Dynamic light scattering in turbid nonergodic media.

We here present a new device based on dynamic light scattering (DLS) for measuring kinetics in turbid and nonergodic systems. This flat cell light scattering instrument has been developed in our laboratory and is based on an original flat cell instrument employing cells of varying thickness in order to measure the static structure and dynamics of a system. The smallest cell thickness is 10 microm. To this original instrument, we have integrated the three-dimensional (3D)-DLS technology as well as the echo method, and in comparison with other 3D-DLS instruments, ours show the best performance; the maximum intercept was 0.6 as opposed to 0.15 for regular 3D-DLS devices (recently we reached beta=0.75). This was made possible by using crossed polarization filters for the two laser beams, thereby allowing the scattered light from both laser beams to be decoupled and the intercept to no longer be limited at the theoretical value of 0.25. The maximum weight fraction of the sample that is measurable with such a setup is more than ten times higher than with a standard 3D-DLS setup or with the flat cell instrument without the 3D technology. Consequently, with the 3D-DLS flat cell instrument presented here, it truly becomes possible to investigate turbid systems. Moreover, the echo method was integrated to enable measurements of nonergodic systems. Here, a new mechanical design of the echo-DLS component was necessary due to the different geometries of the flat cell in comparison with that of a standard cylindrical cell. The performance of our echo device was compared to that of our multispeckle instrument, and the results were in good agreement for correlation times up to 30,000 s and more. The main limitation of this instrument in its current version is the maximum scattering angle of about 50 degrees (or 30 degrees if echo is used).