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Polymer brushes are being increasingly used to tailor surface physicochemistry for diverse applications such as wetting, adhesion of biological objects, implantable devices and much more. Here we perform Dissipative Particle Dynamics simulations to study the behaviour of dense polymer brushes under flow in a slit-pore channel. We discover that the system displays flow inversion at the brush interface for several disconnected ranges of the imposed flow. We associate such phenomenon to collective polymer dynamics: a wave propagating on the brush surface. The relation between the wavelength, the amplitude and the propagation speed of the flow-generated wave is consistent with the solution of the Stokes equations when an imposed traveling wave is assumed as the boundary condition (the famous Taylor's swimmer).

Introduzione: Lo studio dell’empowerment individuale e gruppale ha motivato la strutturazione di uno strumento di valutazione delle aree di intervento lungo le dimensioni (self-efficacy, partecipazione/coinvolgimento, progresso/autonomia). Lo strumento è stato somministrato ad un gruppo di 23 soggetti affetti da disturbo schizofrenico denominato “Albatros” costituito presso la U.O. di Psichiatria dell’A.U.O.P. di Palermo e fruenti di percorsi terapeutici riabilitativi multidisciplinari, confrontato con un gruppo di controllo di soggetti psicotici in esclusivo trattamento farmacoterapico . Materiali e metodi: Lo strumento permette la valutazione dei livelli di empowerment raggiunti tramite l’utilizzo di schede di ricognizione compilate da operatori esterni al gruppo. Step del processo: 1) identificazione e articolazione delle dimensioni da analizzare; 2) elaborazione delle schede di ricognizione da parte dell’equipe multiprofessionale; 3) raccolta dati; 4) valutazione dell’efficacia dello strumento in un intervallo di tempo semestrale in tre rilevazioni. Conclusioni: I dati ad oggi raccolti ed elaborati raffrontanti gli utenti del gruppo con i controlli, lungo le dimensioni analizzate, permettono il monitoraggio e la gestione dei punti di forza e di debolezza dei percorsi riabilitativi. Le risultanze assumono una valenza pro-attiva per la modulazione degli interventi e dei progetti, mentre la progettazione e l’uso di uno specifico strumento valutativo favorisce l’identità del gruppo di lavoro limitando i rischi di autoreferenzialità.

A detailed simulation of the packing behaviour of deformable particles settles the debate about whether soft matter can adopt an unconventional crystal structure at high densities -- it can. The hunt is now on for a real-world example.

In order to predict the equilibrium phase behaviour of colloidal particles, one should first identify the “candidate” structures in which the particles may assemble. Subsequently, the free energy of the candidate structures should be determined to establish the thermodynamically stable phases and to map out the bulk phase diagram. Here, we describe a simple method based on a simulated annealing approach to predict candidate structures and several techniques to calculate the free energy of a thermodynamic system and to map out the phase diagram. Exemplarily, we present phase diagrams of several shape-anisotropic hard particles, e.g., hard dumbbells, hard bowl-shaped particles, and hard oblate spherocylinders.

Experiments on gels have provided contradictory results concerning the relation between correlation and response functions during aging. To clarify this puzzle, we numerically investigate the fluctuation-dissipation plot in equilibrium polymers and in network forming gels employing two distinct observables, (i) the density Fourier transform and (ii) the single-particle potential energy, to probe (i) diffusional processes and (ii) the development of a bond network. The plot behaves very differently for the two cases. Violation from the equilibrium behavior is found only for the second observable. The experimental implications of the discovered sensitivity to the choice of the probe are discussed, in particular, with respect to the existing experimental results.

