Finite-size effects in systems with diverging characteristic lengthscale have been addressed via state-of-the-art Monte Carlo and molecular dynamics simulations of various models exhibiting solidsolid, liquid-liquid and vapor-liquid transitions. Our simulations, combined with the appropriate application of finite-size scaling theory, confirm various non-trivial singularities in equilibrium dynamic critical phenomena and non-equilibrium domain coarsening phenomena, as predicted by analytical theories. We convincingly demonstrate that the finite-size effects in the domain growth problems, with conserved order parameter dynamics, is weak and universal, irrespective of the transport mechanism. This result is strikingly different from the corresponding effects in critical dynamics. In critical phenomena, difference in finite-size effects between statics and dynamics is also discussed.PACS numbers: 64.70.JaIn computer simulations, finite size of the systems poses enormous difficulty in studying problems where characteristic length scales diverge [1,2]. E.g., in equilibrium critical phenomena [3] the correlation length (ξ) diverges aswhere ǫ = |T − T c |/T c , T c being a critical temperature. On the other hand, when a homogeneous system is quenched inside the miscibility gap, the phase separation progresses via divergence of average domain-size, ℓ(t), as a function of time (t) [3-6] aswhere the exponent α depends upon the transport mechanism. This difficulty can, of course, be overcome via application of finite-size scaling theory [1,2,7]. Nevertheless, it is of immense importance to learn the effects of finite system size, e.g., the study of nucleation and growth in nano-scopic systems, structure and dynamics in ultrathin films, etc., are of great independent interest. Also, an appropriate knowledge of the size effects helps judicial choice of the system size for the direct understanding of the problem in the thermodynamic limit so that any unexpected deviation from a prediction is not inappropriately attributed to the deficiency in system size. While in static critical phenomena such problems are well addressed, the situation appears challenging in dynamics. It is certainly of fundamental importance to make a comparative study of finite-size effects in statics and dynamics. However, there are only a few computational studies [8][9][10][11][12] of dynamic critical phenomena due to the fact that here, in addition to finite-size effects, the critical slowing down brings in another major hurdle. So finite-size effects are not appropriately probed and were thought to be same as in statics. On the other hand, despite a lot of simulation studies over several decades, the finite-size scaling theory in non-equilibrium domain coarsening problems found only rare application [13-15] and the finite-size effects in this type of problems remained a challenging issue.In this letter, in addition to confirming results for various singular behaviors in critical and coarsening phenomena, we address the issue of finite-size effects in th...
We present results from extensive 3-d molecular dynamics (MD) simulations of phase separation kinetics in fluids. A coarse-graining procedure is used to obtain state-of-the-art MD results. We observe an extended period of temporally linear growth in the viscous hydrodynamic regime. The morphological similarity of coarsening in fluids and solids is also quantified. The velocity field is characterized by the presence of monopole-like defects, which yield a generalized Porod tail in the corresponding structure factor. The nonequilibrium evolution of a phase-separating binary mixture, A+B, is a complex nonlinear process [1]. This problem has attracted much research interest both computationally [2] and experimentally [3]. The growth of A-rich and B-rich domains during phase separation is a scaling phenomenon. The two-point equal-time correlation function, C ψψ (r, t), which characterizes the domain morphology and growth, scales as C ψψ (r, t) = g(r/ℓ(t)) [4]. Here, g(x) is a scaling function independent of time. The average domain size ℓ(t) grows with time t asThe growth exponent α depends upon the transport mechanism which drives segregation. For diffusive dynamics, ℓ ∼ t 1/3 , which is referred to as the LifshitzSlyozov (LS) law [1]. The LS behavior is the only growth law expected for phase-separating solid mixtures. However, for fluids and polymers, one expects faster growth at large length scales where hydrodynamic effects are dominant. For d = 3, convective transport yields additional growth regimes [5] withIn Eq. (2), the inertial length ℓ in [≃ η 2 /(ργ), η, ρ and γ being the shear viscosity, density and interfacial tension] marks the crossover from a low-Reynolds-number viscous hydrodynamic regime to an inertial regime. There has been experimental evidence [6] for a crossover from diffusive to viscous growth. However, no experimental observation of an inertial regime has been reported. While recent focus has turned to systems with realistic interactions and boundary conditions [2,3], our understanding of segregation kinetics in bulk fluids remains far from complete. The viscous regime has been observed in numerical studies using the phenomenological Model H [7,8]. Further, both viscous and inertial regimes have been observed in lattice Boltzmann simulations [9,10]. However, molecular dynamics (MD) methods, where hydrodynamics is automatically inbuilt, have rarely been used to study domain growth, primarily due to heavy computational requirements. To the best of our knowledge, the first MD study was by Ma et al. [13] used a similar model to study the crossover from diffusive to viscous dynamics. However, they do not observe linear growth in the post-crossover regime. In all these cases, MD results have been obtained for low-density fluids over very limited time-windows, and conclusions drawn from these should not be taken seriously. In related work, Kabrede and Hentschke [14] found α ≃ 0.5 in MD simulations of gas-liquid phase separation. In this letter, we present results from largescale MD simulations in co...
Deubiquitinases (DUBs) are key regulators of complex cellular processes. HIV-1 Tat is synthesized early after infection and is mainly responsible for enhancing viral production. Here, we report that one of the DUBs, USP7, stabilized the HIV-1 Tat protein through its deubiquitination. Treatment with either a general DUB inhibitor (PR-619) or USP7-specific inhibitor (P5091) resulted in Tat protein degradation. The USP7-specific inhibitor reduced virus production in a latently infected T-lymphocytic cell line J1.1, which produces large amounts of HIV-1 upon stimulation. A potent increase in Tat-mediated HIV-1 production was observed with USP7 in a dose-dependent manner. As expected, deletion of the USP7 gene using the CRISPR-Cas9 method reduced the Tat protein and supported less virus production. Interestingly, the levels of endogenous USP7 increased after HIV-1 infection in human T-cells (MOLT-3) and in mammalian cells transfected with HIV-1 proviral DNA. Thus, HIV-1 Tat is stabilized by the host cell deubiquitinase USP7, leading to enhanced viral production, and HIV-1 in turn up-regulates the USP7 protein level.
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