X-ray Photon Correlation Spectroscopy (XPCS) offers unprecedented sensitivity to the dynamics of structural changes in materials. However, XPCS facilities have generally been limited to microstructure length scales smaller than ≈ 50 nm, thus eliminating large classes of materials that are of major technological importance. In recent years, we have been able to extend the range of this technique dramatically (into the micrometer scale regime) by combining XPCS speckle measurements with Bonse-Hart ultrasmall-angle scattering (USAXS) studies at the Advanced Photon Source. [1-4] While USAXS characterizes microstructures over the nanometer-to-micrometer scale range, use of a small entrance slit allows the coherence of the undulator X-ray beam to be exploited to give XPCS measurements of the internal microstructure dynamics. At the large end of the scale range, the slower material dynamics are well matched to the time resolution offered by USAXS-based XPCS. Using a point-counting configuration at selected Q values, we have established that phenomena previously observed for nanoparticle dispersions, including de Gennes narrowing, extend to these coarser length scales.  This is important because the slower relaxation times at mesoscale lengths in aqueous colloidal suspensions can be followed directly using USAXS-XPCS. Thus, phenomena such as bimodal interparticle interactions or suspension liquid phase transformations can be studied at the mesoscale while retaining relevance for nanoscale phenomena, where much shorter relaxation times make direct studies difficult.
USAXS-XPCS can also be configured to make repeated, short USAXS scans to detect incipient (precursor) microstructure changes under non-equilibrium conditions by following associated changes in the observed speckles.  We have applied this approach to study amorphous-to-crystalline phase transformations in dental composites. 
Finally, we have explored the feasibility of conducting simultaneous multiple USAXS-XPCS measurements using a nanofabricated slit array with each partially coherent X-ray beam paired to a group of pixels on a position-sensitive detector. This would allow rapid measurement of the dynamics in a heterogeneous material or could be used to follow a reaction front advancing across the sample.
 F. Zhang, A.J. Allen, L.E. Levine, J. Ilavsky, G.G. Long A.R. Sandy; J. Appl. Cryst., 44, 200-212 (2011).
 F. Zhang, A.J. Allen, L.E. Levine, J. Ilavsky, G.G. Long; Metall. Mater. Trans. A,43, 1445-1453 (2012).
 F. Zhang, A.J. Allen, L.E. Levine, L. Espinal, J.M. Antonucci, D. Skrtic, J.N.R. O’Donnell, J. Ilavsky; J. Biomed. Mater. Res. A, 100, 1293-1306 (2012).
 F. Zhang, A.J. Allen, L.E. Levine, J. Ilavsky, G.G. Long; Langmuir 29, 1379-1387 (2013).