Entropic Attraction and Repulsion in Binary Colloids Probed with a Line Optical Tweezer

The long-range entropic forces that arise between two micrometer-sized colloidal spheres in a fluid of much smaller colloidal spheres were directly measured using a line-scanned optical tweezer. This new technique allowed us to measure the functional form of the potential with sub- k B T energy and 15 nm spatial resolution. At the lowest small sphere concentrations; the potential was monotonically attractive; while at higher concentrations an oscillatory potential was observed; due to the liquid structure of the small spheres. Surprisingly; the large spheres came together only rarely at the higher concentrations; suggesting a new means for stabilizing suspensions using entropy alone.