Soft Report - Dipartimento di Fisica - Sapienza

Missionstate, we aim to measure the correlation function C(t) of a certain observable A(t)and measure the response function R(t) of the system after a perturbation(conjugate to A(t) in the system Hamiltonian) is applied. The study of C(t) wouldallow to verify the MCT prediction, while the comparison of C(t) and R(t) will giveinformation on the generalization of the FD relation. Among the possibleobservable, we plan to study (at least) a scalar observable (number density), avector one (dipolar polarization) and a pure rank two tensor one (the polarizabilityanisotropy).A2. Generalized fluctuation-dissipation relation in aging systems.The aim of this Proget is the study of the off-equilibrium dynamics in supercooledliquids, polymers and colloids. In particular our goal is the experimental test of welldefined theoretical predictions: the existence of scaling laws for the physicalobservable in the aging regime, the existence of a relation between the responseand the (conjugate) correlation functions and, consequently, the measurement ofan effective temperature [2]. Moreover, it is our aim to verify (as much as possible)the universality of these prediction by investigate different kind of systems (rangingfrom molecular glass formers to colloids) and different kind of observable. Amongthe latter, and according to our experimental capabilities, we selected a scalarobservable (number density), a vector one (dipolar polarization) and a pure ranktwo tensor one (the polarizability anisotropy). To reach our goal one needs to 1)bring the selected sample out of equilibrium 2) measure the correlation functionC(t1,t2) of a certain observable A(t), as a function of both times and 3) measurethe response function R(t2) of the system after a perturbation (conjugate to A(t) inthe system Hamiltonian) is applied at time t1. Steps 2) and 3) alone can be used tostudy the scaling laws expected for correlation and response functions respectively,while their comparison (or better the simultaneous measure of 2 and 3) bringsinformation on the generalized Fluctuation-Dissipation Theorem (FDT) and on theeffective temperature. The steps 1-3 should be repeated for the three differentobservable listed above: a) density, b) dipolar polarization and c) polarizabilityanisotropy (and, of course, for different samples).[1] C. B. Holmes, M. Fuchs, M. E. Cates, cond-mat/0210321[2] L. F. Cugliandolo, cond-mat/0210312B. Self Assembly, Clustering, Structural ArrestWithin soft-matter research, the CRS SOFT privileges studies on self-assembly,clustering and structural arrest on colloidal systems, including biological mattersystems. Studies are focused on the three relevant levels of comprehensionrequested when dealing with colloidal systems: the microscopic level (chemicalphysics of the basic interactions), the mesoscopic level (search for effectivepotentials between colloidal particles tracing out the solvent, predictions of thestructure and dynamics) and the macroscopic level (structural and dynamicalproperties of the compounds, equilibrium and non-ergodic properties). Theactivities are organized around complementary research projects.SOFT Scientific Report 2004-0610