Direct measurements of polymer-induced forces

Colloid-polymer mixtures are found in dispersions that are an important part of people's everyday lives. The dynamics and phase stability of colloid-polymer mixtures depend on the interactions that are present in these systems. Therefore, knowledge of interactions is of basic interest. Depending on their adsorption affinity polymers added to the colloidal suspension can cause steric stabilization or flocculation due to depletion or adsorption (bridging). This paper reviews theoretical and experimental work performed on polymer-induced interactions in colloidal suspensions. Theoretically, polymers have mainly been treated as ideal flexible chains or even generalized as non-interacting (phantom) spheres. Many relevant experiments, however, have been performed with polymer chains, which are polydisperse and/or charged and/or self-interacting. These cases are challenging for theoreticians: a limited amount of work performed on these systems is also discussed here. We particularly concentrate in this review on the direct experimental measurement of polymer-induced interactions. A brief description of techniques which enable these measurements is given and their strengths and weaknesses are discussed. (topical review)