José Rafael Bordin

Diffusion enhancement in core-softened fluid confined in nanotubes.

We study the effect of confinement in the dynamical behavior of a core-softened fluid. The fluid is modeled as a two length scales potential. This potential in the bulk reproduces the anomalous behavior observed in the density and in the diffusion of liquid water. A series of NpT molecular dynamics simulations for this two length scales fluid confined in a nanotube were performed. We obtain that the diffusion coefficient increases with the increase of the nanotube radius for wide channels as expected for normal fluids. However, for narrow channels, the confinement shows an enhancement in the diffusion coefficient when the nanotube radius decreases. This behavior, observed for water, is explained in the framework of the two length scales potential.

Relation Between Flow Enhancement Factor and Structure for Core- Softened Fluids Inside Nanotubes

The relationship between enhancement flow and structure of core-softened fluids confined inside nanotubes has been studied using nonequilibrium molecular dynamics simulation. The fluid was modeled with different types of attractive and purely repulsive two length scale potentials. Such potentials reproduce in bulk the anomalous behavior observed for liquid water. The dual control volume grand canonical molecular dynamics method was employed to create a pressure gradient between two reservoirs connected by a nanotube. We show how the nanotube radius affects the flow enhancement factor for each one of the interaction potentials. The connection between structural and dynamical properties of the confined fluid is discussed, and we show how attractive and purely repulsive fluids exhibit distinct behaviors. A continuum to subcontinuum flow transition was found for small nanotube radius. The behavior obtained for the core-softened fluids is similar to what was recently observed in all-atom molecular dynamics simulations for classical models of water and also in experimental studies. Our results are explained in the framework of the two length scale potentials.

Distinct dynamical and structural properties of a core-softened fluid when confined between fluctuating and fixed walls

Molecular dynamics simulations were used to study the structural and dynamical properties of a water-like core-softened fluid under confinement when the confining media is rigid or fluctuating. The fluid is modeled using a two-length scale potential that reproduces, in the bulk, the anomalous behavior observed in water. We perform simulations in the NVT ensemble for fixed flat walls and in the NpT ensemble using a fluctuating wall control of pressure to study how the fluid behavior is affected by fixed and non-fixed walls. Our results indicate that the dynamical and structural properties of the fluid are strongly affected by the wall mobility.

New structural anomaly induced by nanoconfinement.

We explore the structural properties of anomalous fluids confined in a nanopore using molecular dynamics simulations. The fluid is modeled by core-softened (CS) potentials that have a repulsive shoulder and an attractive well at a further distance. Changing the attractive well depth of the fluid-fluid interaction potential, we studied the behavior of the anomalies in the translational order parameter t and excess entropy s(2) for the particles near to the nanopore wall (contact layer) for systems with two or three layers of particles. When the attractive well of the CS potential is shallow, the systems present a three to two layers transition and, additionally to the usual structural anomaly, a new anomalous region in t and s(2). For attractive well deep enough, the systems change from three layers to a bulk-like profile and just one region of anomaly in t and s(2) is observed. Our results are discussed on the basis of the fluid-fluid and fluid-surface interactions.

Enhanced flow of core-softened fluids through narrow nanotubes

We investigate through non-equilibrium molecular dynamic simulations the flow of anomalous fluids inside rigid nanotubes. Our results reveal an anomalous increase of the overall mass flux for nanotubes with sufficiently smaller radii. This is explained in terms of a transition from a single-file type of flow to the movement of an ordered-like fluid as the nanotube radius increases. The occurrence of a global minimum in the mass flux at this transition reflects the competition between the two characteristic length scales of the core-softened potential. Moreover, by increasing further the radius, another substantial change in the flow behavior, which becomes more evident at low temperatures, leads to a local minimum in the overall mass flux. Microscopically, this second transition is originated by the formation of a double-layer of flowing particles in the confined nanotube space. These nano-fluidic features give insights about the behavior of confined isotropic anomalous fluids.

Self-Assembly and Water-like Anomalies in Janus Nanoparticles.

