Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Non Equilibrium Dynamics and ComplexityColloidal Suspensions Under ShearResearch activities in the field of colloidalsuspensions have recently exhibited rapid growth,for their relevance in both technical applications andfundamental science. On one hand, they areessential in many industrial application ranging frompaints to lubrificants, or in pharmaceuticalapplications such as drug delivery. On the otherhand, they play a preminent role as model systemsfor the principles of phase transitions in condensedmatter. One of the most peculiar behavoir of softcolloidal matter is the strong sensitivity of its flowproperties to external deformation. A systematicscientific comprehension about this topic is stillimmature.In the last few years we investigated the physicalmechanism governing the interaction between agingdynamics and shear flow in a Laponite suspension,which represents a prototype of a soft glassymeterial [2]. Dynamic light scattering has been usedto probe the relaxation of the intermediate scatteringfunction of the aging sample and the effect of shearon the non-equilibrium structural dynamics has beeninvestigated during various protocols of appliedshear. The shear flow influences significantly theaging dynamics as soon as structural relaxationenters the timescale set by the inverse shear rate(Fig. 1). Aging is strongly reduced in this sheardominated region, while for a fixed waiting time theaverage structural relaxation scales as the inverseshear rate.Fig. 2: Velocity profile of the Laponite sample putunder shear in a cone-plate cell, as a function ofthe distance along the gap of width H. Shearlocalization shows up next to the rotating cone wall.Shear rejuvenation of gelled samples has also beeninvestigated and we observed a substantiallydifferent aging regime characterized by a fasterrelaxation dynamics. In particular, the slowrelaxation time shows a power law dependence onthe waiting time after shear cessation.Another aspect of the soft glassy sample weinvestigated is the shear banding phenomena,characterizing yield stress fluids at low applied shearrates. Shear localization [4] has been observed inthe Laponite sample through heterodyne dynamiclight scattering technique (Fig. 2). The velocityprofile has been studied as a function of both thewaiting time and the applied shear rate. Theevolution of both the flow curve and the yield stressvalue is reflected locally in the evolution of the shearbanding velocity profile as the system ages. Finally,a periodic oscillation between two different velocityprofiles has also been observed, showing a stick-slipbehavior in the flow.Fig. 1 Slow relaxation time as a function of waitingtime during aging under different shear rates. Frombottom to top the shear rate value is: 446, 223, 67,22 s -1 . Solid symbols refer to aging without shear.Arrows in top frame indicate the inverse shear ratevalue corresponding to each curve. Deviation fromthe aging evolution is evident as soon as the slowrelaxation time reaches the timescale of the inverseshear rate.References[1] D. Bonn, S. Tanase, B. Abou, H. Tanaka, J.Meunier, Phys. Rev. Lett., 89, 015701 (2002).[2] P. Sollich, F. Lequeux, P. Hèbraud, M.E. Cates,Phys. Rev. Lett., 78, 2020 (1997).[3] R. Di Leonardo, F. Ianni, G. Ruocco, Phys. Rev.E, 71, 011505 (2005)[4] F. Varnik, L. Bocquet, J.-L. Barrat, L. Berthier,Phys. Rev. Lett. 90, 095702 (2003).AuthorsR. Di Leonardo, S. Gentilini, F. Ianni and G. Ruocco.SOFT CNR-INFM – Dipartimento di Fisica Universita’Roma ‘‘La Sapienza’’ – 00185 Roma – ItalySOFT Scientific Report 2004-0668