I discuss the forces driving nanoparticles to interfaces and how it is possible to control their segregation in thin polymer films (ca. 100 nm thick) placed on a flat, solid substrate. Consider a mixture of nanoparticles suspended in a pure polymer melt which have no interparticle or intermolecular forces between them. This is equivalent to hard spheres mixed with a polymer chain comprised of smaller hard spheres chained together like a pearl necklace. Fluids density functional theory proves that a monolayer of nanoparticles forms at the solid substrate through a first order phase transition driven purely by entropy. This thermodynamic assembly force is so large that it stabilizes the film allowing it to go up and over protrusions on the substrate defying surface tension and dewetting forces. In fact, a Lagrange multiplier technique was used with a simple model to incorporate intermolecular forces demonstrating the stabilizing force is more general than expected. Exploring naoparticle assembly has allowed us to more fully understand this phenomenon and has lead us to its use in the manufacture of polymer based solar cells which will be discussed in the seminar.