Fritschi, Sebastian

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Rheology of colloidal and metallic glass formers

2020-07, Voigtmann, Thomas, Siebenbürger, Miriam, Amann, Christian P., Egelhaaf, Stefan U., Fritschi, Sebastian, Krüger, Matthias, Laurati, Marco, Mutch, Kevin J., Samwer, Konrad H.

Colloidal hard-sphere suspensions are convenient experimental models to understand soft matter, and also by analogy the structural-relaxation behavior of atomic or small-molecular fluids. We discuss this analogy for the flow and deformation behavior close to the glass transition. Based on a mapping of temperature to effective hard-sphere packing, the stress–strain curves of typical bulk metallic glass formers can be quantitatively compared with those of hard-sphere suspensions. Experiments on colloids give access to the microscopic structure under deformation on a single-particle level, providing insight into the yielding mechanisms that are likely also relevant for metallic glasses. We discuss the influence of higher-order angular signals in connection with non-affine particle rearrangements close to yielding. The results are qualitatively explained on the basis of the mode-coupling theory. We further illustrate the analogy of pre-strain dependence of the linear-elastic moduli using data on PS-PNiPAM suspensions.

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Stress-strain relations in bulk metallic glasses and colloidal dispersions

2013, Amann, Christian P., Ballauff, Matthias, Egelhaaf, Stefan U., Fritschi, Sebastian, Krüger, Matthias, Fuchs, Matthias, Laurati, Marco, Mutch, Kevin J., Samwer, Konrad, Siebenbürger, Miriam, Voigtmann, Thomas, Weysser, Fabian

A comparison is made between the nonlinear rheological response of bulk metallic glass formers and of colloidal dispersions. Stress-strain curves measured after switch-on of constant deformation rates are analyzed quantitatively using a schematic model of mode coupling theory generalized to homogeneous and incompressible flows. A mapping between metallic and dispersion rheology is possible when stresses are rescaled by an entropic scale, accumulated strains by geometrical factors, and rates by the intrinsic relaxation time. Exploiting this similarity and the possibility to directly observe individual colloidal particles, we investigate the structural distortions in the colloidal system using confocal microscopy. The distortions exhibit the (from elasticity theory) expected quadrupolar but also a strong isotropic component.