Sign change in the net force in sphere-plate and sphere-sphere systems immersed in nonpolar critical fluid due to the interplay between the critical Casimir and dispersion van der Waals forces

摘要

We study systems in which both long-ranged van der Waals and critical Casimirinteractions are present. We study the interplay between these forces, as wellas the {\it total} force (TF) between a spherical colloid particle and a thickplanar slab, and between two spherical colloid particles. We do that usinggeneral scaling arguments and mean-field type calculations utilizing theDerjaguin and the surface integration approaches. They both are based on dataof the forces between two parallel slabs separated at a distance $L$ from eachother, confining the fluctuating fluid medium characterized by its temperature$T$ and chemical potential $\mu$. The surfaces of the colloid particles and theslab are coated by thin layers exerting strong preference to the liquid phaseof the fluid, or one of the components of the mixture, modeled by strongadsorbing local surface potentials, ensuring the so-called $(+,+)$ boundaryconditions. On the other hand, the core region of the slab and the particles,influence the fluid by long-ranged competing dispersion potentials. Wedemonstrate that for a suitable set of colloids-fluid, slab-fluid, andfluid-fluid coupling parameters the competition between the effects due to thecoatings and the core regions of the objects involved result, when one changes$T$, $\mu$ or $L$, in {\it sign change} of the Casimir force (CF) {\it and} theTF acting between the colloid and the slab, as well as between the colloids.This can be used for governing the behavior of objects, say colloidalparticles, at small distances, say in colloid suspensions for preventingflocculation. It can also provide a strategy for solving problems withhandling, feeding, trapping and fixing of microparts in nanotechnology. Datafor specific substances in support of the experimental feasibility of thetheoretically predicted behavior of the CF and TF have been also presented.