Here - Institute for Building Materials - ETH Zürich

Intermittent dynamics of confined fluids, first passage statistics and diffusionprocess in colloidal and porous media.P. Levitz 11 PECSA, University Pierre et Marie Curie- CNRS, Paris FranceINTRODUCTION: Porous materials,concentrated colloidal suspensions are example ofconfining systems developing large specificsurface, presenting a rich variety of shapes andexhibiting complex and irregular morphologies ona large length scale. Such a confinement stronglyinfluences the molecular dynamics of embeddedfluids and the diffusive motion of particlesentrapped inside these materials. The particletrajectory can be described as an alternatesuccession of surface adsorption steps andconfining bulk relocations. The full transportprocess appears as an intermittent dynamics. Thisquestion related to a first passage problem isdiscussed in this presentationSPECTRAL ANALYSIS OF THEINTERMITTENT DYNAMICS: The timedependence of the intermittent dynamics of aconfined fluid near a pore interface can beanalysed using the density probability distribution(p.d.f.) of the time either spend in the bulk before areadsorption on the surface or characterizing theway that an adsorbed molecules is released in thebulk [1,2]. These two p.d.f. reflect two firstpassage statistics, one for the surface adsorptionthe other for the bulk relocation. A spectralanalysis of this dynamics is discussed for severalinterfacial geometries [1-3]. Direct comparisonswith recent molecular dynamics simulationsperformed for confined liquid water inside variouspore shapes are discussed [4].PROBING INTERMITTENT DYNAMICS:The Nuclear Magnetic Relaxation Dispersiontechnique (NMRD) is an effective experimentalmethod to follow an intermittent dynamics near aninterface. Under some general conditions ofrelaxation [1]; the related spin-lattice relaxationrate R 1 (ω) provides a direct evaluation of thespectral density of the intermittent dynamics. Inthis part, we will present several experimentalstudies of the water intermittent dynamics, usingNMRD. Various colloidal and/or porous interfaceswill be dicussed (plaster pastes [5], flat colloidalclay particles [1], long and stiff nanometric strands[3]) with a special emphasis on the fluid-surfaceinteraction ( “nano-wettability” [5]).DISCUSSION & CONCLUSIONS: The formeranalysis of the intermittent dynamics provides asimple description of the local exploration of aninterfacial medium by a particle and/or a molecule.This coarse grained description allows to quantifythe influence of the “local” confining geometryand also to estimate the degree of interactionduring the adsorption step. On larger length scale,real interfacial media such as pore networks orconcentrated colloidal suspensions generallyexhibit a complex interfacial geometry. Atraditional way to follow the geometrical upscalingis to look first at the pore scale, second, at a porenetwork, etc… Iteration of the intermittent processat these different levels of organization can bepursued using First Passage Time statistics.Finally, another important aspect of theintermittent dynamics is related to its efficiency tooptimize the research of a surface target [6], whichputs forward a general mechanism of enhancementand regulation of chemical reactivity. We willemphasis these two topics and comment somepreliminary results.REFERENCES:1 P. Levitz; 2005; J. Phys.Condens. Matt. 17: S4059 2 P. Levitz, D. S.Grebenkov, M. Zinsmeister, K. Kolwankar; B.Sapoval, 2006; Phys. Rev. Lett. 96: 180601 3 P.Levitz, M. Zinsmeister, P. Davidson, D.Constantin, and O. Poncelet; 2008; Phys. Rev. E78: 030102 (R) 4 P. A. Bonnaud, B. Coasne and R.J.-M. Pellenq, J. 2010, Phys. Condens.Mat., 22:284110 5 J-P Korb and P Levitz;2008; Magnetic Resonance in Porous Media. 1081;55-58 6 O. Bénichou, D. Grebenkov, P. Levitz, C.Loverdo and R. Voituriez; 2010; PRL 105:150606ACKNOWLEDGEMENTS: This work wasperformed in several collaborations with M.Zinsmeister, J-P Korb, D. Grebenkov, O.Benichou, M Han, M. Fleury, P. Bonnaud, P.-A.Cazade, R. J.-M. Pellenq 2,3 , and B. Coasne. Manythanks to all of them.20