Long‐lived yet bare bubbles in ultrasound

A bare water bubble, without the stabilization of any surfactant, can remain intact for more than 7 min in ultrasound. By contrast, once the sound power is turned off, the bubble will burst within several seconds. Scale bar = 1 mm. The figure is reproduced from fig. 1b in the article by Xiaoliang Ji et al. published in Droplet (https://doi.org/10.1002/dro2.119). Bubbles, often seen as metaphors for transience due to their ephemeral nature, have fascinated artists, engineers, and scientists for centuries. Despite centuries of research, new and often surprising bubble physics and applications continue to emerge. In particular, bubble-oriented fluid dynamics has been shown to play an important role in various disciplines such as chemistry, colloidal science, catalysis, electrolysis, medicine, biology, geophysics, and many others.1, 2 There has been a strong motivation for researchers to investigate and manipulate the lifetime of bubbles, which could help address important questions related to chemical engineering, food/cosmetics industry, and even geophysics. For instance, the lifetime of bubbles is linked to the stabilization of foams and the size of bubble-bursting aerosols. To this end, as the recent discovery of Xiaoliang Ji and coworkers reveals, bubbles suspended in ultrasound could show extraordinary stability without any chemical stabilizers.3 With the bubble lifetime effectively prolonged by approximately two orders of magnitude, their work proposes a new method to produce a long-lived bubble. Not only do bubbles with longer lifetimes empower enthusiasts of giant bubble creation to fabricate larger, more durable bubbles, but they also provide a scaffold for long-lasting foams vital in a variety of industrial applications. However, bubbles are susceptible to various physical processes that cause them to burst. For instance, the liquid film of the bubble may drain due to gravity and/or capillary driving forces, and it may also evaporate in unsaturated air with relatively low humidity. The demise of bubbles becomes inevitable as the film continues to thin. Additionally, bubble films are susceptible not only to external nuclei (e.g., dust) but also to inherent instability driven by van der Waals attractions. To produce bubbles with a long lifetime, mitigating the above effects becomes crucial. In previous studies, various additives were used in bubble recipes to alleviate the thinning of the film and to enhance bubble stability, including ethanol, glycerin, surfactants, proteins, polymers, and particles. Such additives can introduce various effects that slow the film drainage, including higher fluid viscosity and surface rigidity, or strong solutocapillary effects.4 Additionally, the film evaporation could be counterbalanced by hygroscopicity, and the film can be stabilized by electric double-layer forces that compete with van der Waals attractions. Bare bubbles consisting of pure liquids hardly survive over seconds, and only a few studies have investigated methods to sustain a bubble without the above additives. It was demonstrated that a low-frequency vibration can significantly increase the lifetime of bubbles in a water bath by supplying additional energy to keep bubbles bouncing.5 In addition, the bubble lifetime could be enhanced by a thermal Marangoni effect when the bath temperature is higher than that of the surface.6 Both methods, though, studied surface bubbles in a liquid bath. In this issue of Droplet, Xiaoliang Ji and coworkers document a novel phenomenon where bubbles, made of pure water and acoustically levitated in the air, survive for more than 7 min, which is a duration approximately 100 times longer than their counterpart without an acoustic field.3 Levitation represents a useful method in the research of small-volume samples like droplets and bubbles, as it prevents chemical and thermal contaminations from solid surfaces and eliminates contact line effects. With carefully designed experiments, the authors generated acoustically levitated bubbles by pulling down the center of levitated oblate droplets (Figure 1). The levitated bubbles show excellent stability and remain intact for minutes without significant film drainage until they finally rupture, and they even survive the puncture of needles. Such stable bubbles can be reproduced whether or not the solution contains glycerin or surfactants. By controlling the spatial distribution of energy potential, the acoustic field also tunes the shape of the levitating bubble as well as its film thickness distribution. To understand why the levitated bubbles remain stable, the authors simulate the sound field and show that a higher acoustic pressure exists at the bottom of the bubble. The acoustic pressure is not only crucial for bubble levitation but also creates a hydrostatic pressure gradient, effectively suppressing drainage and prolonging the bubble's lifespan. The long-lived bubble composed of a pure liquid in acoustic levitation advances our understanding of the stability of bubble films as well as the bubble–ultrasound interaction. More importantly, such a controllable levitation platform for a long-lived, floating bubble may enable careful investigation of bubble fluid mechanics for future work concerning film drainage, foam stability, and aerosol emission/suppression. Indeed, studies have investigated the stability of liquid films with ultrasonic vibration,8 but much remains to be done to understand the intriguing behavior of bubbles under levitation in an ultrasonic field, such as how the film thickness distribution could be precisely tuned by the ultrasonic field. In summary, we would like to quote a statement from Lhuissier and Villermaux,9 which was paraphrased from Michael Faraday's Chemical History of a Candle10: "There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a bubble (originally: candle)." The authors declare no conflict of interest.