Self assembly in soft matter
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The term “soft matter” applies to a variety of physical systems, such as liquids, colloids, polymers, foams, gels, and granular materials. The most fascinating aspect of soft matter lies in the fact that they are not atomic or molecular in nature. They are instead macromolecular aggregates, whose spatial extent lies in the domain 1 nm to 1 ¹m. Some of the most important examples of soft matter are polymers, which exhibit intriguing and useful physical properties. In this work, the adsorption and self assembly of linear and star polymers on smooth surfaces are studied using coarse-grained, bead-springmolecular models and Langevin dynamics computer simulations. The aim is to gain insight on atomic-forcemicroscopy images of polymer films on mica surfaces, adsorbed from dilute solution following a good solvent-to-bad solvent quenching procedure. In the case of linear polymers, under certain experimental conditions, a bimodal cluster distribution is observed. It is demonstrated that this type of distribution can be reproduced in the simulations, and rationalized on the basis of the polymer structures prior to the quench. In addition to providing insight on experimental observations, the simulation results support a number of predicted scaling laws such as the decay of the monomer density as a function of distance from the surface, and the scaling of the filmheight with the strength of the polymer-surface interactions. Star polymers represent a special class of polymers, in which one end of each linear chain is tethered to a small central core to forma single particle. The discovery of these molecules led to the synthesis of a wide range of new materials. Their structures are effectively considered as intermediate between those of colloids and linear polymers. We explore the behaviour of the star polymers (which are like “soft colloids”) in the proximity of a surface, using Langevin dynamics simulations. A number of different measurements such as the height, radius of gyration, and asphericity of adsorbed stars with different number of arms, are shown to provide valuable insights on experimental findings. The simplest soft matter systems consist of spherical, rigid colloidal particles. Examples of such particles are chemically synthesized polystyrene or silica particles. We investigated the neighbour distribution in a two-dimensional polydisperse harddisk fluid, corresponding physically to a colloidal monolayer. The disk diameter distribution was defined by a power-law with the aim of realizing a scale-free nearneighbour network. Scale-free (power-law) behaviour is found in many important networks, for example, in transportation systems, biochemical reactions, scientific and movie-actor collaborations, and sexual contacts. We have provided the first example of a scale-free network in amodel condensed-matter system. Finally, we use genetic algorithms, a method for efficiently searching for minima on energy landscapes, to investigate the ordered equilibrium structures formed by binary mixtures of anisotropic dipolar particles confined on a plane, under the presence of an external magnetic field. The anisotropy of the interparticle forces is controlled by tilting the external magnetic field with respect to the plane. Initially, as the field is tilted the structures are only slightly perturbed, but once the anisotropy exceeds a critical value, completely new structures emerge.