冷たいスパークリングウォーター。 浜辺に打ち寄せる波。 たき火のパチパチという音。 やかんから蒸気。
エアロゾルとは、空気中に浮遊する液体または細かい固体粒子の小さな液滴を指します。 エアロゾルはさまざまな方法で生成できますが、最も重要な発生源の 1 つは、液体と空気の境界での気泡の破裂です。 汚染された界面がより一般的であるにもかかわらず、以前の研究は主に「きれいな」気泡に焦点を当てていました.
イリノイ大学アーバナ シャンペーン校の機械科学および工学助教授の Jie Feng と大学院生の Zhengyu Yang による新しい研究は、薄い油層で覆われた破裂気泡がより小さいサイズの液滴を生成し、液滴の総数が多くなり、排出されることを示しました。きれいな水で発生する泡と比較して、より速い速度で。 この研究は最近、
Aerosols are quite ubiquitous in our environment, and they can be natural or anthropogenic in nature. Bubble-bursting aerosols play a key role in the transfer of mass across liquid interfaces. Sea spray aerosols, for example, are primarily generated by bubble bursting at the ocean surface. The drops that are produced from bubble bursting can impact air pollution, global climate, and even the transmission of infectious diseases. One important parameter of these droplets is their size since that is indicative of residence time and transport in the atmosphere- small drops are more easily lifted by winds and can travel much further.
Feng says, “We have contaminated water everywhere. When the bubble rises from deeper water to the surface, it will collect contaminants and form an organic layer around it. We call this a contaminated bubble. When it reaches the surface and bursts, it can actually aerosolize these contaminants into small droplets.”
Feng and Yang investigated the impact a thin layer of oil has on bursting bubbles, as a model system for contaminated bubbles. The bursting of a millimeter-sized bare bubble at an aqueous surface produces drops with a typical size of around 100 micrometers (µm) (a typical human hair is around 100-200 µm). In this work, they found that drops can be as small as a few µm when the bursting bubble is coated by a thin layer of oil. Additionally, bare bubble bursting produces drops with a typical ejection velocity of 1 meter per second (m/s), whereas oil-coated bubble bursting produces drops with an ejection velocity as large as 10 m/s. Feng summarizes, “The main conclusion of our work is that we found, for these contaminated bubbles, they can quite effectively aerosolize the contaminants into micron-size droplets.”
In an industrial setting like a wastewater treatment plant, smaller contaminated drops can pose a significant risk to those that work in the plant. Bubble bursting in these settings may generate acidic mists and bioaerosols. Understanding the effect of contaminated bubbles on size distribution and ejection speeds is crucial to designing effective personal protective equipment and implementing additional guidelines on air and water quality near such facilities.
On a broader scale, aerosols have an effect on weather, climate, and even human health.
“These droplets can transport pathogens, bacteria, and viruses,” Yang said. “When you have these small aerosols, and they can be ejected higher, the small size and higher ejection speed can help them stay in the atmosphere for a longer time.” Understanding size and composition of aerosols is important to improve global modeling efforts. Furthermore, these contaminated drops can pose a greater risk of pollutant spread as well as infection since smaller aerosols are able to penetrate further in the respiratory tract than larger aerosols.
Reference: “Enhanced singular jet formation in oil-coated bubble bursting” by Zhengyu Yang, Bingqiang Ji, Jesse T. Ault and Jie Feng, 23 February 2023, Nature Physics.