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We present a generalization of inertial coupling (IC) [Balboa Usabiaga et al., J. Comput. Phys. 235, 701 (2013)], which permits the resolution of radiation forces on small particles with arbitrary acoustic contrast factor. The IC method is based on a Eulerian-Lagrangian approach: particles move in continuum space while the fluid equations are solved in a regular mesh (here we use the finite volume method). Thermal fluctuations in the fluid stress, important below the micron scale, are also taken into account following the Landau-Lifshitz fluid description. Each particle is described by a minimal cost resolution which consists of a single small kernel (bell-shaped function) concomitant to the particle. The main role of the particle kernel is to interpolate fluid properties and spread particle forces. Here, we extend the kernel functionality to allow for an arbitrary particle compressibility. The particle-fluid force is obtained from an imposed "no-slip" constraint which enforces similar particle and kernel fluid velocities. This coupling is instantaneous and permits the capture of the fast, nonlinear effects underlying the radiation forces on particles. Acoustic forces arise because of an excess either in particle compressibility (monopolar term) or in mass (dipolar contribution) over the fluid values. Comparison with theoretical expressions shows that the present generalization of the IC method correctly reproduces both contributions. Due to its low computational cost, the present method allows for simulations with many [O(104)] particles using a standard graphical processor unit ; We thank Aleksandar Donev, Pep Español, and Ignacio Pagonabarraga for fruitful discussions and suggestions to broaden the scope of this research. We are quite honored to acknowledge funding from the Spanish government FIS2010- 22047-C0S and from the Comunidad de Madrid MODELICO-CM (S2009/ESP-1691)

The following article appeared in Journal of Chemical Physics 139.21 (2013): 214113 and may be found at http://scitation.aip.org/content/aip/journal/jcp/139/21/10.1063/1.4834696 ; The Stokes-Einstein relation for the self-diffusion coefficient of a spherical particle suspended in an incompressible fluid is an asymptotic result in the limit of large Schmidt number, that is, when momentum diffuses much faster than the particle. When the Schmidt number is moderate, which happens in most particle methods for hydrodynamics, deviations from the Stokes-Einstein prediction are expected. We study these corrections computationally using a recently developed minimally resolved method for coupling particles to an incompressible fluctuating fluid in both two and three dimensions. We find that for moderate Schmidt numbers the diffusion coefficient is reduced relative to the Stokes-Einstein prediction by an amount inversely proportional to the Schmidt number in both two and three dimensions. We find, however, that the Einstein formula is obeyed at all Schmidt numbers, consistent with linear response theory. The mismatch arises because thermal fluctuations affect the drag coefficient for a particle due to the nonlinear nature of the fluid-particle coupling. The numerical data are in good agreement with an approximate self-consistent theory, which can be used to estimate finite-Schmidt number corrections in a variety of methods. Our results indicate that the corrections to the Stokes-Einstein formula come primarily from the fact that the particle itself diffuses together with the momentum. Our study separates effects coming from corrections to no-slip hydrodynamics from those of finite separation of time scales, allowing for a better understanding of widely observed deviations from the Stokes-Einstein prediction in particle methods such as molecular dynamics ; A. Donev was supported in part by the Air Force Office of Scientific Research under Grant No. FA9550- 12-1-0356. R. Delgado-Buscalioni and F. Balboa acknowledge funding from the Spanish Government FIS2010-22047- C05 and from the Comunidad de Madrid MODELICO-CM (S2009/ESP-1691). Collaboration between A. Donev and R. Delgado-Buscalioni was fostered at the Kavli Institute for Theoretical Physics in Santa Barbara, California, and supported in part by the National Science Foundation under Grant No. NSF PHY05-51164

