Self -organization of particles and nanofluids under confinements: Applications to spreading, wetting and soil removal

摘要

We studied the self-organization of different-sized particles in different confined geometries. The structure formations of millimeter-sized granules confined to a solid substrate, micrometer-sized colloidal particles confined to a fluid-liquid interface, and nanometer-sized particles confined in the wedge film are discussed. We spread the steel particles (diameter: 1.59mm) on a silicon wafer and a polystyrene surface to form a two-dimensional (2D) hard-sphere system and a 2D charged-sphere system, respectively. The 2D granular structures versus the particle area fraction were monitored and analyzed through the radial distribution function, potential of the mean force, structure factor, and bond orientation order parameter or correlation functions. We observed the particle structural transition from liquid-like to triangular-lattice crystal-like with increasing particle coverage (A = 0.70--0.82) in the 2D hard-sphere system. In a 2D charged system, a 'fluid-like' structure was observed in the particle area fraction range of A = 0.01 to 0.18. We 'investigated' the structuring of micrometer-sized particles confined in a fluid-liquid interface through Monte Carlo simulations using the asymptotic pair potential proposed by Hurd (1985) which includes both the screened Coulomb contribution and the dipole-dipole interactions. The effects of the multi-particle effective interactions and of the particle charge on the 2D particle structuring were elucidated by the radial distribution function. The technological concept of the nanoparticle structuring in the wedge film was explored with regards to its application to the spreading, wetting, and soil removal phenomena. The experimental and theoretical investigations on the cleansing of canola oil from a glass substrate using commercially available nanofluids were pursued. 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. These results confirm the novel mechanism of detergency using nanofluids based on the normal force (i.e., 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).