A rare event

Abstract

When one presents a scientific work that refers to some branch of physics, both at a popular and technical level, usually even non-expert readers have at least a vague idea of what they are dealing with. This happens with a very large set of topics, ranging from gravity to semiconductors. On the contrary, when presenting a thesis on colloids, it is strictly necessary to start this long journey from the mere definition of the system under investigation. This is by the way a rather peculiar phenomenon, as each of us has to deal with colloids in everyday life. So, what are colloids?One possibility is to call a colloid any particle whose size varies between about 1 nm and 1 µm [1]. This term comes from the Greek word κoλλα, which means glue, as it was coined to describe sticky particles on a semipermeable membrane in 1860, by Thomas Graham [2]. Other more familiar examples of colloids include the toothpaste, paints, milk, and various creams.All of the above-mentioned examples involve not only colloids, but more precisely insoluble colloids dispersed in a medium composed of particles that are much smaller than the colloids themselves. This is the most interesting kind of colloidal system (as well as the one on which this thesis focuses), because it gives rise to the other fundamental characteristic of colloids, which also serves as an alternative definition for them. What distinguishes colloids from other systems in fact, more than their spatial dimensions, is their dynamics. Colloids perform trajectories which are random, but still possess very precise characteristics. This dynamics is called Brownian motion, as a tribute to Robert Brown, the botanist who first described the movement of grains of pollen in water in 1827 [3]. At the dawn of the twentieth century, Einstein and Sutherland [4,5] gave an explanation to the random movement of colloidal particles suspended in a medium. They understood that the numerous molecules of the solvent are in constant motion due to their thermal energy, and collide continuously onto the surfaces of the colloids. The result of these interactions is a stochastic force that acts on the colloids themselves. Returning to the first attempt to give a definition of a colloid, based on spatial dimensions, we realise that both lower and upper limits (respectively 1 nm and 1 µm) are imposed by the dynamical characteristics just described. In other words, colloids are particles that are large enough not to be affected by quantum effects, but small enough to experience Brownian motion.In a colloidal system, colloids interact not only with the solvent particles, but also with each other. The resulting collective behaviour is surprisingly rich and diverse. Indeed, thanks to the Brownian motion, a colloidal system effectively explores phase space in search of the minimum free energy. This exploration brings different systems into a multitude of different phasesranging from the fluid phase to crystalline states, to liquid crystals and even to quasicrystalsthrough the process of self-assembly [6]. Th...