Brownian motion question

1. Aug 29, 2014

Warpspeed13

I was reading about ferrofluid and I was wondering how you would go about calculating the maximum size of a particle that could be suspended by Brownian motion in a fluid? Can a denser fluid suspend larger particles?

2. Aug 29, 2014

mpelaez83

The response is: kT>mgd, where kT is thermal energy, m the mass of the particle, g is gravity and s is the particle typical size. When thermal energy is smaller sedimentation will occur.

3. Aug 29, 2014

Warpspeed13

Cool. How would the mass of the carrier fluid particles be accounted for? It seems mercury would be different than water.

4. Aug 29, 2014

mpelaez83

You actually account for that through viscosity in some way. The carrier fluid as you call it is accountef by a viscosity and a temperature, the system temperature

5. Aug 29, 2014

mpelaez83

These are deep questions people dont ask so there are a lot of reasonable misconceptions.

6. Aug 29, 2014

Warpspeed13

Cool thanks for the help

7. Aug 30, 2014

Warpspeed13

Sorry but what units were you're original equation in, I assumed it was jules> kg* 9.82* surface area in m^2

8. Aug 30, 2014

mpelaez83

g has units of m/s^2, d is in meters and m is in kg. Then you get joules.

9. Aug 30, 2014

Warpspeed13

Is kT the energy of one carrier fluid atom or the energy of the system?

10. Aug 30, 2014

mpelaez83

kT is the energy associated to one particle. When i say particle you can think on both the solvent molecules or the dispersed particles. The energy of a thermodynamic system it depends on and adittional thermodynamic variable, partiole number N. For example in an ideal gas the energy of the system is ~NkT

11. Aug 30, 2014

Warpspeed13

Cool thanks for the help

12. Aug 30, 2014

mpelaez83

No problems. Physics is cool.

13. Aug 31, 2014

Yep.