Microscopic theory of the elastic shear modulus and length-scale-dependent dynamic re-entrancy phenomena in very dense sticky particle fluids

Abstract

We apply the hybrid projectionless dynamic theory (hybrid PDT) formulation of the elastically collective nonlinear Langevin equation (ECNLE) activated dynamics approach to study dense fluids of sticky spheres interacting with short range attractions. Of special interest is the problem of non-monotonic evolution with short range attraction strength of the elastic modulus (“re-entrancy”) at very high packing fractions far beyond the ideal mode coupling theory (MCT) nonergodicity boundary. The dynamic force constraints explicitly treat the bare attractive forces that drive transient physical bond formation, while a projection approximation is employed for the singular hard-sphere potential. The resultant interference between repulsive and attractive forces contribution to the dynamic vertex results in the prediction of localization length and elastic modulus re-entrancy, qualitatively consistent with experiments. The non-monotonic evolution of the structural (alpha) relaxation time predicted by the ECNLE theory with the hybrid PDT approach is explored in depth as a function of packing fraction, attraction strength, and attraction range. Isochronal dynamic arrest boundaries based on activated relaxation display the classic non-monotonic glass melting form. Comparisons of these results with the corresponding predictions of ideal MCT, and also the ECNLE and NLE activated theories based on projection, reveal large qualitative differences. The consequences of stochastic trajectory fluctuations on intra-cage single particle dynamics with variable strength of attractions are also studied. Large dynamical heterogeneity effects in attractive glasses are properly captured. These include a rapidly increasing amplitude of the non-Gaussian parameter with packing fraction and a non-monotonic evolution with attraction strength, in qualitative accord with recent simulations. Extension of the microscopic theoretical approach to treat double yielding in attractive glass nonlinear rheology is possible.