From Bulk to Nano-Structured LiquidsPhysical processes occurring at the microscopiclength scales are of fundamental importance inorder to understand the physical proprieties of<strong>di</strong>sordered 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 <strong>di</strong>sordered 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 <strong>di</strong>sorder. 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 <strong>di</strong>mension, <strong>di</strong>spersein 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 interme<strong>di</strong>atebetween the solid and the liquid one.Actually, a large variety of the NSDM (e.g. microemulsion,viscous and binary liquids, meso-phases)inclu<strong>di</strong>ng also materials not properly soft (e.g.liquid-filled porous glasses, binary and complexglasses) are identified as soft matter.The <strong>di</strong>sordered 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 <strong>di</strong>sorder. Indeed, they showseveral mo<strong>di</strong>fication of the macroscopic phenomenainduced by the static nano-structure, like phase<strong>di</strong>agrams and thermodynamic proprieties.The supercooled waterBetween the <strong>di</strong>sordered 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 <strong>di</strong>spersion 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 stu<strong>di</strong>es 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 pre<strong>di</strong>ctions of the mode-coupling theory(MCT). Our data substantially support the MCTscenario, provi<strong>di</strong>ng 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 OKEstu<strong>di</strong>es on the water phase <strong>di</strong>agram. Furthermore westu<strong>di</strong>ed 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 a<strong>di</strong>abatic sound velocity and the possibleexistence of a negative <strong>di</strong>spersion (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. <strong>di</strong> Firenze.(b) INFM-CRS-<strong>Soft</strong> Matter(CNR), Univ. la <strong>Sapienza</strong>. (c) Dip. <strong>di</strong> <strong>Fisica</strong>, Univ. <strong>di</strong>Firenze. (d) Dip. <strong>di</strong> Chimica, Univ. <strong>di</strong> Firenze.81SOFT Scientific <strong>Report</strong> 2004-06
Scientific <strong>Report</strong> – Non Equilibrium Dynamics and ComplexityA porous glass (PG) is a usual silicate glass where ispresent a random network of empty pores. There are<strong>di</strong>fferent techniques to produce such porous glassesand the most common is based on sol-gel method.With this techniques it is possible to generate PGcharacterized by <strong>di</strong>fferent porous shape and<strong>di</strong>mension <strong>di</strong>stribution (see figure fig1). PG arecommercially available and they are known as VycorCCl 4, that it has been very well stu<strong>di</strong>ed in its bulkphase. Our first investigation analyses two transportphenomena of the filled PG, in particular the acousticpropagation and the thermal <strong>di</strong>ffusion, and comparethem with the same processes in bulk matter.Indeed the filled PG must be considered as twostrongly interacting materials, here the liquid andglass matrix, so that the transport phenomena could<strong>di</strong>ffer substantially from the bulk one.References[1] A.Taschin, R.Cucini, P.Bartolini and R.Torre.“Acoustic propagation in liquid filled porous glasses,a test of Biot model”. In preparation (2006).AuthorsA.Taschin (a,c), R.Cucini (c), P.Bartolini (a,c) andR.Torre(a,b,c).(a) INFM-CRS-<strong>Soft</strong> (CNR), Univ. la <strong>Sapienza</strong>. (b) Dip.<strong>di</strong> <strong>Fisica</strong>, Univ. <strong>di</strong> Firenze. (c) LENS, Univ. <strong>di</strong> Firenze.Fig.2. In the figure we show a 3D computer modelof the porous glass structure. The empty pores andcavities have typical <strong>di</strong>mension of severalnanometers.Micro-emulsion: Water in OilMicro-emulsion is a colloidal suspension composed ofdrops of water stabilized by surfactant molecules in aoily solvent. Typically they have <strong>di</strong>ameters of fewnano-meters and represent a soft dynamic nanostructure.In this soft matter example there are threecomponents, a solute (water) a solvent (oil) and astructuring agent (surfactant), that play a role indefining the local structure present in the materialand, furthermore, the long range <strong>di</strong>sorder is dynamic.Thus in micro-emulsion the correlation between microand macro proprieties is more articulated than in thePG glasses, where only two interacting componentsare present and the <strong>di</strong>sorder is static. We areperforming a series of time-resolved and frequencyspectroscopic experiments on a very well stu<strong>di</strong>edFig.3. Here we report two experimental resultswith the relative fitting functions. The sample is aporous glass filled with a molecular liquid (CCl 4 inVycor 7030) and the data are obtained by means oftime resolved transient grating.(a trade mark from Corning Glass company): this PGshow a very narrow <strong>di</strong>stribution of porous <strong>di</strong>ametersaround 40 nm. PG can be easily filled with liquid of<strong>di</strong>fferent nature and it represents a very useful solidmatrix to produce a static and rigid nano-structuringof the liquid. The porous glass filled with a liquid isindeed an interesting nano-structured <strong>di</strong>sorderedmaterial, not only as a remarkable model for thebasic research on interaction at the nano scale butalso as effective example of many material presentin nature and in technological creation. We startfilling the Vycor glass with a simple molecular liquid,Fig. 4. A computer simulation of a inverse micellecharacterizing the AOT microemulsion. Only thesurfactant and water molecules are shown.micro-emulsion: the AOT-water-decane microemulsion,being AOT the surfactant and decane theoil. We focus on the investigation of acousticpropagation in this system as a function oftemperature, in order to measure and characterizethe interplay between the colloidal phase/structuremo<strong>di</strong>fications and sound propagation. In particular weinvestigated the effect of the “percolation line”present in the phase <strong>di</strong>agram of this microemulsion(i.e. a supposed transition between two <strong>di</strong>fferentstates of dynamic aggregation), on the soundvelocity. Our preliminary results shown a weak effecton the elastic modulus of the <strong>di</strong>spersed phase (AOT-Water).SOFT Scientific <strong>Report</strong> 2004-0682
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