Keywords: diffusing wave spectroscopy, photon correlation spectroscopy, dynamic light scattering, non–ergodic samples, gels, nanotechnology, multispeckle measurements
Rotating angled plate diffusing wave spectroscopy
Solid–like non–ergodic samples can be difficult to study using standard single speckle diffusing wave spectroscopy as the measurement may lack requisite ensemble averaging. We demonstrate here that this required ensemble averaging can be induced by translating the speckle over the collection optics so that multiple speckles are observed over the duration of the experiment. Translation is achieved by refraction through a transparent rotating plate with a surface normal angled slightly with respect to the axis of rotation. As with other ensemble averaging methods based on rotation, the measured time averaged correlation is that of the ensemble average correlation modulated by a periodic Gaussian–like function. By rotating the plate slowly, the short time behaviour of the ensemble averaged intensity correlation can be measured with good fidelity using a standard multi–tau correlator. Because the method combines easily with camera–based multispeckle approaches, the long–time behaviour can be recovered using concurrent multispeckle measurements if necessary. No additional scattering is required beyond that produced by the sample and the technique also works in the single scattering regime which is not possible with the other commonly used alternatives for ensemble averaging: two cell or rotating diffusor approaches.