Settling vs. Suspension
Learning Objectives
Students will be able to:
- Explain why some particles settle while others stay suspended
- Describe the forces acting on particles in air
- Predict settling time based on particle size
- Apply understanding to real-world scenarios (droplets vs. aerosols)
The Battle of Forces
Every particle in air experiences two main forces:
Gravity
Pulls particle down
Air Resistance (Drag)
Pushes back against motion
Why Size Determines Fate
| Factor | Large Particles | Small Particles |
|---|---|---|
| Mass | Higher (gravity wins) | Lower (air resistance wins) |
| Surface area to mass ratio | Lower | Higher (more drag per unit mass) |
| Settling speed | Fast | Very slow or none |
| Behavior | Falls like a ball | Drifts like a feather (or floats) |
Settling Times: The Numbers
How long does it take for a particle to fall from ceiling height (~3 meters) in still air?
| Particle Type | Size | Settling Time |
|---|---|---|
| Large droplet (cough) | 100 μm | ~10 seconds |
| Pollen grain | 30 μm | ~2 minutes |
| Large dust | 10 μm | ~8 minutes |
| PM2.5 particle | 2.5 μm | ~4 hours |
| Smoke particle | 0.5 μm | ~41 hours |
| Virus-carrying aerosol | 0.1 μm | Days to weeks |
Key insight: Particles below about 1 μm essentially never settle on their own—they stay airborne indefinitely unless removed by ventilation or filtration!
The Droplet vs. Aerosol Distinction
Droplets (>100 μm)
- Fall to ground within seconds
- Travel short distances (<2 meters)
- Ballistic trajectory (like thrown objects)
- The "6-foot rule" was based on these
Aerosols (<100 μm)
- Stay suspended in air
- Can travel across entire rooms
- Move with air currents
- This is why ventilation matters!
Important: Large droplets can evaporate and become smaller aerosol particles! A 100 μm droplet can become a 10 μm aerosol within seconds.
What Keeps Small Particles Floating?
- Brownian Motion — Random collisions with air molecules bounce tiny particles around, counteracting gravity
- Air Currents — Any air movement (HVAC, doors, people moving) keeps particles aloft
- High Surface-to-Mass Ratio — More surface area relative to weight means more drag
- Convection — Warm air rising can carry particles upward
Demonstration: Glitter vs. Flour
Materials:
- Large glitter (represents large droplets)
- Fine flour or cornstarch (represents aerosols)
- Clear container or darkened room with flashlight
Procedure:
- Release glitter from shoulder height
- Time how long it takes to reach the floor
- Release flour from the same height
- Observe how long flour stays visible in air
Discussion:
- What differences did you observe?
- How does this relate to infectious disease transmission?
- Why might this explain why ventilation is important for small particles but not large ones?
Real-World Application: The Smoke Test
Smoke particles are typically 0.1-1 μm—similar in size to exhaled respiratory aerosols.
Think about it: When you see cigarette smoke or incense smoke in a room, notice how it:
- Drifts and swirls rather than falling
- Spreads throughout the room over time
- Follows air currents
- Lingers for a long time
This is exactly how respiratory aerosols behave!
Key Takeaway
Gravity pulls all particles down, but air resistance fights back. For particles smaller than about 10 μm, air resistance wins—they stay suspended for minutes to hours. For particles below 1 μm, they essentially float indefinitely. This is why PM2.5 and respiratory aerosols are an air quality problem that requires air cleaning (ventilation and filtration), not just waiting for particles to settle.