Entropic attraction and repulsion in binary colloids probed with a line optical tweezer

The long-range entropic forces that arise between two micrometer-sized coll oidal spheres in a fluid of much smaller colloidal spheres were directly me asured 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 1 5 nm spatial resolution. At the lowest small sphere concentrations, the pot ential was monotonically attractive, while at higher concentrations an osci llatory potential was observed, due to the liquid structure of the small sp heres. Surprisingly, the large spheres came together only rarely at the hig her concentrations, suggesting a new means for stabilizing suspensions usin g entropy alone. [S0031-9007(99)09246-7].