We develop an inertial coupling method for modeling the dynamics of point-like "blob" particles immersed in an incompressible fluid, generalizing previous work for compressible fluids (Balboa Usabiaga et al., 2013). The coupling consistently includes excess (positive or negative) inertia of the particles relative to the displaced fluid, and accounts for thermal fluctuations in the fluid momentum equation. The coupling between the fluid and the blob is based on a no-slip constraint equating the particle velocity with the local average of the fluid velocity, and conserves momentum and energy. We demonstrate that the formulation obeys a fluctuation–dissipation balance, owing to the non-dissipative nature of the no-slip coupling. We develop a spatio-temporal discretization that preserves, as best as possible, these properties of the continuum formulation. In the spatial discretization, the local averaging and spreading operations are accomplished using compact kernels commonly used in immersed boundary methods. We find that the special properties of these kernels allow the blob to provide an effective model of a particle; specifically, the volume, mass, and hydrodynamic properties of the blob are remarkably grid-independent. We develop a second-order semi-implicit temporal integrator that maintains discrete fluctuation–dissipation balance, and is not limited in stability by viscosity. Furthermore, the temporal scheme requires only constant-coefficient Poisson and Helmholtz linear solvers, enabling a very efficient and simple FFT-based implementation on GPUs. We numerically investigate the performance of the method on several standard test problems. In the deterministic setting, we find the blob to be a remarkably robust approximation to a rigid sphere, at both low and high Reynolds numbers. In the stochastic setting, we study in detail the short and long-time behavior of the velocity autocorrelation function and observe agreement with all of the known behavior for rigid sphere immersed in a fluctuating fluid. The proposed inertial coupling method provides a low-cost coarse-grained (minimal resolution) model of particulate flows over a wide range of time-scales ranging from Brownian to convection-driven motion ; A. Donev was supported in part by the Air Force Office of Scientific Research under grant number FA9550-12-1-0356. B. Griffith acknowledges research support from the National Science Foundation un-der awards OCI 1047734 and DMS 1016554. R. Delgado-Buscalioni and F. Balboa acknowledge funding from the Spanish government FIS2010-22047-C05 and from the Comunidad de Madrid MODELICO-CM (S2009/ESP-1691). Collaboration between A. Donev and R. Delgado-Buscalioni was fostered at the Kavli Institute for Theoretical Physics in Santa Barbara, California, and supported in part by the National Science Foundation under Grant No. NSF PHY05-51164

We develop numerical schemes for solving the isothermal compressible and incompressible equations of fluctuating hydrodynamics on a grid with staggered momenta. We develop a second-order accurate spatial discretization of the diffusive, advective, and stochastic fluxes that satisfies a discrete fluctuation-dissipation balance and construct temporal discretizations that are at least second-order accurate in time deterministically and in a weak sense. Specifically, the methods reproduce the correct equilibrium covariances of the fluctuating fields to the third (compressible) and second (incompressible) orders in the time step, as we verify numerically. We apply our techniques to model recent experimental measurements of giant fluctuations in diffusively mixing fluids in a microgravity environment [A. Vailati et al., Nat. Comm., 2 (2011), 290]. Numerical results for the static spectrum of nonequilibrium concentration fluctuations are in excellent agreement between the compressible and incompressible simulations and in good agreement with experimental results for all measured wavenumbers ; These authors' research was supported by the Spanish government FIS2010-22047-C0S and the Comunidad de Madrid MODELICO-CM (S2009/ESP-1691). This author's research was supported by the DOE Applied Mathematics Program of the DOE Office of Advanced Scientific Computing Research under the U.S. Department of Energy under contract DE-AC02-05CH11231. The fourth author's research was supported by the DOE Applied Mathematics Program of the DOE Office of Advanced Scientific Computing Research under the U.S. Department of Energy under contract DE-AC02-05CH11231 and by the National Science Foundation under grant DMS-1115341. The fifth author's research was supported by the DOE Computational Science Graduate Fellowship under grant DE-FG02-97ER25308. This author's research was supported by the National Science Foundation under awards OCI 1047734 and DMS 1016554.

