On the effect of capillarity and polarity in colloidal suspensions of nanoparticles

Abstract

Chapter 1 Phenomenological background 1.ColloidsThis thesis deals with a series of phenomena occurring in systems composed of particles with dimensions ranging from a few nanometers up to several microns. The structural properties of these so-called colloidal systems can easily be altered by mechanical or thermal stress of the order of thermal fluctuations. In this sense they belong to the broad branch of physics known as soft matter [1]. Examples of colloidal systems are present in tremendous numbers in every day life: suspensions/emulsions such as milk, blood, paint, liquid crystal displays in electronic equipment, gels and lotions, polymers, biological entities such as bacteria, sperm cells, tissues, etc.A very important property of colloids is that they perform a random motion, when suspended in a solvent such as water or oil. This random motion is commonly referred to as Brownian motion, since observed for the first time by R. Brown in 1827 [2] for pollen particles. The theoretical explanation of Brown's observations was provided by A. Einstein in 1905 [3] amongst others, and, a few years later, definitive evidence of Brownian motion was provided in experiments by J. Perrin [4]. Because of the Brownian motion, colloidal systems are well suited to be studied by statistical mechanics. Indeed, colloids in a solvent are able to move around in the solvent and can interact with each other or with the solvent molecules. As a consequence they are in principle able to explore all possible configurations in which the system (colloids+solvent) can be, thus satisfying the ergodic hypothesis, which is the central assumption of statistical mechanics. However, as in atomic and molecular systems, ergodicity also breaks in colloidal systems when approaching the glass transition [5,6].On the experimental side, the possibility of manipulating colloidal systems has widely increased in the last decades. Indeed, important advancements have been made in preparing colloidal suspensions and tuning their properties [7,8]. Moreover, because of their size and slow dynamics, colloids are well suited to be studied in real space and real time using advanced microscopy techniques [9].Out of the endless possibilities of colloidal systems, we will focus in this thesis on colloids trapped at a fluid-fluid interface, and on liquid crystalline colloidal systems. Chains of colloids at fluid-fluid interfacesA situation in which colloidal particles display striking features is when they are trapped at a fluid-fluid interface, i.e. at the interface of two immiscible fluids. Indeed, as firstly experimentally observed at the beginning of the twentieth century [10,11], and successively theoretically shown in a seminal paper by Pieranski [12], sub-millimiter sized particles strongly adsorb at fluid-fluid interfaces in order to reduce the interfacial area, and hence the free energy of the interface [13]. Once adsorbed, colloidal particles can give rise to stable monolayers.Recently, capillarity has been invoked to explain this intriguing phenomenon [1...