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Brownian motion question

  1. Aug 29, 2014 #1
    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. jcsd
  3. Aug 29, 2014 #2
    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.
  4. Aug 29, 2014 #3
    Cool. How would the mass of the carrier fluid particles be accounted for? It seems mercury would be different than water.
  5. Aug 29, 2014 #4
    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
  6. Aug 29, 2014 #5
    These are deep questions people dont ask so there are a lot of reasonable misconceptions.
  7. Aug 29, 2014 #6
    Cool thanks for the help
  8. Aug 30, 2014 #7

    Sorry but what units were you're original equation in, I assumed it was jules> kg* 9.82* surface area in m^2
  9. Aug 30, 2014 #8
    g has units of m/s^2, d is in meters and m is in kg. Then you get joules.
  10. Aug 30, 2014 #9

    Is kT the energy of one carrier fluid atom or the energy of the system?
  11. Aug 30, 2014 #10
    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
  12. Aug 30, 2014 #11
    Cool thanks for the help
  13. Aug 30, 2014 #12
    No problems. Physics is cool.
  14. Aug 31, 2014 #13
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