Cleansing dynamics of oily soil using nanofluids.

We explored the technological concept of the nanoparticle structuring in the wedge film with regards to its application to the oily soil removal phenomena. The experimental and theoretical investigations on the cleansing of canola oil from a glass substrate using commercially available nanofluids were pursued. Five commercially-available nanofluids with pHs varying from 9.3 to 9.9 were used in the experiments. Experimental results clearly indicated that the time to separate the oily soil from the glass surfaces by nanofluids was much shorter than that for the reference alkaline solution at the same pH. The positive contributions of the nanoparticles to the soil cleaning performance were rationalized in terms of the decrease in the contact angle and the interfacial tension, positive second virial coefficient, and high osmotic pressure of the nanofluid. The effective nanoparticle diameter and the effective volume (i.e., concentration) of the nanoparticles were determined using our novel capillary force balance technique in conjunction with the microinterferometric method. Using the experimentally measured values of the effective particle diameter, effective volume, and the osmotic pressure, the structural disjoining pressure in the wedge film was calculated from a theoretical model based on the statistical mechanics theory. The experimental data for the oil cleaning performance correlated well with the calculated values of the disjoining pressure, the spreading coefficient, and the film tension. We used the drop profile analysis based on the Laplace equation augmented with the extra term of the disjoining pressure to theoretically analyze the nanofluid spreading and wetting phenomena, and the detachment of the oil drop from the solid surface. These results confirm the novel mechanism of detergency using nanofluids based on the normal force (i.e., structural disjoining pressure) arising from the ordered nanoparticle structure formation in the confined space between the soil and the solid substrate (i.e., the wedge film).