This equation reveals a balance of the gravity, buoyancy force, and viscous drag that are acting on the suspended nanoparticles.
According to Eq. (1), the following measures can be taken to decrease the speed of nanoparticles’ sedimentation in nanofluids, and henceforth to produce an improvement for the stability of the nanofluids: (1) reducing R, the nanoparticles size; (2) increasing l, the base fluid viscosity and (3) lessening the difference of density between the nanoparticles and the base fluid (qP qL).
Clearly reducing the particle size should remarkably decrease the sedimentation speed of the nanoparticles and improve the stability of nanofluids, since V is proportional to the square of R.
According to the theory in colloid chemistry, when the size of particle decreases to a critical size, Rc, no sedimentation will take place because of the Brownian motion of nanoparticles (diffusion).