Forces on a colloidal particle in a polymer solution: A study using optical tweezers

We report a study of the dynamical behaviour of a polystyrene latex sphere in a telechelic poly(ethylene oxide) solution using optical tweezers. With this new technique, we use a position-sensing detector and a lock-in amplifier to measure the displacement magnitude and phase responses of one latex sphere driven sinusoidally by optical tweezers. For a single particle in solution, the equation of motion of the particle is simply that of the forced oscillation problem with damping from viscous drag and the restoring force from the elasticity of the solution medium and that of the optical tweezers. Because the system is overdamped, it is not feasible to probe the high-frequency regime. Thus we cannot measure the viscosity and elastic moduli separately from frequency-dependent measurements alone. At low polymer concentration, measurements of the viscosity have been achieved. We compared the measured viscosity with that obtained with other measurements. Key issues for further development of this technique, such as measuring the elastic modulus, are briefly discussed.