Why 20–50 micron bubbles are the heart of DAF — surface-area-to-volume ratio, Laplace over-pressure, number density and the size–velocity trade-off that governs flotation.
The Science Behind Dissolved Air Flotation
The performance of dissolved air flotation begins with bubble size. Micro-bubbles of 20–50 microns present an enormous surface area per unit volume, rise slowly enough to contact particles yet quickly enough to carry them, and exist in vast numbers — the physics that make DAF far more effective than coarse-bubble aeration.
A sphere’s surface-area-to-volume ratio is 6/d, so a 30 µm bubble offers roughly 33× the interfacial area per unit volume of a 1 mm bubble. Its terminal rise follows Stokes’ law, v = g(ρw−ρa)d²/18µ — small enough to rise gently, large enough to lift attached solids.
Why the 20-50 micron window is optimal
20–50 µm balances contact opportunity against sufficient buoyant lift.
The 6/d law means small bubbles present vast interfacial area for attachment.
Internal pressure rises as 4σ/d, keeping micro-bubbles stable and discrete.
Stokes’ law sets a gentle rise that maximises particle contact time.
Millions of bubbles per mL give an enormous collision probability.
Saturator and nozzle design (verified by CFD) set a tight, fine distribution.
| Bubble diameter | Stokes rise velocity | Surface area / volume |
|---|---|---|
| 20 µm | ~0.22 mm/s | ~300,000 m²/m³ |
| 30 µm | ~0.49 mm/s | ~200,000 m²/m³ |
| 50 µm | ~1.4 mm/s | ~120,000 m²/m³ |
| 100 µm | ~5.4 mm/s | ~60,000 m²/m³ |
| 500 µm | ~140 mm/s | ~12,000 m²/m³ |
Continue across the DAF science series
The micro-bubble DAF overview and hub.
Read MoreCollision and attachment mechanics that capture particles.
Read MoreAggregate density and Stokes rise that size the tank.
Read MoreSize a DAF and check A/S, loading and recycle.
Read MoreOur dissolved air flotation systems range.
Read MoreOur process engineers design dissolved air flotation from first principles — bubble size & surface area and the full bubble physics — for guaranteed removal at minimum lifecycle cost.
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