We evaluate the phase diagram of the Kern–Frenkel patchy model with four interaction sites for four different values of the radial interaction range, all in the single-bond-per-patch regime, keeping the area of the interaction patches fixed. Four stable crystal phases are investigated, namely diamond cubic (DC), bcc, fcc, and plastic fcc. The DC is favored at low temperatures and pressures, while the bcc is favored at low temperatures and intermediate to high pressures. At low temperatures and very high pressures an ordered fcc phase is found, while—as expected—at high temperatures, the only stable crystal is a plastic fcc phase. We find a rich phase diagram with several re-entrant coexistence lines, which can be brought in the equilibrium phase diagram by a proper choice of the range. We also show that the gas-liquid phase separation becomes metastable as the range narrows, and it takes place in a region of the phase diagram where the low density diamond crystal is the thermodynamically stable phase.

We report simulations of a simple model of particles with limited valence in two dimensions and compare the numerical results with recent predictions based on the application of the Wertheim theory. The predictions for the fraction of formed bonds are rather accurate, except at low densities and temperatures, where enhanced bonding is found. Such differences are traced-back to the break-down of the approximation of absence of intra-cluster bonding. The enhanced bonding is thus attributed to the growing entropic cost of merging different clusters when the density is low, compared to the free-energy gain of forming an intra-cluster bond. The presence of closed bond loops in finite size clusters affects the location of the percolation locus, which is located at temperature lower than expected on the basis of the Flory–Stockmayer theory. Similarly, the critical region is shifted to temperatures smaller than the ones accessible with present time numerical resources, despite the implementation of efficient cluster moves. Only a weak evidence of a low-T gas–liquid phase separation between a very dilute gas phase and a low-density percolating liquid phase is found.

We report on a computer simulation and integral equation study of a simple model of patchy spheres, each of whose surfaces is decorated with two opposite attractive caps, as a function of the fraction ? of covered attractive surface. The simple model explored—the two-patch Kern–Frenkel model—interpolates between a square-well and a hard-sphere potential on changing the coverage ?. We show that integral equation theory provides quantitative predictions in the entire explored region of temperatures and densities from the square-well limit ? = 1.0 down to ? ? 0.6. For smaller ?, good numerical convergence of the equations is achieved only at temperatures larger than the gas-liquid critical point, where integral equation theory provides a complete description of the angular dependence. These results are contrasted with those for the one-patch case. We investigate the remaining region of coverage via numerical simulation and show how the gas-liquid critical point moves to smaller densities and temperatures on decreasing ?. Below ? ? 0.3, crystallization prevents the possibility of observing the evolution of the line of critical points, providing the angular analog of the disappearance of the liquid as an equilibrium phase on decreasing the range for spherical potentials. Finally, we show that the stable ordered phase evolves on decreasing ? from a three-dimensional crystal of interconnected planes to a two-dimensional independent-planes structure to a one-dimensional fluid of chains when the one-bond-per-patch limit is eventually reached.

Nanoparticles tethered with DNA strands are promising building blocks for bottom-up nanotechnology, and a theoretical understanding is important for future development. Here we build on approaches developed in polymer physics to provide theoretical descriptions for the equilibrium clustering and dynamics, as well as the self-assembly kinetics of DNA-linked nanoparticles. Striking agreement is observed between the theory and molecular modeling of DNA-tethered nanoparticles.

We analyze a set of 10 M-step molecular dynamics (MD) data of low-temperature SPC/E model water with a phenomenological analytical model. The motivation is twofold: to extract various k-dependent physical parameters associated with the single-particle or the self-intermediate scattering functions (SISFs) of water at a deeply supercooled temperature and to apply this analytical model to analyse of new high resolution quasielastic neutron scattering data presented elsewhere. The SISF of the center of mass computed from the MD data show clearly time-separated two-step relaxations with a well defined plateau in between. We model the short time relaxation of the test particle as a particle trapped in a harmonical potential well with the vibrational frequency distribution function having a two-peak structure known from previous inelastic neutron scattering experiments. For the long time part of the relaxation, we take the alpha relaxation suggested by mode-coupling theory. The model fits the low-temperature SISK over the entire time range from 1 fs to 10 ns, allowing us to extract peak positions of the vibrational density of states, the structural relaxation rats 1/tau of the cage (the potential well) and the stretch exponent beta. The structural relaxation rate has a power law dependence on the magnitude of the wave vector transfer k and the stretch exponent varies from 0.55 at large k to unity at small k.

