I am trying to study diffusion and Brownian motion, I found that diffusion is the macroscopic version of Brownian motion and also that if particle in a fluid medium is really small then the force excreted by the random motion of molecules will be unequal in some dimensions leading to net force and net motion. My question is related to this concept: Do we have a formula or quantitative analysis to tell that particle of 'this' size in terms of molecules' size of fluid media or Diffusion coefficient, or some other quantities, can undergo Brownian motion but not larger than that? Please also include your references.


1 Answer 1


The colloidal particels of a colloidal solution when viewed through a ultramicroscope show a constant zig-zag motion known as Brownian movement.

$Size\space of\space colloidal\space particles :1nm - 100nm$

Brownian movement is a characteristic property of colloidal particles. This motion is independent of the nature of the colloid but depends in the size of particles and the viscosity of the solution. Smaller the size and lesser the viscosity, faster the motion. The motion becomes intense at higher temperature.

The Brownian movement is due to the unbalanced bombardment of the particles by the molecules of the dispersion medium. As the size of particles increases, the probability of uneven bombardment decreases and the Brownian movement becomes slow. (Here particles refer to colloidal particles)

The Brownian movement has a string effect which does not permit the particles to settle and thus, is responsible for the stability of sols.

  • $\begingroup$ Is there a formula showing its exact relation ship with viscosity, or is it a general rule that for particle of size 1nm−100nm there will be Brownian motion irrespective of anything(viscosity, diffusion coefficient) etc. Also does increase in viscosity decreases randomness of Brownian motion, and is there quantitative analysis for this too? $\endgroup$
    – Userhanu
    Commented Feb 21, 2017 at 10:05
  • $\begingroup$ Einstein gave the formula for Brownian motion. I dont understand the formula part but the theory that is have posted in my answer is correct. Check this out en.wikipedia.org/wiki/Brownian_motion $\endgroup$
    – Mitchell
    Commented Feb 21, 2017 at 10:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.