Entropically driven self–assembly and interaction in suspension

In this paper we present fundamental studies elucidating the role of entropy in parti- cle suspensions. We focus on systems composed of large colloidal particles along with a second, usually smaller species such as a particle or polymer. We describe direct measurements of these interactions in suspension, and we systematically show how these forces can be used to control the self-assembly of colloidal particles. The paper provides a unified review of the experiments from our laboratory, and in a few cases touches on very recent results. Entropic forces between macromolecules in suspension are sometimes produced by the addition of other constituents to the background solvent. These added con- stituents are often other (usually smaller) particles, rods or polymers. The resulting entropic forces influence suspension stability, and are of considerable importance in a wide variety of practical materials ranging from frozen desserts to paints to motor oils to living cells. During the last few years we have been studying entropic effects in suspension. We have quantified these forces by direct measurement in a variety of complex fluids, we have identified some novel manifestations of these entropic phe- nomena, and we have used these effects to control the self-assembly of particles. In this paper we provide a unified review of the experiments from our laboratory, and in a few cases touch on very recent results. The starting point of our exposition naturally begins with mixtures of large- and small-diameter hard spheres. Hard-sphere colloids lack attractive and long-range interactions, which typically compete with entropic effects to produce ordered phases. Nonetheless, as Asakura & Oosawa (1958) first noted, in mixtures of different-sized spherical particles, an ordered arrangement of large spheres can increase the total entropy of the system by increasing the entropy of the small spheres. This phe- nomenon is depicted in figure 1. Because the centre of mass of the small sphere