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 sp here in a telechelic poly(ethylene oxide) solution using optical tweez ers. With this new technique, we use a position-sensing detector and a lock-in amplifier to measure the displacement magnitude and phase res ponses of one latex sphere driven sinusoidally by optical tweezers. Fo r a single particle in solution, the equation of motion of the particl e is simply that of the forced oscillation problem with damping from v iscous drag and the restoring force from the elasticity of the solutio n medium and that of the optical tweezers. Because the system is overd amped, it is not feasible to probe the high-frequency regime. Thus we cannot measure the viscosity and elastic moduli separately from freque ncy-dependent measurements alone. At low polymer concentration, measur ements of the viscosity have been achieved. We compared the measured v iscosity with that obtained with other measurements. Key issues for fu rther development of this technique, such as measuring the elastic mod ulus, are briefly discussed.