Theory of the antibubble collapse

New insights into antibubble formation by single drop impact on a same-liquid pool

HYPOTHESIS

Secondary drops (SDs) generated when falling drops impact a same-liquid bath can potentially generate antibubbles. Different mechanisms of antibubble formation can be identified and their size and formation probability (PAb) can be predicted.

EXPERIMENTS

Surfactant solutions were dropped from various heights using a highly stable pulseless microfluidic pump in a same-liquid bath. The impact was recorded using a high-speed camera. The formation of SDs and antibubbles as well as their sizes were evaluated considering the falling-drop height (HFD) and dimensionless parameters.

FINDINGS

This study reports new mechanisms for antibubble formation from SDs. A decrease in the surface tension yielded a thinner central jet, thereby yielding more SDs. Larger values of the HFD, impact velocity (U), and Weber number (We) increased the SD size and decreased the SD count; the increase in size increased the antibubble size. The number of SDs correlated with the formation of two distinct antibubbles or a single (coalesced) antibubble. The plots for PAb versus HFD, U, and We exhibited two distinct peaks. A moderate increase in the surfactant concentration enhanced PAb in the first regime, whereas an excessive concentration limited antibubble formation. Artificial neural modeling can successfully predict antibubble formation. These findings provide valuable insights for the research on controlled antibubble generation.

Preparation of acid-responsive antibubbles from CaCO3-based Pickering emulsions

HYPOTHESIS

Hydrophobized fumed silica particles were previously reported for producing antibubbles that are quite stable in neutral as well as in acidic media. To produce acid-responsive antibubbles (e.g., for gastric drug delivery), the silica nanoparticles must be replaced by suitable particles, e.g., calcium carbonate (CaCO3), which can degrade at low pH to release the encapsulated drug.

EXPERIMENTS

Two variants of CaCO3-stabilized antibubbles were prepared (by using CaCO3 particles pre-coated with stearic acid, or by using native CaCO3 particles in combination with sodium stearoyl lactylate) and drug release was compared with classic antibubbles produced with hydrophobized fumed silica particles.

FINDINGS

CaCO3 particles (pre-coated with stearic acid) can be used to produce stable antibubbles, which provided an entrapment efficiency of a model drug (methylene blue, MB) of around 85%. A burst release of MB (∼60%) from the antibubbles was observed at pH 2 (i.e., the pH of the stomach), which was further increased to 80% during the next 30 min. On the contrary, at neutral pH, about 70% of the drug remained encapsulated for at least 2 h. We further demonstrated that the acidic conditions led to the desorption of CaCO3 particles from the air-liquid interface resulting in the destabilization of the antibubbles and the release of drug-containing cores.

Advances in antibubble formation and potential applications

Antibubbles are unusual physical objects consisting of a liquid core(s) surrounded by a thin air film/shell while in a bulk liquid. Antibubbles carry two air-liquid interfaces, i.e., one with the inner liquid and the other with the outer liquid. The distinct structure of antibubbles makes them quite attractive for drug and therapeutic delivery, although their potential applications have not been realized so far. The major challenge in this regard is a short-lived span of antibubbles, which is usually in the order of a few minutes to a few hours based on the stabilization mechanism used. We present a critical overview of different techniques that can be used to generate antibubbles. This includes a more commonly applied conventional approach in which the air-film is created through surface entrapment when a liquid jet/drop falls on a bulk liquid. The other available options rely on entirely different mechanisms for antibubble formation, for instance, through drop encapsulation by a submerged air bubble, or through evaporation/sublimation of volatile oil from a W/O/W double emulsion. Furthermore, the mechanisms of antibubble formation and collapse, and the factors affecting their stability have been discussed explicitly; and wherever required, the concept is correlated to other allied physical objects such as bubbles, liquid marbles, etc. Finally, the potential applications, research gaps in the existing knowledge, and some directions for future research are provided towards the end of this article.

To Split or Not to Split: Dynamics of an Air Disk Formed under a Drop Impacting on a Pool

When a drop falls and impacts on a liquid pool, it entraps an air disk below the drop, which then contracts into a central bubble. Here, we use high-speed imaging and high-resolution numerical simulations to characterize the air-disk contraction dynamics for different liquid properties. We show that the air disk can contract into a single central bubble, form a toroidal bubble, or split vertically into two smaller bubbles. We demonstrate that the transitions between the different regimes can be separated by an Ohnesorge number, Oh_{e}, based on the air-disk thickness. For the lowest Oh_{e}, we find a new regime, where vortex shedding from the rim of the contracting air disk breaks the vertical symmetry and prevents the bubble from splitting in two.

Controlling the lifetime of antibubbles

An antibubble is a liquid droplet wrapped by a thin layer of gas, inside a bulk liquid usually of the same composition. The lifetime of an antibubble is governed by the drainage of the gas between the two liquid-gas interfaces populated by surfactants. Depending on the relative magnitude of surface viscosity and elastic moduli, which directly depend on or are determined by the nature of surfactants, the lifetime of an antibubble may vary a lot, from few seconds to few minutes. While such a difference can be predicted with models that include the role of interfacial properties, they were not observed experimentally in previous studies, due to important sources of dispersion. In this review, the main sources of dispersion are identified, such as (i) the initial amount of gas embedded in the antibubble, (ii) the level of saturation of gas in the bulk liquid, (iii) the presence of dust particles (<0.5 μm) in the gas, and (iv) three-dimensional flow effects. By accounting for these various effects, we obtain a coherent view on the lifetime of an antibubble, as a function of its radius and the surface rheology, with excellent consistency between experiments and modeling. Results thus demonstrate that controlling the size and lifetime of antibubbles is achievable.

The acoustical signals produced by antibubble formation

An antibubble is an unusual object: a submerged water drop encapsulated in a thin shell of air that is stable underwater for 10-100 s. They are often thought of as the inverse of a soap bubble because they are a spherical shell of air in water in contrast to a shell of water in air. Antibubbles may be formed when water droplets impact the surface of surfactant-covered water, within a limited range of drop radius and drop impact velocity. In this paper, the range of drop size and impact velocity over which large antibubbles (radius 1-3 mm) are generated by the impact of falling drops is characterised, and the relationship of these parameters to the size of the antibubble formed is shown. Measurements of the two acoustical signals that may be produced as an antibubble is formed by drop impact are reported, and their relationship to the antibubble radius and shell thickness is established. Acoustical measurements taken are interpreted in the context of a modified Rayleigh-Plesset equation that provides a good fit to the frequency data for air shells greater than 100 μm in thickness. However, these results highlight the need for future work on the damping mechanisms associated with these larger antibubbles.