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Microscopic Theory of the Influence of Strong Attractive Forces on the Activated Dynamics of Dense Glass and Gel Forming Fluids

We theoretically study the non-monotonic (re-entrant) activated dynamics associated with a repulsive glass to fluid to attractive glass transition in high density particle suspensions interacting via strong short range attractive forces. The classic theoretical projection approximation that replaces all microscopic forces by a single effective force determined solely by equilibrium pair correlations is revisited based on the projectionless dynamic theory (PDT) that avoids force projection. A hybrid-PDT is formulated that explicitly quantifies how attractive forces induce dynamical constraints, while singular hard core interactions are treated based on the projection approach. Both the effects of interference between repulsive and attractive forces, and structural changes due to attraction-induced bond formation that competes with caging, are included. Combined with the microscopic Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of activated relaxation, the resultant approach appears to properly capture both the re-entrant dynamic crossover behavior and the strong non-monotonic variation of the activated structural relaxation time with attraction strength and range at very high volume fractions. Qualitative differences with ECNLE theory-based results that adopt the full projection approximation are identified, and testable predictions made. The new formulation appears qualitatively consistent with multiple experimental and simulation studies, and provides a new perspective for the overall problem that is rooted in activated motion and interference between repulsive and attractive forces. This is conceptually distinct from empirical shifting or other ad hoc modifications of ideal mode coupling theory which do not take into account activated dynamics. Implications for thermal glass forming liquids are briefly discussed.