We investigate the behavior of a colloidal suspension using active, oscillatory microrheology. The experimental system is an aqueous suspension of index matched fluorinated ethylene propylene (FEP) particles embedded with either 2 um silica or 3 um polystyrene probe particles. The probes are trapped and oscillated using laser tweezers at frequencies of 5-1000 Hz and at amplitudes of 25-400 nm. The oscillation amplitude and phase of the probe are measured using a photo diode and lock-in amplifier, which are used to compute the frequency dependent microviscosity of the suspension. Frequency thinning is observed at all concentrations and is most significant for volume fractions above 0.3. The results are in quantitative agreement with both previous measurements of the suspension microviscosity [1] and recently developed theory [2].

[1] Meyer, A., et al., Journal of Rheology. 50(1), 77-92, (2006). [2] Khair, A. S., and J. F. Brady, Journal of Rheology. 49(6), 1449-1481, (2005).