Samy Merabia

Critical heat flux around strongly heated nanoparticles.

We study heat transfer from a heated nanoparticle into surrounding fluid using molecular dynamics simulations. We show that the fluid next to the nanoparticle can be heated well above its boiling point without a phase change. Under increasing nanoparticle temperature, the heat flux saturates, which is in sharp contrast with the case of flat interfaces, where a critical heat flux is observed followed by development of a vapor layer and heat flux drop. These differences in heat transfer are explained by the curvature-induced pressure close to the nanoparticle, which inhibits boiling. When the nanoparticle temperature is much larger than the critical fluid temperature, a very large temperature gradient develops, resulting in close to ambient temperature just a radius away from the particle surface. The behavior reported allows us to interpret recent experiments where nanoparticles can be heated up to the melting point, without observing boiling of the surrounding liquid.

Thermal conductance at the interface between crystals using equilibrium and nonequilibrium molecular dynamics

In this article, we compare the results of non-equilibrium (NEMD) and equilibrium (EMD) molecular dynamics methods to compute the thermal conductance at the interface between solids. We propose to probe the thermal conductance using equilibrium simulations measuring the decay of the thermally induced energy fluctuations of each solid. We also show that NEMD and EMD give generally speaking inconsistent results for the thermal conductance: Green Kubo simulations probe the Landauer conductance between two solids which assumes phonons on both sides of the interface to be at equilibrium. On the other hand, we show that NEMD give access to the out-of-equilibrium interfacial conductance consistent with the interfacial flux describing phonon transport in each solid. The difference may be large and reaches typically a factor 5 for interfaces between usual semi-conductors. We analyze finite size effects for the two determinations of the interfacial thermal conductance, and show that the equilibrium simulations suffer from severe size effects as compared to NEMD. We also compare the predictions of the two above mentioned methods -EMD and NEMD- regarding the interfacial conductance of a series of mass mismatched Lennard-Jones solids. We show that the Kapitza conductance obtained with EMD can be well described using the classical diffuse mismatch model (DMM). On the other hand, NEMD simulations results are consistent with a out-of-equilibrium generalisation of the acoustic mismatch model (AMM). These considerations are important in rationalizing previous results obtained using molecular dynamics, and help in pinpointing the physical scattering mechanisms taking place at atomically perfect interfaces between solids, which is a prerequesite to understand interfacial heat transfer across real interfaces.

Dynamical study of bubble expansion following laser ablation in liquids

This work examines the initial growth and collapse stages of bubbles induced by laser ablation in liquids. First, the bubble shape and size are tracked using an ultrafast camera in a shadowgraph imaging setup. The use of an ultrafast camera ensures a high control of the reproducibility, because a thorough measurement of each bubble lifetime is performed. Next, an analytical cavitation-based model is developed to assess the thermodynamic bubble properties. This study demonstrates that the bubble evolution is adiabatic and driven by inertial forces. Surprisingly, it is found that the bubbles consist of significantly more solvent molecules than ablated matter. These results are valuable to the field of nanoparticle synthesis as they provide insight into the mechanics of laser ablation in liquids.

Thermal properties of amorphous/crystalline silicon superlattices.

Thermal transport properties of crystalline/amorphous silicon superlattices using molecular dynamics are investigated. We show that the cross-plane conductivity of the superlattices is very low and close to the conductivity of bulk amorphous silicon even for amorphous layers as thin as ≃ 6 Å. The cross-plane thermal conductivity weakly increases with temperature which is associated with a decrease of the Kapitza resistance with temperature at the crystalline/amorphous interface. This property is further investigated considering the spatial analysis of the phonon density of states in domains close to the interface. Interestingly, the crystalline/amorphous superlattices are shown to display large thermal anisotropy, according to the characteristic sizes of elaborated structures. These last results suggest that the thermal conductivity of crystalline/amorphous superlattices can be phonon engineered, providing new directions for nanostructured thermoelectrics and anisotropic materials in thermal transport.

Thermal boundary conductance across rough interfaces probed by molecular dynamics

(Received 14 March 2013; revised manuscript received 29 January 2014; published 25 February 2014)Wereportontheinfluenceoftheinterfacialroughnessonthethermalboundaryconductancebetweentwosolids,using molecular dynamics. We show evidence of a transition between two regimes, depending on the interfacialroughness: When the roughness is small, the boundary conductance is constant, taking values close to theconductanceofthecorrespondingplanarinterface.Whentheroughnessislarger,theconductancebecomeslargerthantheplanarinterfaceconductanceandtherelativeincreaseisfoundtobeclosetotheincreaseoftheinterfacialarea. The cross-plane conductivity of a superlattice with rough interfaces is found to increase in a comparableamount, suggesting that heat transport in superlattices is mainly controlled by the boundary conductance. Theseobservations are interpreted using the wave characteristics of the energy carriers. We characterize also the effectof the angle of the asperities and find that the boundary conductance displayed by interfaces having steep slopesmay become important if the lateral period characterizing the interfacial profile is large enough. As a result,triangular-shapedinterfacesmaybeusedtoenhancetheconductanceofplanarinterfacesbyafactorgreaterthanthree.Finally,weconsidertheeffectoftheshapeof theinterfaces andshowthatthesinusoidalinterfacedisplaysthe highest conductance because of its large true interfacial area. All of these considerations are relevant to theoptimization of nanoscale interfacial energy transport.DOI: 10.1103/PhysRevB.89.054309 PACS number(s): 68

