Department of Physics & Astronomy, Johns Hopkins University X-ray Photon Correlation Spectroscopy Studies of Nanoscale Particle Motion within Heterogeneous Complex Fluids Robert L. Leheny Department of Physics & Astronomy, Johns Hopkins University We are pursuing a research program that applies x-ray photon correlation spectroscopy (XPCS) to microrheology by investigating the dynamics of nanometer-scale particles within polymer melts. At high temperatures the dynamic structure factors, f(q,t), we extract from the XPCS measurements indicate diffusive motion of the nanoparticles. Specifically, f(q,t) has an exponential lineshape with a characteristic decay time that depends on ther wave vector as t = 1/(Dq2). At low temperature the nature of the observed dynamics changes and can be modeled as strain in the melt resulting from localized stress relaxation events.. The signature features of these dynamics are a compressed-exponential lineshape for f(q,t) with a decay time that varies inversely with wave vector, t ~ 1/q. Such non-diffusive motion has been observed in numerous disordered soft solids, and our results show this phenomena is a general feature not only of soft but also hard disordered materials. Correlation times versus wave vector for nonoparticle motion in a polymer melt Schematic of nanoparticle in polymer melt