What are Nanobubbles
General Definition of Nanobubbles
Nanobubbles are extremely small gas bubbles- typically smaller than 200-nm in diameter, that are suspended in a liquid like water.
Despite their tiny size, nanobubbles behave very differently from larger bubbles and have unique physical, chemical, and interfacial properties that make them valuable in many industries.
Scientific Definition of Nanobubbles
A nanobubble is a spherical gas cavity in a liquid medium with a diameter typically ranging from 20-nm to 200-nm, possessing high internal pressure, negative surface charge (zeta potential), and unusually long stability, even though classical theory would suggest they should dissolve almost instantly.
Scientific Mechanisms Supporting Nanobubble Stability
- Laplace Pressure: Internal pressure increases as bubble size decreases, preventing collapse.
- Zeta Potential: Surface charge repels coalescence and enables stability in suspension.
- Brownian Motion: Keeps nanobubbles in uniform dispersion without rising.
- High Surface Area-to-Volume Ratio: Enhances gas–liquid reactivity and transfer kinetics.
Key Characteristics of Nanobubbles
- Size: < 200 nanometers (1,000× smaller than a grain of sand).
- Stability: Can remain stable in liquid for days, weeks, or even months.
- Buoyancy: Essentially neutrally buoyant (they don't rise like normal bubbles).
- Surface Charge: Typically carry a negative zeta potential, reventing coalescence.
- High Internal Pressure: Due to surface tension, pressure inside can be several atmospheres.
- Large Surface Area: Extremely high surface area-to-volume ratio.
- Disappearance: They shrink and dissolve (do not burst like large bubbles).
How Do Nanobubbles Work
Nanobubbles interact physically and chemically with their surroundings in several ways:
- Dissolution: They gradually release gas (like O₂, H₂, CO₂) into the liquid, improving oxygenation or altering pH.
- Collapse Effects: If triggered to collapse (e.g., via ultrasound), they generate localized heat, pressure, and free radicals (oxidizing agents).
- Surface Interaction: Their negative charge and nanoscale size help them adhere to surfaces, disrupt biofilms, and loosen contaminants.
Correlation Between Nanobubbles and Brownian Motion
The correlation between nanobubbles and Brownian motion lies in the scale, behavior, and dynamics of nanobubbles in fluid environments.
- Size Matters: Nanobubbles are typically less than 200 (200-nm) nanometers in diameter. At this nanoscale, they are small enough to be significantly influenced by Brownian motion - random collisions with surrounding fluid molecules.
Result: Nanobubbles exhibit random jostling movements within the fluid, similar to other nanoparticles or colloids, especially in low-turbulence or stagnant environments.
- Brownian Motion Keeps Nanobubbles Suspended: Unlike larger bubbles (which rise and burst quickly due to buoyancy), nanobubbles do not rise rapidly or coalesce.
This is because Brownian Motion counteracts buoyancy for extremely small objects. The random collisions with water molecules keep nanobubbles in suspension. The net effect is that Nanobubbles remain stable in liquid for hours, days, or even weeks, rather than immediately floating to the surface and dissipating.
- Nanobubbles Exhibit Diffusion & Transport Behavior: Nanobubbles exhibit diffusion and transport behavior primarily due to their extremely small size and the physics of how particles move in a fluid. Because nanobubbles are less than 200 nanometers (< 200-nm) in diameter, they are constantly jostled by collisions with surrounding water molecules - a phenomenon known as Brownian Motion.
This random motion causes the nanobubbles to drift in all directions rather than rise quickly to the surface like larger bubbles. As a result, nanobubbles spread out naturally throughout the fluid, diffusing evenly without the need for external agitation. Additionally, their small size makes them nearly neutrally buoyant, allowing them to remain suspended for long periods. This stability enables them to travel through water and reach confined or low-flow areas, enhancing gas delivery and treatment efficiency. By exhibiting this type of passive movement, nanobubbles effectively transport dissolved gases - such as oxygen, ozone, or hydrogen - throughout a system, supporting applications like wastewater treatment, agriculture, and biomedical processes.
- Interfacial Stability: Brownian Motion Delays Coalescence: Interfacial stability refers to the ability of nanobubbles to resist merging (coalescence) with one another, and Brownian motion plays a key role in this. Because nanobubbles are so small, they move randomly and rapidly due to constant collisions with water molecules. This random motion reduces the chances of nanobubbles coming close enough and staying in contact long enough to merge. Even when they do collide, their surface charge and stability at the gas-liquid interface often cause them to repel each other. As a result, Brownian motion helps maintain their separation and prevents coalescence, contributing to the long-term stability of nanobubbles in suspension.
