Nanofluids: A New Class of Materials Produced from Nanoparticle Assemblies** By Ramesh Jagannathan*and Glen C. Irvin, Jr. 1. Introduction Withinthelasttwodecades,thescienceofmolecularclusters hascapturedtheseriousinterestofacademicgroupsand,more recently, some technologists. [1] The interest in inorganic clus- tersismainlyduetotheirsize-dependentproperties,forexam- ple,opticalandelectrical.Oneofthegoalsforclusterresearch is to be able to custom-build solids that use these clusters as basic building blocks (like atoms), in which they retain their special properties or collectively exhibit new phenomena. This capability would, in principle, lead to the creation of a wide rangeofnewmaterialsintheoptical,electronic,andbiological fields,whichwouldnotbepossibleotherwise.Todate,mostof the emphasis has been on clusters of inorganic materials, for example, silicon, [2] silver, [3] etc. Molecular clusters of organic materials have significance in biological [4,5] systems. Clathrate hydratesarehighlyordered,vanderWaals'-stabilizedmolecu- larclusters. [6] Thepropertiesofmolecularclustersappeartobe intimately linked to the number of molecules in the clusters (effective size) as well as the geometrical configuration (molecular orientation and intermolecular/interatomic dis- tances)ofsuchmolecules. [7±10] Herein, we report the discovery of a novel material built usingorganicmolecularclustersthatdisplayuniquephaseand optical properties. When produced using certain experimental and formulation conditions, these organic molecular clusters resultinnovelfluids,whichwehavetermednanofluids.Nano- fluids of this type are constructed from a mixture of organic molecular clusters and surfactant molecules in the absence of any apparent chemical reaction and liquid solvent. Concep- tually,theyarenottraditionaldispersions,andnosolventswere used. We define a nanofluid as a collection of molecular clus- ters.Inourobservations,theyareclear,coloredthinfilmsthat coattheglasswallsofthevialsthatwereusedtocollectthem. Theapparentviscosityofthefilmseemstodependonthetype ofsurfactantused.InthesurfactantFomblin,ithadtheappar- ent viscosity of ªnatural honeyº at room temperature and floweddownthewallsofthevialundertheinfluenceofgravity when tilted. The same behavior was seen with the surfactant Fluorolink, but after the vial was heated to 40C. Pictures ofthenanofluidareshowninFigure1.Weshowapictureofa Fluorolink-based nanofluid in a vial, held upside down after heating to 40C. The digital still image shows the nano- fluid flowing down the walls of the vial under its own weight. We also show images of the dye and surfactant powders usedtocreatethenanofluid.Typically,theamountofnanofluid Adv. Funct. Mater. 2005, 15, 1501±1510 DOI: 10.1002/adfm.200400474  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1501 ± [*] Dr. R. Jagannathan,Dr. G. C. Irvin Eastman Kodak Company Rochester, NY 14652-3701 (USA) E-mail: ramesh.jagannathan@kodak.com [**] We acknowledge Dr. David Tuschel for the optical and Raman charac- terizations of the nanofluid, and Dr. Thomas Penner for reviewing the manuscript. We present evidence of a novel nanostructured fluid, a nanofluid, composed of molecular clusters of a polar organic dye and surfactant. These are not nanoparticles dispersed in a solvent; there are no solvent molecules present. These materials, which aresolidsunderambientconditions,arenon-reactivelyprecipitatedfromacompressedCO 2 solution,resultinginaliquid-like material,whichwecallananofluid.Theprecipitateddye±surfactantclustersare1±4nminsize.Thisnanofluidexhibitsintense luminescentsignatures,whicharesignificantlyblue-shiftedwithrespecttothedyepowderorasolutionofit.TheX-raydiffrac- tion pattern did not show any structure in the low-angle regime. The fluorinated surfactant is highly soluble in compressed CO 2 .ThepolardyedoesnotdissolveincompressedCO 2 butissolubilizedbyelectrostaticinteractionswiththesurfactanthead groups. We believe that the ultrafast and controlled precipitation from compressed CO 2 preserves the electrostatic coupling and promotes a structured molecular cluster. Additionally, we demonstrate the formation of organic nanoparticles using this controlledprecipitationprocessfromcompressedCO 2 . Figure 1. Digital image of the Flurolink surfactant powder, Duasyn Acid Blue AE2 dye powder, and the liquid-like nanofluid. FULL PAPER