Microrheology of soft materials using oscillating optical traps

This dissertation addresses the microrheology of soft materials using an oscillating optical tweezers technique. In the technique, a lock-in amplifier is used to detect the in-phase and out-of-phase motions of trapped colloidal probe particles as a function of the trap's oscillation frequency: this allows for the direct determination of the viscoelasticity of soft materials. As proof of principle, the viscoelasticity for a homogeneous polymer solution (as a function of oscillation frequency, 0.01–6000 Hz) is determined; the results compare well to the mechanical properties measured by other methods. The study is extended to polymers that are composed of a water-soluble backbone terminated by hydrophobic moieties. In aqueous solutions, these polymers form micelle like structures, prompting measurements of the viscoelastic properties. The Maxwell model, combined with the Rouse model can describe the measured viscoelasticity. The Maxwell model accounts for hydrophobic associations between the polymers and the Rouse type model accounts for the micellar structure of the polymer. In addition a study is presented of the viscoelasticity of intrinsically inhomogeneous systems, namely the interior of biological cells. In this study, it is found that the viscoelastic properties are weakly dependent on the oscillation frequency.; The oscillating optical tweezers technique is also used for dual particle microrheology. The dual particle microrheology study begins with an investigation of the correlated motions of two hydrodynamically coupled colloidal particles, each of which is optically trapped. By setting one of the particles into forced oscillation using oscillating optical tweezers, the position of each of the particles is measured as a function of frequency. From the in-phase and out-of-phase motions of both of the particles in the 1 traps, correlated motions of the coupled mechanical system are determined as a function of frequency. A theoretical model based on the hydrodynamic coupling of the trapped particles is developed to calculate the response tensor of the system. The experimental results agree with the prediction of the model. This method is extended to investigate the viscoelasticity of heterogeneous media. The mechanical properties surrounding a particle are in good agreement with the properties between two particles for the polymer solutions investigated in this thesis.