We explore the pressure versus temperature phase diagram of a system of dimeric Janus nanoparticles using molecular dynamics simulations. Each nanoparticle is modeled as a dumbbell which has one monomer that interacts by a standard Lennard-Jones potential while the other monomer interacts by a core-softened potential. The systems composed by particles interacting only by core-softened potential exhibit the density and the diffusion anomalous behavior observed in water while if the particles interact only by the Lennard-Jones potential no anomaly is present. Here we explore if the anomalous behavior is present when half of the particles are modeled by a core-softened potential and half with Lennard-Jones potential. We show that the diffusion anomaly is preserve, while the density anomaly can disappear depending on the nonanomalous monomer characteristics. We also show that the self-assembly structures characteristics of the dumbbell systems are affected by the balance between core-softened and non-core-softened monomers.

Effects of confinement on anomalies and phase transitions of core-softened fluids

We use molecular dynamics simulations to study how the confinement affects the dynamic, thermodynamic, and structural properties of a confined anomalous fluid. The fluid is modeled using an effective pair potential derived from the ST4 atomistic model for water. This system exhibits density, structural, and dynamical anomalies, and the vapor-liquid and liquid-liquid critical points similar to the quantities observed in bulk water. The confinement is modeled both by smooth and structured walls. The temperatures of extreme density and diffusion for the confined fluid show a shift to lower values while the pressures move to higher amounts for both smooth and structured confinements. In the case of smooth walls, the critical points and the limit between fluid and amorphous phases show a non-monotonic change in the temperatures and pressures when the nanopore size is increase. In the case of structured walls, the pressures and temperatures of the critical points varies monotonically with the pore size. Our results are explained on basis of the competition between the different length scales of the fluid and the wall-fluid interaction.

High pressure induced phase transition and superdiffusion in anomalous fluid confined in flexible nanopores.

The behavior of a confined spherical symmetric anomalous fluid under high external pressure was studied with Molecular Dynamics simulations. The fluid is modeled by a core-softened potential with two characteristic length scales, which in bulk reproduces the dynamical, thermodynamical, and structural anomalous behavior observed for water and other anomalous fluids. Our findings show that this system has a superdiffusion regime for sufficient high pressure and low temperature. As well, our results indicate that this superdiffusive regime is strongly related with the fluid structural properties and the superdiffusion to diffusion transition is a first order phase transition. We show how the simulation time and statistics are important to obtain the correct dynamical behavior of the confined fluid. Our results are discussed on the basis of the two length scales.

Waterlike features, liquid–crystal phase and self-assembly in Janus dumbbells

We explore the phase diagram of Janus nanoparticles using Molecular Dynamics simulations. Each monomer in the dimer has distinct characteristics. One type of monomer interacts by a Lennard Jones potential, while the other type interacts through a two length scale potential. Previous studies for the monomeric system using this specific two length scale potential do not indicate the presence of waterlike anomalies. However, our results show that the combination of two length scales potential and LJ potential in the Janus nanoparticle will lead to thermodynamic and dynamic anomalies. The self-assembly properties were also explored. We observe distinct kinds of self-assembled structures and a liquid–crystal phase. This result indicates that it is possible to create Janus nanoparticles with waterlike features using monomers without anomalous behavior. The anomalies and structures are explained with the two length scale potential characteristics.

Confinement effects on the properties of Janus dimers

Confinement has been suggested as a tool to tune the self-assembly properties of nanoparticles, surfactants, polymers and colloids. In this way, we explore the phase diagram of Janus nanoparticles confined between two parallel walls using molecular dynamics simulations. A nanoparticle was modeled as a dimer made by one monomer that interacts via a standard Lennard Jones potential and another monomer that is modeled using a two-length scale shoulder potential. This specific design of the nanoparticle exhibits distinct self-assembled structures and a water-like diffusion anomaly in the bulk. Our results indicate that besides the aggregates observed in bulk, new structures are observed under confinement. Also, the dynamic and thermodynamic behavior of the fluid phase is affected. The systems show a reentrant fluid phase and density anomaly. None of these two features were observed in bulk. Our results show that geometrical confinement leads to new structural, thermodynamical and dynamical behavior for this Janus nanoparticle.