1.1 Some Basic Concepts

Colloidal particles are an intrinsic part of systems in which finely divided matter (particles) is dispersed in a liquid or gas. Their size usually ranges from 1 nm to several tens of micrometres, thus covering a broad size domain.1 3 They are not necessarily solid and examples of soft colloidal particles (microgels and bacteria) will be briefly considered later in this chapter. Colloidal particles, similar to surfactant molecules, can spontaneously accumulate at the interface between two immiscible fluids (liquid gas or liquid liquid); they are therefore surface active.4 This fact was realised in the beginning of the last century by Ramsden5 and Pickering6 whose merit for instigating the field of particles at liquid interfaces will be discussed later. It is important to emphasise that the surface activity of these particles is not necessarily due to their amphiphilic nature. Solid particles with homogeneous chemical composition and properties everywhere on their surface (Figure 1.1(a)) can strongly attach to liquid interfaces and the reason for their surface activity is made clear below. There is, however, another class of particles with two distinct surface regions with different chemical composition and wetting properties (Figure 1.1(b)). These so-called Janus particles are both surface active and amphipilic7 which makes them very similar to surfactant molecules (Figure 1.1(c)). This class of amphiphilic particles will not be considered here.