A molecular dynamics simulation in a time interval of 1 fs to 50 ns has been performed to study the single-particle dynamics of supercooled SPC/E water. In spite of the fact that, upon supercooling, SPC/E water progressively evolves into a more open structure locally, the single particle dynamics is shown to be dominated by the so called cage effect. The structural relaxation of the cage at low temperatures leads to the phenomenon of slow dynamics that can only be completely studied by following the trajectories into the ns range. Such results can be interpreted in the framework of the mode-coupling theory for supercooled liquids. The so-called critical Angell temperature of supercooled water can thus be interpreted as kinetic glass transition temperature. For this simulation various approximation previously used in literature for the evaluation of the intermediate scattering function from the experimental results are no longer valid. In fact upon supercooling the stochastic single-particle diffusion looses its validity and the diffusion process is then progressively controlled by the structural relaxation of the cage. The possibility of an experimental test of such a behaviour on real supercooled water is explored.

We made a molecular dynamics study of single-particle dynamics of water molecules in deeply supercooled liquid states. We find that the time evolution of various single-particle time correlation functions is characterized by a fast initial relaxation toward a plateau region, where it shows a self-similar dynamics, then followed by a slow, stretched exponential decay to zero at much later times. We interpret these results in the frame-work of a mode-coupling theory for supercooled liquids. We relate the apparent anomalies of transport coefficients in this model water, on lowering the temperature, to the formation of a long-lived cage around each water molecule and the associated slow dynamics of the cages. The experimentally observed so-called Angell temperature, which is an apparent limit of supercooling in liquid water, could thus be interpreted as a kinetic glass transition temperature predicted by the mode-coupling theory. We then discuss to what extent the experimental incoherent quasi-elastic neutron scattering data from supercooled bulk water support the idea of the existence of the slow dynamics.

A molecular-dynamics simulation of extended simple point charge water in a time interval of 1 fs to 50 ns has been performed to study the single-particle dynamics of water at supercooled temperatures, In spite of the fact that upon supercooling water progressively evolves into a more open structure locally, the single-particle dynamics is nevertheless shown to be dominated by the so-called cage effect experienced by the test particle, The slow structural relaxation of the cage at low temperatures leads to the phenomenon of slow dynamics that can only be completely studied by following the trajectories into the nanosecond range. The objective df this paper is twofold. First, we test the accuracy of various approximations used in-previous analyses of. spectra from incoherent quasielastic neutron-scattering experiments. Second, we explore the possibility of an alternative method of analysis of high-resolution quasielastic neutron-scattering spectra, taking into account the slow dynamics in supercooled water. The approximations tested are the decoupling of the self-intermediate scattering function into a product of rotational and translational components, the physical interpretation of the origin of the experimentally;observed Debye Waller factor, the rotational-diffusion approximation of the rotational; intermediate scattering function, and the random jump diffusion approximation of the translational intermediate scattering function; Various approximations used previously for the component intermediate scattering functions are not sufficiently accurate. The reason for this is that at supercooled temperatures, due to the dominant cage: effect, the conventional picture of the stochastic single-particle diffusion loses its validity. The diffusion process is then progressively controlled by the structural relaxation of the cage.

We report a study of the mode-coupling theory (MCT) glass transition line for the Girifalco model Of C-60 fullerene. The equilibrium static structure factor of the model, the only required input for the MCT calculations, is provided by molecular dynamics simulations. The glass transition line develops inside the metastable liquid-solid coexistence region and extends down in temperature, terminating on the liquid side of the metastable portion of the liquid-vapor binodal. The vitrification locus does not show re-entrant behavior. A comparison with previous computer simulation estimates of the location of the glass line suggests that the theory accurately reproduces the shape of the arrest line in the density-temperature plane. The theoretical HNC and MHNC structure factors (and consequently the corresponding MCT glass line) compare well with the numerical counterpart. Our results confirm the conclusion drawn in previous works about the existence of a glassy phase for the fullerene model at issue.