Séminaire SIMM : Greg McKenna (Texas Tech University, Lubbock, TX)

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28 mai 2020 16:00 » 17:00

Colloids as Glasses : How well does the paradigm hold ?

Gregory B. McKenna
Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409-3121 USA

It is common in the area of colloidal systems to make the analogy between high concentration and the glass transition that occurs at low temperatures in molecular systems. A major short-coming of this perspective is the lack of experimental methods to directly interrogate the dynamics over the same large range of viscosities or relaxation times, viz., approximately 17 decades in molecular systems and 3-5 in most studies of colloidal systems. Therefore, alternative methods of establishing the validity of the colloidal glass transition and its applicability to the understanding of molecular glasses is required. To this end we have developed a novel concentration-jump methodology to investigate the aging or structural recovery of colloids in the so-called "Kovacs signatures" that are observed in temperature-jump situations in molecular glasses. Our results elucidate several aspects of the behavior of the colloidal systems. First, we have been able to show that the Kovacs signatures of "intrinsic isotherm-like" response, asymmetry of approach, and memory do not fully replicate the behavior of the molecular glasses. For example, the intrinsic isotherm (intrinsic volume fraction) experiments in molecular glasses show rapidly diverging behavior for the time to reach equilibrium teq, something not observed in the colloidal systems. Furthermore, the asymmetry of approach in colloidal systems is very weak, suggesting little to no dependence of the dynamics on the glassy structure itself, unlike molecular glasses where the structure dependence is very strong. The second point of the work has been the examination of equilibrium dynamics as a function of concentration using diffusing wave light scattering spectroscopy (DWS). Here we establish that for soft colloidal systems the dynamics deviate from the nominal VFT-type of dynamics and reach a plateau rather than diverging as high concentrations are reached. Such responses are contrary to expected true glassy behavior. Finally, there is a developing body of work suggesting that colloidal glasses as well as some molecular (metallic) glasses can exhibit compressed exponential relaxation behavior for the autocorrelation function g2(t) in light scattering and x-ray photon correlation spectroscopy (XPCS) measurements. Such results have been interpreted to imply a ballistic regime of behavior at long times that gives rise to super-diffusive dynamics for the mean-square-displacement (MSD). We have found in our DWS measurements that the determination of the MSD itself from a compressed exponential response of g2(t) is not necessarily due to a super-diffusive behavior, but is rather a result of the specific scattering geometry that needs to be interpreted with caution. These results and others will be described and their meaning for the glassy colloid paradigm addressed.

ID de réunion : 491 788 8199
Mot de passe : ASSIMM

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