Spreading of nanofluids on solids
Wasan, D.T., Nikolov, A.D., Nature 423 (2003).
nanofluid, adhesion, spreading coefficient, disjoining pressure
The authors of the paper study adhesion and spreading of suspensions of nanometer-size particles, or nanofluids. When a gas bubble dispersed in an aqueous nanofluid approaches a smooth hydrophilic solid surface, a microscopic transition exists between the liquid film and the meniscus. This transition region has a wedge-like profile, and the nanofluid film can change in steps inside this region. The aim of the paper is to find how structural disjoining pressure affects the spreading of colloidal fluids on solid surfaces.
In the main experiment, a wedge-film was formed by blowing an air bubble (diameter 200um) against a smooth glass plate in a suspension of 1um-latex spheres. The volume fraction of latex spheres was 7%. It was found that the latex particles form a 2D colliod crystal at a thickness of the wedge film equal to twice the particle diameter, but the structure changes to a disordered structure when the film grows in excess of three particle diameters. Figure 1 shows the particle in-layer distribution function. This corresponds with the theoretical prediction of disjoining pressure shown in Figure 2. The peaks are due to when integral multiples of the diameter of particles can be accommodated by the wedge. The spreading coefficient is also shown in Figure 2, and it was found that the spreading coefficient increases with a decrease in film thickness. Thus, the authors found that the in-layer particle structuring can enhance the spreading of nanofluids on solids.
This article brings sheds light on the inadequacy of well-established concepts of spreading and adhesion in simple liquids. Whereas in simple fluids, the spreading coefficient would be independent of film thickness, in nanofluids, the spreading coefficient is highly dependent on the thickness of the film. This is due to the oscillatory disjoining pressure, which is caused by the particle structure in the wedge film.