On the role of polydispersity on the phase diagram of colloidal rods.

The rich and complex phase diagram typical of anisotropic biological or synthetic nanoparticles, has brought a great deal of interest over the equilibrium phase behaviour of non-spherical colloids. Amongst the class of anisotropic nanoparticles, hard spherocylindrical colloids have been, over the years, extensively studied because of their optical properties, for their rich phase diagrams, and their important industrial applications, as model particles for biological systems (viruses), or for example as potential drug carriers having the ability of surviving the attacks of the immune systems. As real anisotropic nanoparticles are often polydisperse in size and/or in shape, unveiling the effect of such a perturbation over their equilibrium phase diagram is of paramount importance. This work focuses on the effects of polydispersity over the full equilibrium phase diagram of hard spherocylindrical colloids (HSCs). Previous studies showed that a polydispersity in L alters the equilibrium phase diagram of HSCs. With this work we determine, both theoretically as well as computationally, the effects due to a generic polydispersity, namely in D, in L and, in both ones, on the equilibrium phase diagram and introduce a viable theoretical generalisation of the Onsager theory that allows us to get some insight into the observed phase behaviour.

Unveiling adsorption generality in polymeric macromolecules.

Unveiling a general law for adsorption within macromolecules sets an important step forward in the design of nanomaterials with tunable and controllable properties. Reaching such a generalised control would have an important outcome in a plethora of possible fields, from biomedical applications up to materials science. In fact, the definition of classes regrouping adsorbing macromolecules with different geometrical or chemical properties would enormously simplify the design of controllable adsorbing materials, especially when geometrical or chemical constraints are set by the particular application. In this work we derive, through a combination of Scaling Theories and Molecular Dynamics Simulations, a general law for adsorption of spherical non-deformable colloidal nanoparticles within polymeric macromolecules of different geometries. Starting from the case of adsorption of a single colloid within macromolecular systems, we extend the results to the case in which finite adsorption takes place. We then derive simple predictions linking the adsorption potential to general properties of classes of macromolecules, and introduce a set of measurable quantities that can be exploited as an indirect measurement for loading.

Shape Matters in Magnetic-Field-Assisted Assembly of Prolate Colloids.

An anisotropic colloidal shape in combination with an externally tunable interaction potential results in a plethora of self-assembled structures with potential applications toward the fabrication of smart materials. Here we present our investigation on the influence of an external magnetic field on the self-assembly of hematite-silica core-shell prolate colloids for two aspect ratios ρ = 2.9 and 3.69. Our study shows a rather counterintuitive but interesting phenomenon, where prolate colloids self-assemble into oblate liquid crystalline (LC) phases. With increasing concentration, particles with smaller ρ reveal a sequence of LC phases involving para-nematic, nematic, smectic, and oriented glass phases. The occurrence of a smectic phase for colloidal ellipsoids has been neither predicted nor reported before. Quantitative shape analysis of the particles together with extensive computer simulations indicate that in addition to ρ, a subtle deviation from the ideal ellipsoidal shape dictates the formation of this unusual sequence of field-induced structures. Particles with ρ = 2.9 exhibit a hybrid shape containing features from both spherocylinders and ellipsoids, which make their self-assembly behavior richer than that observed for either of the "pure" shapes. The shape of the particles with higher ρ matches closely with the ideal ellipsoids, as a result their phase behavior follows the one expected for a "pure" ellipsoidal shape. Using anisotropic building blocks and external fields, our study demonstrates the ramifications of the subtle changes in the particle shape on the field-directed self-assembled structures with externally tunable properties.