Vibration spectroscopy of weakly interacting mesoscopic colloids

Brillouin spectroscopy has been used to study the vibrational dynamics of clusters of spherical polystyrene (PS) particles with different size. In a first approximation, the spectra can be described by a single particle model within the continuum approximation of the Lamb theory. The model yields excellent results for particles with diameter dT500 nm, but fails in accounting for the lineshapes of the observed lowest frequency signals in the vibration spectrum of smaller particles due to a broadening and shift of the lines of confined vibrations. The model also fails to predict an additional very low frequency broad band in the case of small particles (d ( 400nm). This band is attributed to phonon propagation in multiple-particle clusters governed by the interactions among particles. These interactions also produce extended modes in close relation to the Lamb modes of the single sphere. A simple model for the particle interaction allows to represent the new spectral features and estimate the strength of the interactions and the long wavelength longitudinal velocity in the colloidal clusters. Their thermal annealing near the glass transition of PS enhances the interactions which are manifested in the low frequency particle vibration spectrum and the phonon bands associated with the vibrations of the individual spheres. The emerging particle vibration spectroscopy becomes a sensitive tool of the colloids’ thermo-mechanical properties as well their interactions.