Characterization of Turbid Colloidal Suspensions Using Light Scattering Techniques Combined with Cross-Correlation Methods Claus Urban and Peter Schurtenberger 1 Polymer Institute, ETH Zurich, CH-8092 Zurich, Switzerland Received March 31, 1998; accepted July 21, 1998 The ability to characterize colloidal suspensions by means of dynamic light scattering is in general limited to systems with negligible contributions from multiple scattering. For larger par- ticle sizes with high scattering contrast this immediately limits the technique to very low concentrations. A promising solution of this problem is to suppress multiple scattering in dynamic light scat- tering experiments using cross-correlation schemes. Based on these considerations we have constructed a so-called 3D cross- correlation experiment with which we are able to characterize extremely turbid suspensions. We have measured monomodal and bimodal suspensions of latex particles of relatively high volume fraction. The results show clearly that we are able to measure the dynamic structure factor in concentrated polydisperse suspensions with dynamic light scattering. Combining static and dynamic light scattering measurements forcharacterizing turbid suspensions the single scattering particle form factorand also the static structure factor can be evaluated. We demonstrate that the implementation of a 3D cross-correlation scheme is a powerful method in sup- pressing multiple scattering contributions in light scattering ex- periments and opens a wide field of characterization of colloidal dispersions with high turbidity without having to resort to high dilution. © 1998 Academic Press Key Words: dynamic light scattering; multiple scattering; cross- correlation; turbid suspensions; concentrated colloid suspensions. INTRODUCTION Dynamic light scattering (DLS) is one of the most popular experimental techniques in the characterization of colloidal suspensions. DLS provides a measure of the time scale for fluctuations in the index of refraction of a complex fluid, and it probes these fluctuations on the length scale of the inverse of the scattering vector, q -1 . In the case of colloidal particles intensity fluctuations are predominantly caused by the diffusive motion of the particles. Based on the correlation between diffusion coefficient and particle size, DLS is now widely used as a very convenient and nondestructive method for particle sizing. The technique is suitable for the characterization of colloidal particles over a wide range of sizes from a few nanometers to several micrometers. Moreover, DLS is also frequently used for investigations of the dynamic behaviour of concentrated and/or strongly interacting colloidal suspensions and glasses, and it allows us to follow processes such as aggregation and gelation. However, its application to many systems of industrial relevance has often be considered to be too complicated due to the very strong multiple scattering in undiluted solutions. The interpretation of a DLS experiment becomes exceedingly difficult for systems with nonnegligible contributions from multiple scattering. Particularly for larger particles with high scattering contrast this limits the technique to very low concentrations, and a large variety of systems are therefore excluded from investigations with dynamic light scat- tering. One quite recent approach to this problem, which has already been implemented in commercial products, works in the limit of very strong multiple scattering, where a diffusion model can be used in order to describe the propagation of the light across the sample (1). Unfortunately, this so-called “dif- fusing wave spectroscopy” does generally not allow us to determine important features such as polydispersity of the particles due to the inherent averaging over a large range of spatial length scales. A very interesting opposite approach aims at sufficiently suppressing contributions from multiple scatter- ing from the measured photon correlation data. There exist a number of different theoretical and experimental approaches to this problem (2– 4). The general idea is to isolate singly scat- tered light and suppress undesired contributions from multiple scattering in a dynamic light scattering experiment (for details see (2)). This can be achieved by performing two scattering experiments simultaneously on the same scattering volume (with two laser beams, initial wave vectors k il and k i2 , and two detectors positioned at final wave vectors k f1 and k f2 ) and cross-correlating the signals seen by the two detectors. If both experiments then have the same scattering vector q, but use different scattering geometries, and in the absence of multiple scattering, the corresponding relations between the dynamic structure factor S( q, ) and the measured auto-correlation ( G 11 (1) (  )) and cross-correlation ( G 12 (1) (  )) function, where (1) indicates singly scattered photons, can then be written as 1 Corresponding author. E-mail: pschurt@ifp.mat.ethz.ch. JOURNAL OF COLLOID AND INTERFACE SCIENCE 207, 150 –158 (1998) ARTICLE NO. CS985769 150 0021-9797/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.