Generation mechanisms and stability

How nanobubbles are generated

Bulk nanobubbles are made mainly in two ways: by cavitation, where a local pressure drop forms bubbles inside the liquid, and by forcing gas through a porous membrane into a flowing liquid Wang & Wang, 2023. The two routes differ in what controls the bubble size, so they are described separately below.

Hydrodynamic cavitation

Hydrodynamic cavitation (HC) works on the Bernoulli principle. As liquid passes through a constriction its velocity rises and its static pressure falls; where the local pressure drops below the saturated vapor pressure of the liquid, cavities (bubbles) form Ahmed et al., 2018; Akshit et al., 2024. Reviews describe HC as widely adopted and suited to scale-up.

A Venturi tube is a common HC device. Liquid accelerates through a conical convergent section, and the negative-pressure zone that forms draws in gas. The gas–liquid mixture then meets intense hydraulic shear, which breaks the gas into micro- and nanobubbles Li et al., 2021.

Swirling-flow generators rotate the liquid fast enough to open a vortex with a low-pressure core, where nanobubbles nucleate Alam et al., 2022. During generation the liquid often turns milky with microbubbles. Those rise and burst, leaving a transparent suspension of the nanobubbles that remain Foudas et al., 2023.

Ceramic membranes

Membrane generators force pressurized gas through a porous ceramic membrane, for example one with 100 nm pores, into a flowing liquid. The gas pressure must exceed the capillary pressure of the pores (the bubble point), and shear from the flowing liquid detaches the bubbles from the membrane surface Wang & Wang, 2023.

In this route the membrane and the operating pressure set the bubble size. Pore size, the hydrophobicity of the membrane surface and the gas injection pressure all influenced nanobubble size, and higher injection pressures yielded smaller bubbles Ahmed et al., 2018; Phan et al., 2021.

Why nanobubbles persist

Bulk nanobubbles last far longer than classical theory allows, and three physical models have been proposed to explain why. By the Young–Laplace equation, the smaller a bubble, the higher its internal pressure, so a nanobubble should lose its gas and dissolve within microseconds. Laboratory suspensions have instead been reported to persist for months Nirmalkar et al., 2018; Foudas et al., 2023.

Surface charge and ion shielding

The surface-charge model holds that electric charge at the interface counteracts the internal pressure. Nanobubbles in pure water carry a negatively charged interface, attributed to adsorbed hydroxide ions (OH⁻). The charge attracts positive counterions and forms an electric double layer (EDL), and electrostatic repulsion between ions on the bubble surface acts as an outward pressure against the inward surface tension (the Laplace pressure) Nirmalkar et al., 2018.

The size of this charge is measured as zeta potential, covered in Key physico-chemical properties.

An interfacial "skin"

The skin model holds that material adsorbed at the gas–liquid interface slows the escape of gas. Organic contaminants, surfactants or solid particles collect at the interface and form a rigid or semi-rigid shell. The shell reduces surface tension and acts as a physical barrier to gas diffusion, which inhibits dissolution Yasui et al., 2019; Jin et al., 2022.

Supersaturation and dynamic equilibrium

The third model holds that gas flowing into and out of the bubble reaches a balance. Local gas supersaturation in the liquid around the bubble, or hydrophobic surface interactions that trap gas, would then prevent a net loss of gas from the bubble Akshit et al., 2024; Lasek et al., 2023.

The cited sources present these as alternative explanations. They also depend on different conditions: the skin model needs contaminants, surfactants or particles at the interface, while the surface-charge model is described for pure water.