Icosahedral virus capsids are closed shells built up with a hexagonal lattice of proteins, which incorporate pentamers at their fivefold vertices. Human adenovirus particles lose pentamers (pentons) during infection under a variety of physicochemical cues, including mechanical pulling of molecular motors and the viscous drag of the cytoplasm. By combining atomic force microscopy experiments with survival analysis and Markovian transition state theory, we investigate the sequence of adenovirus penton disassembly that reveals the aging of the virus structure. We show evidence that the lifetime of pentons gradually decreases, accompanied by capsid softening as neighboring pentons are lost. This cooperative dismantling process, which involves first-neighbor penton-penton distances of at least 45 nm, leads to a 50% increase in the virus disassembling rate of the virus particle. Theory and experiments fit remarkably well, allowing us to obtain the spontaneous escape rate and the energy barrier of penton disassembly (∼30 kBT). The observed increase in the penton's loss rate reveals long-range structural correlations within the capsid. Our estimations suggest that the mechanical cues arising from the strokes of protein motors carrying the virus to the nucleus could help penton disassembly and warrant the timely delivery of weak-enough capsids for adenovirus infection. ; Agencia Estatal de Investigación and European Regional Development Fund (BFU2016-74868-P and PID2019–104098 GB-I00/AEI/10.13039/501100011033); the Ministerio de Economía y Competitividad of Spain (BFU2013-41249-P and BIO2015-68990-REDT); and the Agencia Estatal CSIC (2019AEP045). The CNB-CSIC is further supported by a Severo Ochoa Excellence grant (SEV 2017-0712) and Regional Madrid Government (PEJ16/MED/TL-0935). P. J.d.P. acknowledges support by grants from the Spanish Ministry of Economy, Industry and Competitiveness projects (FIS2017- 89549-R; "Maria de Maeztu" Program for Units of Excellence in R&D MDM-2014-0377; and FIS2017-90701- REDT) Spanish Ministry of Econonomy, Industry and Competitiveness FIS2017-86007-C3-1-P

Two-dimensional layered perovskites are attracting increasing attention as more robust analogues to the conventional three-dimensional metal-halide perovskites for both light harvesting and light emitting applications. However, the impact of the reduced dimensionality on the optoelectronic properties remains unclear, particularly regarding the spatial dynamics of the excitonic excited state within the two-dimensional plane. Here, we present direct measurements of exciton transport in single-crystalline layered perovskites. Using transient photoluminescence microscopy, we show that excitons undergo an initial fast diffusion through the crystalline plane, followed by a slower subdiffusive regime as excitons get trapped. Interestingly, the early intrinsic diffusivity depends sensitively on the choice of organic spacer. A clear correlation between lattice stiffness and diffusivity is found, suggesting exciton–phonon interactions to be dominant in the spatial dynamics of the excitons in perovskites, consistent with the formation of exciton–polarons. Our findings provide a clear design strategy to optimize exciton transport in these systems ; This work has been supported by the Spanish Ministry of Economy and Competitiveness through The "María de Maeztu" Program for Units of Excellence in R&D (MDM-2014-0377). M.S. acknowledges the financial support of a fellowship from "la Caixa" Foundation (ID 100010434). The fellowship code is LCF/BQ/IN17/11620040. M.S. has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 713673. F.P. acknowledges support from the Spanish Ministry for Science, Innovation, and Universities through the state program (PGC2018-097236-A-I00) and through the Ramón y Cajal program (RYC-2017-23253), as well as the Comunidad de Madrid Talent Program for Experienced Researchers (2016-T1/IND-1209). N.A., M.M. and R. D.B. acknowledges support from the Spanish Ministry of Economy, Industry and Competitiveness through Grant FIS2017-86007-C3-1-P (AEI/FEDER, EU). E.P. acknowledges support from the Spanish Ministry of Economy, Industry and Competitiveness through Grant FIS2016-80434-P (AEI/FEDER, EU), the Ramón y Cajal program (RYC-2011- 09345) and the Comunidad de Madrid through Grant S2018/ NMT-4511 (NMAT2D-CM). S.P. acknowledges financial support by the VILLUM FONDEN via the Centre of Excellence for Dirac Materials (Grant No. 11744)

Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. Here, we study exciton transport in phase pure mixed-halide 2D perovskites of (PEA)2Pb(I1-xBrx)4. Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport. We find a significant reduction even for relatively low alloying concentrations. By performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites ; This work has been supported by the Spanish Ministry of Economy and Competitiveness through the "Mari ́ a de Maeztu" Program for Units of Excellence in R&D (MDM-2014-0377). M.S. acknowledges the financial support through a Doc.Mobility Fellowship from the Swiss National Science Foundation (SNF) with grant number 187676. In addition, M.S. acknowledges the financial support of a fellowship from "la Caixa" Foundation (ID 100010434). The fellowship code is LCF/BQ/IN17/11620040. Further, M.S. has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 713673. F.P. acknowledges support from the Spanish Ministry for Science, Innovation, and Universities through the state program (PGC2018-097236-A-I00) and through the Ramón y Cajal program (RYC-2017-23253), as well as the Comunidad de Madrid Talent Program for Experienced Researchers (2016-T1/IND-1209). M.M., N.C., and R.D.B. acknowledge support from the Spanish Ministry of ...