Effect of gravity on colloidal deposition studied by atomic force microscopy

Atomic force microscopy (AFM) was used to study the effect of gravitational forces on the deposition of submicrometer colloidal particles onto solid s urfaces to test the usual assumption that the contribution of gravity to th e behavior of particles with diameters <1 mum is negligible. The effects of both particle size and density were examined using polystyrene sulfate lat ex, silica, and colloidal gold particles of several sizes ranging from 10 n m to 1 mum. The results show that significant differences are observed in t he deposition of colloidal particles onto horizontal and vertical surfaces, under identical suspension conditions and exposure times, showing that gra vitational forces can have a considerable effect. This effect was quantifie d by analysis of the AFM images. The experimental results are compared to c alculations of the expected coverage and particle surface concentration ass uming diffusion-limited adsorption and deposition by sedimentation. Gravity can be negligible for low-density particles with diameters considerably sm aller than I gin. However, if the density of the colloidal particles is hig h, as in the case of colloidal gold, gravity can become a significant drivi ng force for particle transport to the surface, even for particles with dia meters much smaller than 100 nm. (C) 2001 Academic Press.