Arrested dynamics of the dipolar hard sphere model

Abstract

We report the combined results of molecular dynamics simulations and theoretical calculations concerning various dynamical arrest transitions in a model system representing a dipolar fluid, namely, N (soft core) rigid spheres interacting through a truncated dipole-dipole potential. By exploring different regimes of concentration and temperature, we find three distinct scenarios for the slowing down of the dynamics of the translational and orientational degrees of freedom: At low (η = 0.2) and intermediate (η = 0.4) volume fractions, both dynamics are strongly coupled and become simultaneously arrested upon cooling. At high concentrations (η ≥ 0.6), the translational dynamics shows the features of an ordinary glass transition, either by compressing or cooling down the system, but with the orientations remaining ergodic, thus indicating the existence of partially arrested states. In this density regime, but at lower temperatures, the relaxation of the orientational dynamics also freezes. The physical scenario provided by the simulations is discussed and compared against results obtained with the self-consistent generalized Langevin equation theory, and both provide a consistent description of the dynamical arrest transitions in the system. Our results are summarized in an arrested states diagram which qualitatively organizes the simulation data and provides a generic picture of the glass transitions of a dipolar fluid. PACS numbers: xxxxxxxx yyyyyyyy

“Inner clocks” of glass-forming liquids

“…( 28), this condition can be fulfilled if either both b T (t) and bR(t) vanish asymptotically or, for l = 0, only b T (t) vanishes since β lm (k) ∝ l vanishes in this case. These two possibilities, in fact, correspond to totally or partially arrested states of matter as predicted by the NE-SCGLE and observed in extensive molecular dynamics simulations: 65 in the fully arrested state, both mobilities vanish, while in mixed-glass states, rotational degrees of freedom equilibrate, and thus, bR(t) never vanishes.…”

“…( 27) and has been systematically employed 62 to understand the dynamically arrested states already detected in extensive molecular dynamics (MD) simulations of this model. 65,66 We summarize the relevant ingredients of the non-spherical version of the NE-SCGLE theory in the Appendix.…”

“…Similar to the spherical case discussed previously, the most central equation of the extended NE-SCGLE describes the timeevolution of the non-equilibrium structure factor, which for liquids involving non-spherical interactions can be defined as S(k, μ; k ′ , μ ′ ; t) ≡ ⟨δn(k, μ; t) δn * (k ′ , μ ′ ; t)⟩. Here, δn(k, μ; t) is the Fourier transform of the fluctuation in the local number density n(r, μ; t) ≡ N −1/2 ∑ N a=1 δ(r − ra(t))δ(μ − μa(t)) of particles at the position r and with orientation μ at time t. 67 In practice, however, one focuses on the coefficients of the spherical harmonic expansion of n(k, μ; t), 63,[65][66][67][68] commonly referred to as tensorial density modes and denoted as nlm (k, t) [see Eq. (A1)].…”

“…(A1)]. These coefficients, in turn, allow us to define the corresponding projections of S(k, μ; k ′ , μ ′ ; t), namely, 65,66…”

How chains and rings affect the dynamic magnetic susceptibility of a highly clustered ferrofluid

“…This study was confined to very low concentration ferrofluids, so that the distinct contributions from different clusters could be clearly discerned. An obvious extension is to study more concentrated systems with strong interactions, where gel-like structures are to be anticipated [46][47][48][49]. On the one hand, χ(ω) may still show the high-frequency features arising from particle motions within self-assembled structures.…”