Nanoparticles at fluid interfaces are becoming a central topic in colloid science studies. Unlike in the case of colloids in suspensions, the description of the forces determining the physical behavior of colloids at interfaces still represents an outstanding problem in the modern theory of colloidal interactions. These forces regulate the formation of complex two-dimensional structures, which can be exploited in a number of applications of technological interest; optical devices, catalysis, molecular electronics or emulsions stabilization. From a fundamental viewpoint and typical for colloidal systems, nanoparticles and microparticles at interfaces are ideal experimental and theoretical models for investigating questions of relevance in condensed matter physics, such as the phase behavior of two-dimensional fluids. This review is a topical survey of the stability, self-assembly behavior and mutual interactions of nanoparticles at fluid interfaces. Thermodynamic models offer an intuitive approach to explaining the interfacial stability of nanoparticles in terms of a few material properties, such as the surface and line tensions. A critical discussion of the theoretical basis, accuracy, limitations, and recent predictions of the thermodynamic models is provided. We also review recent work concerned with nanoparticle self-assembly at fluid interfaces. Complex two-dimensional structures varying considerably with the particle nature have been observed in a number of experiments. We discuss the self-assembly behavior in terms of nanoparticle composition, focusing on sterically stabilized, charged and magnetic nanoparticles. The structure of the two-dimensional assemblies is a reflection of complex intercolloidal forces. Unlike the case for bulk colloidal suspensions, which often can be described reasonably well using DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, the description of particles at interfaces requires the consideration of interfacial deformations as well as interfacial thermal fluctuations. We analyze the importance of both deformation and fluctuations, as well as the modification of electrostatic and van der Waals interactions. Finally, we discuss possible future directions in the field of nanoparticles at interfaces.

We report on a large scale computer simulation study of crystal nucleation in hard spheres. Through a combined analysis of real- and reciprocal-space data, a picture of a two-step crystallization process is supported: First, dense, amorphous clusters form which then act as precursors for the nucleation of well-ordered crystallites. This kind of crystallization process has been previously observed in systems that interact via potentials that have an attractive as well as a repulsive part, most prominently in protein solutions. In this context the effect has been attributed to the presence of metastable fluid-fluid demixing. Our simulations, however, show that a purely repulsive system (that has no metastable fluid-fluid coexistence) crystallizes via the same mechanism.

We study the excess free energy due to phase coexistence of fluids by Monte Carlo simulations using successive umbrella sampling in finite L×L×L boxes with periodic boundary conditions. Both the vapor-liquid phase coexistence of a simple Lennard-Jones fluid and the coexistence between A-rich and B-rich phases of a symmetric binary (AB) Lennard-Jones mixture are studied, varying the density ρ in the simple fluid or the relative concentration xA of A in the binary mixture, respectively. The character of phase coexistence changes from a spherical droplet (or bubble) of the minority phase (near the coexistence curve) to a cylindrical droplet (or bubble) and finally (in the center of the miscibility gap) to a slablike configuration of two parallel flat interfaces. Extending the analysis of Schrader et al., [Phys. Rev. E 79, 061104 (2009)], we extract the surface free energy γ(R) of both spherical and cylindrical droplets and bubbles in the vapor-liquid case and present evidence that for R→∞ the leading order (Tolman) correction for droplets has sign opposite to the case of bubbles, consistent with the Tolman length being independent on the sign of curvature. For the symmetric binary mixture, the expected nonexistence of the Tolman length is confirmed. In all cases and for a range of radii R relevant for nucleation theory, γ(R) deviates strongly from γ(∞) which can be accounted for by a term of order γ(∞)/γ(R)−1∝R−2. Our results for the simple Lennard-Jones fluid are also compared to results from density functional theory, and we find qualitative agreement in the behavior of γ(R) as well as in the sign and magnitude of the Tolman length.

For partially wetting, ellipsoidal colloids trapped at a fluid interface, their effective, interface-mediated interactions of capillary and fluctuation-induced type are analyzed. For contact angles different from 90( degrees ) , static interface deformations arise which lead to anisotropic capillary forces that are substantial already for micrometer-sized particles. The capillary problem is solved using an efficient perturbative treatment which allows a fast determination of the capillary interaction for all distances between and orientations of two particles. Besides static capillary forces, fluctuation-induced forces caused by thermally excited capillary waves arise at fluid interfaces. For the specific choice of a spatially fixed three-phase contact line, the asymptotic behavior of the fluctuation-induced force is determined analytically for both the close-distance and the long-distance regime and compared to numerical solutions.