isothermal assumption, the study found that nanobubble oscillations are closer to adiabatic. Adiabatic processes involve changes in temperature and are characterized by no heat exchange with the surroundings They highlighted that the adiabatic behavior observed in nanobubbles could be misinterpreted due to the conventional approach of using a polytropic process to analyze thermal responses. High Laplace pressure inside the nanobubbles leads to nonideal gas behavior, which the vdW equation of state aptly describes.  A significant revelation was the temperature jump at the liquid-gas interface, indicating nonequilibrium effects that are crucial in understanding nanobubble behavior.  The findings pose challenges in characterizing nanobubble sizes using ultrasound in experiments, as the oscillatory behavior differs from previous assumptions.

The new model, integrating the vdW equation and temperature jump, showed excellent agreement with the MD simulations, reinforcing its accuracy. The study provides a refined understanding of how nanobubble oscillations occur, bridging the gap between theoretical predictions and actual behaviors.

The research significantly advances our understanding of nanobubbles by challenging the conventional wisdom of isothermal oscillations. It underscores the importance of considering nonideal gas behavior and nonequilibrium effects at the nanoscale. This deeper understanding is crucial for the effective application of nanobubbles in various technological and medical fields.

The implications of this study are vast, particularly in fields where nanobubbles are employed. In wastewater treatment, the understanding of nanobubble dynamics can lead to more efficient methods for removing contaminants. In cancer therapy and diagnosis, the precise control over nanobubble behavior could enhance the delivery and efficacy of therapeutic agents. Similarly, in microfluidic cleaning, a deeper understanding of nanobubble oscillations can lead to more effective cleaning processes, crucial in industries where contamination control is paramount.

One of the most significant contributions of this study is the challenge to the isothermal paradigm in nanobubble dynamics. By demonstrating that nanobubble oscillations are closer to their isothermal limit but influenced by nonideal gas behaviors, the research opens up new avenues for exploring the physics of nanoscale phenomena. This shift in understanding could lead to the development of new models and theories that more accurately describe the behavior of nanobubbles and other nanoscale entities.

From a technological and industrial perspective, this research has the potential to revolutionize how nanobubbles are used in various applications. By providing a more accurate model of nanobubble behavior, engineers and scientists can design more efficient and effective technologies for a range of applications, from medical devices to environmental cleanup. The study lays the groundwork for further research into the dynamics of nanobubbles and their applications. Future work could explore the kinetic modeling of the internal gas phase of nanobubbles, particularly in cases where they are stabilized by organic shells. Additionally, incorporating mass transfer into the model could capture the evaporation and condensation of gas/vapor molecules, further enhancing our understanding of these phenomena. In conclusion, the study by Dr. Dockar, Gibelli, and Dr. Borg represents a significant advancement in our understanding of nanobubble dynamics. By challenging existing paradigms and introducing a new theoretical model, the research has opened up new possibilities for the application of nanobubbles in various fields. The implications of this study are far-reaching, with the potential to impact numerous industries and lead to the development of more efficient and effective technologies. The future of nanobubble research is bright, and this study will undoubtedly be a cornerstone in the field for years to come.

Breaking Bubbles: The Isothermal Illusion in Nanobubble Dynamics - Advances in Engineering

About the author

Dr Matthew Karl Borg

Deputy Head of Research Institute for Multiscale Thermofluids (IMT)
School of Engineering, University of Edinburgh

I am Reader in Mechanical Engineering at the University of Edinburgh since 2020, and lecturer before this from 2015.  My research focuses on non-equilibrium and non-continuum fluid flows characteristic of micro- and nano-scale devices, granular flows and crowd modelling. In these types of multiscale flow problems, conventional fluid equations are generally inaccurate, so the research challenge has been to develop the new engineering science needed to understand and technologically exploit these complex flows.

Key example areas I work on include: water filtration in nanostructured membranes to deal with the global water crisis; low pressure gas flows to deal with the demands of the silicon chip photolithography industry and the growing aerospace industry; and cavitation surface nanobubbles that could clean precision surfaces or reduce marine drag.

I have developed computational particle simulation tools (e.g. dsmcFoam, mdFoam) and hybrid methodologies in the third-party open-source software OpenFOAM since 2006 to deal with these problems, which run on UK supercomputing facilities, such as ARCHER2, Cirrus and ARCHIE-WeSt.   Since 2009 several of these models have been chosen by OpenCFD Ltd, the founders of the OpenFOAM project to be incorporated into releases of the code for distribution worldwide.

About the author

Dr Duncan Dockar,

University of Edinburgh
Engineering bulk nanobubbles for enhanced water disinfection and quality

Nanobubbles are an emerging branch of nanotechnology being employed in water treatment to improve bacterial disinfection efficiency and for precision surface cleaning. Dr Dockar’s research will reveal the atomic-level phenomena surrounding the nanobubbles’ stability, disinfection mechanisms, and deployment in microfluidic engineering devices, through novel molecular-based simulations.

Reference

Dockar D, Gibelli L, Borg MK. Thermal Oscillations of Nanobubbles. Nano Lett. 2023 Dec 13;23(23):10841-10847. doi: 10.1021/acs.nanolett.3c03052.

Go to Nano Lett.

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