Thermal conductivity and thermal boundary resistance of nanostructures

AbstractWe present a fabrication process of low-cost superlattices and simulations related with the heat dissipation on them. The influence of the interfacial roughness on the thermal conductivity of semiconductor/semiconductor superlattices was studied by equilibrium and non-equilibrium molecular dynamics and on the Kapitza resistance of superlattices interfaces by equilibrium molecular dynamics. The non-equilibrium method was the tool used for the prediction of the Kapitza resistance for a binary semiconductor/metal system. Physical explanations are provided for rationalizing the simulation results.PACS68.65.Cd, 66.70.Df, 81.16.-c, 65.80.-g, 31.12.xv

Atomistic amorphous/crystalline interface modelling for superlattices and core/shell nanowires

In this paper we present a systematic and well controlled procedure for building atomistic amorphous/crystalline interfaces in silicon, dedicated to the molecular dynamics simulations of superlattices and core/shell nanowires. The obtained structures depend on the technique used to generate the amorphous phase and their overall quality is estimated through comparisons with structural information and interfacial energies available from experimental and theoretical results. While most of the related studies focus on a single planar interface, we consider here both the generation of multiple superlattice planar interfaces and core/shell nanowire structures. The proposed method provides periodic homogeneous and reproducible, atomically sharp and defect free interface configurations at low temperature and pressure. We also illustrate how the method may be used to predict the thermal transport properties of composite crystalline/amorphous superlattices.

Effective temperatures of a heated Brownian particle

We investigate various possible definitions of an effective temperature for a particularly simple nonequilibrium stationary system, namely a heated Brownian particle suspended in a fluid. The effective temperature based on the fluctuation dissipation ratio depends on the time scale under consideration, so that a simple Langevin description of the heated particle is not possible on short time scales. The short- and long-time limits of this effective temperature are shown to be consistent with the temperatures estimated from the kinetic energy and Einstein relation, respectively. The transient fluctuation theorem provides still another definition of the temperature, which is shown to coincide with the short-time value of the fluctuation dissipation ratio.

Entanglement-induced reinforcement in polymer nanocomposites

We propose a coarse-grained model able to describe filled entangled polymer melts. Our purpose is to study the reinforcement caused by the effect of fillers on the entanglement network, as speculated in previous experimental work, and also observed in molecular dynamics studies. In this work, the filler volume fraction effect, the distribution of the fillers (cubic lattice, randomly dispersed, and small clusters randomly dispersed) and the presence or absence of grafted chains on the fillers are investigated. Our model is based on a “slip-link” model initially developed to study the entanglements in pure polymer melts and offers a less costly computational method than molecular dynamics simulations for the study of entangled polymer melts. The polymer chains are described as Rouse chains of Brownian particles connected by Hookean springs, and are subject to friction and random forces. Entanglements are artificially imposed by objects (slip-links) exhibiting statistical fluctuations that do not modify the equilibrium statistics of the melt. In addition we introduced excluded volume interactions between chain segments, to take into account the incompressibility of the melt. These excluded volume interactions do not perturb the dynamics of the chains in the homogeneous limit as expected from theoretical considerations on short range interactions. Finally, the fillers are modeled by immobile spherical objects, with or without grafted chains, which interact with a repulsive potential with the chain monomers. The chains grafted onto the fillers are represented by “additional slip links” confined in the vicinity of each filler. We first present the effect of the filler distribution and filler volume fraction, considering only bare fillers. Then, the effect of grafted chains via the additional slip-links is also shown as a function of the same parameters. Our results show that the presence of grafted chains induces an important change in the viscosity, calculated by integrating the stress autocorrelation function. Both the plateau value and the terminal relaxation time depend on the density of fillers and on the number of grafted chains. Moreover, we find that a disordered filler configuration induces confinement effects that amplify reinforcement compared to the case of a perfectly ordered configuration.

Numerical study of a slip-link model for polymer melts and nanocomposites

We present a numerical study of the slip link model introduced by Likhtman for describing the dynamics of dense polymer melts. After reviewing the technical aspects associated with the implementation of the model, we extend previous work in several directions. The dependence of the relaxation modulus with the slip link density and the slip link stiffness is reported. Then the nonlinear rheological properties of the model, for a particular set of parameters, are explored. Finally, we introduce excluded volume interactions in a mean field such as manner in order to describe inhomogeneous systems, and we apply this description to a simple nanocomposite model. With this extension, the slip link model appears as a simple and generic model of a polymer melt, that can be used as an alternative to molecular dynamics for coarse grained simulations of complex polymeric systems.