Soft Report - Dipartimento di Fisica - Sapienza

From Bulk to Nano-Structured LiquidsPhysical processes occurring at the microscopiclength scales are of fundamental importance inorder to understand the physical proprieties ofdisordered systems, either liquid or solid. Themacroscopic structural and transport phenomena,characterized by long lengths and slow dynamics,are intrinsically correlated with the physical eventstaking place at the microscopic scale, occurring atshort length and on fast time scale. The interplayand correlation between the micro (10 -6 m), or evennano (10 -9 m), phenomena and the macroscopic onehas been, and still it is, the basic problem in thedefinition of the disordered phase. This old andfundamental physical problem is rising recently newinterest for its relevance in the science andtechnology of Nano-Structured Disordered Matter(NSDM). We include in this name all theheterogeneous materials where a nano-structure ispresent but there is a long range disorder. In factany study or characterization of a NSDM undergoesto the previous outlined question: how this nanostructureaffects the macroscopic proprieties of thematter?A typical example of a NSDM are the colloidalsuspensions: these are composed by colloidalparticles, typically of sub-micro dimension, dispersein a solvent liquid. Each colloidal particle defines alocal nano-structure in the solvent liquid. OftenNSDM presents very peculiar proprieties, notpresent in standard bulk matters. For example,colloids show mechanical proprieties intermediatebetween the solid and the liquid one.Actually, a large variety of the NSDM (e.g. microemulsion,viscous and binary liquids, meso-phases)including also materials not properly soft (e.g.liquid-filled porous glasses, binary and complexglasses) are identified as soft matter.The disordered system group, present at LENS(DSG@LENS), during this year pursuits the researchon supercooled water and glass-formers, alreadyoutlined in the previous reports, in order to clarifythe open questions. Furthermore, DSG starts a newexperimental study of some NSDM by means oftime-resolved spectroscopy. In particular we selectthree classes of systems that are appropriate foroptical spectroscopy: liquid-filled porous glasses,micro-emulsions, and mixture of molecular liquidspresenting unexpected mesophases. All thesematerials present a definite local structure,characterized by a length scale of few nano-meters,and a long range disorder. Indeed, they showseveral modification of the macroscopic phenomenainduced by the static nano-structure, like phasediagrams and thermodynamic proprieties.The supercooled waterBetween the disordered materials, the supercooledand glassy water represents a very special case.Indeed, water is characterized by the anomalousbehavior of a number of physical properties. Theseanomalies become more and more marked in thesupercooled phase of water resulting in theunexpected temperature dependence of severaldynamic, transport and thermodynamic properties.This remarkable phenomenon is the sign of a strongLiquid-filled Porous Glassesc s [Km/s]1.601.551.501.451.401.351.301.251.20-30 -20 -10-40 -20 0 20 40 60 80 100Temperature [°C]Fig. 1. Here we report the sound velocity of waterobtained from the transient grating experiments (•)with the other data available in the literature. Inthe inset we report a magnification of the lowtemperature data, where it appears clearly theunexpected dispersion effect for ultrasonic data.variation in relevant features of the water structureand dynamics occurring at temperatures below themelting point. Despite the effort of researchers toelucidate this issue, many of its aspects still are tobe clarified. We have undertaken a series ofexperimental studies of supercooled water by meansof time-resolved non-linear spectroscopic techniques.With the ultra-fast optical Kerr effect spectroscopy(OKE) we have been able to measure the structuralrelaxation proprieties in an unprecedented timeinterval and data quality. We compared our data withthe main predictions of the mode-coupling theory(MCT). Our data substantially support the MCTscenario, providing unambiguous evidence thatweakly supercooled water can be described by a fullydynamic model successfully, with no need for athermodynamic origin. In order to complete thisresearch we are attempting to extend the OKEstudies on the water phase diagram. Furthermore westudied the acoustic and thermal phenomena in thesupercooled water dynamics by means of timeresolvedtransient grating experiments. In particular,acoustic phenomena in supercooled water showsseveral anomalous behaviours, as a steep decreaseof the adiabatic sound velocity and the possibleexistence of a negative dispersion (sound velocitywould decrease with increasing frequency) see fig. 1.References[1] R.Torre, P.Bartolini and R.Righini, Nature 248,296 (2004).[2] A. Taschin, P. Bartolini, M. Ricci and R. Torre,Philos. Mag. 84, 1471 (2004).[3] A.Taschin, P.Bartolini, R.Eramo and R.Torre.“Supercooled water relaxation dynamics byheterodyne transient grating experiment”, Phys.RevE, submitted.AuthorsP.Bartolini (a,b), R.Eramo (a,b), R.Righini (a,d) andR.Torre (a,b,c).(a) LENS, Univ. di Firenze.(b) INFM-CRS-Soft Matter(CNR), Univ. la Sapienza. (c) Dip. di Fisica, Univ. diFirenze. (d) Dip. di Chimica, Univ. di Firenze.81SOFT Scientific Report 2004-06