Oscillatory Perturbation of Dense Colloidal Suspensions

Using microscopic magnetic particles, we locally perturb dense colloidal suspensions by moving an external magnet in a rotational motion. We use confocal microscopy to track the position of the magnetic probe particle and surrounding colloidal particles. We use the known applied external force on the probe particle and the probe’s measured oscillatory amplitude to calculate the storage and loss moduli of colloidal suspensions with various volume fractions. We find that these quantities agree well with prior results obtained from traditional rheology. To characterize the response of the colloidal particles to the external perturbation, we obtain the oscillatory amplitude of the colloidal particles as a function of distance r from the probe particle and find that the amplitudes decay as 1/r. Finally, we investigate the possibility of determining the speed of sound in colloidal suspensions by measuring the phase lag of the colloidal particles’ oscillations with respect to the phase of the probe particle.