Wetting of glass surfaces by ionic magnetic fluids: Effect of the concentration and the pH of the solution and of the size of the magnetic grains on a surface stabilized birefringent layer

The wetting of glass surfaces by ionic magnetic fluids (ferrofluids) is stu died, using an improved birefringence technique, as a function of the conce ntration and size of the colloidal grains, and the pH of the solution. The problem is considered as a phase transition induced by the surface field on a thin ferrofluid layer which is in contact with the glass walls. The indu ced optical phase shift increases with the grain concentration. The penetra tion length in the bulk, induced by the ordered surface ferrofluid layer, c alculated assuming that the order parameter at the surface is independent o f grain concentration, is of the order of 2-4 grain diameters. The grain si ze, in the range from 2.5 to 9.5 nm, and the solution pH do not seem to be relevant parameters for the induced phase shift. (C) 2000 American Institut e of Physics. [S0021-9606(00)70346-1].