Abstract
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.
Highlights
Well-characterized Brownian colloidal dispersions with deliberately tuned interactions serve as invaluable modelDedicated to Matthias Ballauff on occasion of his retirementLaboratory, Universitatsstraße 1, 40225 Dusseldorf, Germany 5 Universitat Konstanz, 78457, Konstanz, Germany 6 Universitat Gottingen, Friedrich-Hund-Platz 1, 37077, Gottingen, Germany 7 Department of Chemistry “Ugo Schiff” and CSGI, University of Florence, Sesto Fiorentino, Florence, I-50019, Italy systems on two accounts: they are models of more complex suspensions that are of application interest in their own right and at the foundation of the field of soft-matter physics
The long-time dynamics close to the glass transition provides a notable exception: here, the theoretical description of the microscopic dynamics of the systems differs (Newtonian for metallic melts, Brownian for colloidal particles), the near-equilibrium structural relaxation is equivalent due to Colloid Polym Sci (2020) 298:681–696 the dominance of slow relaxation processes that are driven by local density fluctuations [5, 6]
While mode-coupling theory of the glass transition (MCT) implies that the glass-transition point itself is unchanged by the kinetic parameters of a system, the dynamics in its vicinity might show different dependence on, for example, mobility ratios in mixtures depending on whether one is on the liquid side or the glass side of the MCT transition [12]
Summary
The relevant size ratios are in the range of 0.8 to 1.0, and for binary mixtures of such size ratios, MCT calculations predict that the glass-transition point shifts to slightly lower packing fraction [52] This effect has been found in molecular-dynamics simulations [58] and is not in disagreement with the trend observed in Fig. 1 for the metallic-alloy data. The leading non-trivial spherical-harmonic projections of δg(r) confirm that on distances corresponding to nearestneighbor shells around particles, different local deformation modes prevail (Fig. 4), both in the MD simulation representative of the metallic system, and in experiment on the colloidal hard-sphere-like suspension To obtain the latter data, we have developed a high-precision setup that allows confocal-microscopy imaging of a flowing colloidal suspension combined with accurate localization of the particles through image analysis. The analytical evaluation from the generalized Maxwell model (lines in Fig. 7) confirms this
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