Line optical tweezers for measurement of entropic forces in complex fluids

Summary form only given.Optical tweezers are exciting and relatively new measurement tools that have been used to exert forces on micron-sized particles and on molecules attached to particles. In this contribution we introduce the optical line tweezer as means to measure interaction potentials between two particles in various complex fluids. Interestingly, the method enables us to study structure and dynamics of the background fluids as well as to probe the direct particle electrostatic interactions. We have used the technique to make sensitive measurements with spatial resolutions of 10 nm and energy resolutions less than 0.1 kT. Entropic effects in hard-sphere colloids and in DNA polymer solutions have been observed and are described.