## 1 Introduction  
Limited solubility of gases in many liquids (typically below Henry's-Law levels), e.g., gases in water, such as oxygen and especially hydrogen, is a fundamental challenge. In ecosystems and the environment, lack of dissolved oxygen (DO) is a major reason for fish kills and water bodies being blighted by algal blooms, in addition to, sometimes, a lack of effectiveness of activated-sludge processes in water treatment or poorer-than-hoped results in irrigation. Nanobubbles (NBs) are gaseous domains on the nanoscale, existing on solid surfaces or in bulk liquid--noted for their long-time (meta)stability and high potential for real-world applications [1; 2; 3], e.g., nanoscopic cleaning [4], boundary-slip control in microfluidics [5], wastewater treatment [6], hetero-coagulation [7], and medical applications [8]. While NBs on surfaces have been observed, NBs in the bulk have been studied less [2; 3]. It is speculated that NBs' long-lived presence arises from negative-charge build-up at the bubble/liquid interface, with the surface having strong electron affinity [9]. Generated properly, nanobubbles offer a chance, promisingly, to overcome fundamental gas-in-liquid solubility \"bottlenecks\". The concept of nanobubbles versus traditional, coarser bubbles, is illustrated on the top right of Figure 1--with the latter subject to buoyancy phenomena, while NBs are relatively impervious to rising--in essence, subverting Stokes' Law of bubble rising [2; 3]. In liquids, bubbles have internal equilibrium pressures at least that of the external environment. Each bubble is surrounded by an interface that possesses different properties to that of the bulk solution. In any event, microbubbles are not stable for long periods (-minutes), rising slowly and indirectly to the surface, but smaller ones (\(\approx\)<20 \(\upmu\)m diameter) will shrink to form more effective and stable NBs [3]. Only these tiny bubbles (<1 um diameter) are stable for significant periods in suspension [1; 2]. The surface area of a volume of bubbles is in inverse proportion to the bubble diameter. Thus, for the same volume of the bubble, their surface area increases proportionally to the reduction in bubble diameter, meaning that NBs have an excellent area-to-volume ratio, as is evident in the top right of Figure 1. In such a way, we can enhance greatly gas-liquid mass transfer--of which we shall consider further anon (vide infra). Naturally, if NBs are generated well--indeed, the art of \"nanobubble engineering\", as it were--the stability of NBs, together with their high surface area per volume, leads to important and useful applications. In terms of the status quo (ante) vis-a-vis industrial NB generation, the bottom left of Figure 1 depicts the \"forced-convection\" approach, whereby traditional (mechanical) generation methods mainly rely on hydrodynamic, acoustic, particle, and optical cavitation, in tandem with injecting pressurised gases through a tubular ceramic membrane lined by nano-pores--which, inevitably, just get biofouled in short order. These NB-generation processes raise issues such as, inter alia, high energy consumption, non-flexibility, and complexity. In stark contrast, on the bottom right, we see the disruptive electric-field invention, whereby the application of static electric fields via sheathed electrodes to water-gas systems promotes the instant and low-energy formation and build-up of ultra-dense gas nanobubbles inside the liquid phase, by enhancing metastable gas solubility through electrostrictive action [10; 11]. This endows these \"thick-skinned\" NBs with unique environmental and industrial applications [12; 13], realising nano-porous liquids (NPLs) as a new class of 'designer' green solvents. Nanobubbles, with greater mass density than mechanical-generation approaches, generated properly by canny and effective electric-field nanobubble engineering [14; 15; 16; 17], offer a chance, promisingly, to overcome fundamental gas-in-liquid solubility \"bottlenecks\"--often with important environmental benefits [18; 19].