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The propagation of frost in an assembly of supercooled dew droplets takes place by the formation of ice protrusions that bridge ice particles and still-liquid droplets. In this work, we develop a Kinetic Monte Carlo (KMC) model to study the formation kinetics of the ice protrusions. The KMC simulations reproduce well the experimental results reported in the literature. The elongation speed of the ice protrusions does not depend on the droplet size but increases when the interdroplet distance decreases, the temperature increases, or the substrate wettability increases. While 2D diffusion of the water molecules on the substrate surface is sufficient to explain the process kinetics, high 3D (vapor) water-molecule concentration can lead to the development of 3D lateral branches on the ice protrusions. A 1D analytical model based on the water-molecule concentration gradient between a droplet and a nearby ice particle reproduces well the simulation results and highlights the relation between the protrusion elongation kinetics and parameters like the interdroplet distance, the water diffusivity, and the concentration gradient. The bridge-formation time has a quadratic dependence on the droplet-ice distance. Comparing the simulations, the analytical model, and the experimental results of the literature, we conclude that the propagation of frost on a flat substrate in an assembly of supercooled dew droplets with interdroplet spacing larger than about 1 μm is limited by water-molecule diffusivity.

International audience; Second-harmonic generation (SHG) is a nonlinear optical method allowing the study of the local structure, symmetry, and ferroic order in noncentrosymmetric materials such as ferroelectrics. The combination of SHG microscopy with local polarization analysis is particularly efficient for deriving the local polarization orientation. This, however, entails the use of tedious and time-consuming modeling methods of nonlinear optical emission. Moreover, extracting the complex domain structures often observed in thin films requires a pixel-by-pixel analysis and the fitting of numerous polar plots to ascribe a polarization angle to each pixel. Here, the domain structure of GeTe films is studied using SHG polarimetry assisted by machine learning. The method is applied to two film thicknesses: A thick film containing large domains visible in SHG images, and a thin film in which the domains' size is below the SHG resolution limit. Machine learning-assisted methods show that both samples exhibit four domain variants of the same type. This result is confirmed in the case of the thick film, both by the manual pixel-by-pixel analysis and by using piezoresponse force microscopy. The proposed approach foreshows new prospects for optical studies by enabling enhanced sensitivity and high throughput analysis.

Vacancies at interfaces between a film and a substrate can affect material properties and could play a role in solid state dewetting. Using kinetic Monte Carlo simulations, we show that interfacial mono-vacancies diffuse and coalesce to form vacancy clusters and voids. The film/substrate excess energy ES, which is related to the apparent contact angle, controls the mechanisms of coalescence. Depending on ES, voids emerging at the film surface form a hole that can be filled by the film or can lead to dewetting of the film from the substrate.

International audience; In the quest for materials for ferroelectrics-based spintronics with a large spin-orbit coupling, it is essential to carefully control the ferroelectric domains structure, their spatial organization and the domain wall type. Here, we perform the growth of GeTe thin lms on Si by molecular beam epitaxy in a large thickness range. We show that the volume fraction along with the size of the ferroelectric nanodomains can be controlled by nely adjusting the deposition thickness and temperature. We evidence the formation of 71 •-type domain walls and in situ measurements during thermal cycling show the hysteretic appearance and decay of ferroelectric domains. In combination with a detailed analysis of the GeTe/Si interface, we demonstrate that the interfacial mist dislocations formed during the growth plays a key role in the stability of the ferroelectric nanodomains.

International audience; Au deposition on Si(110) at temperature higher than 633 K leads to the formation of elongated liquid nano-droplets. In this work we show that under an electric eld, the droplets move along the [1-10] axis towards the positive electrode by electromigration. During the motion, they dissolve the substrate forming a hole underneath and a nanowire behind them. We develop a model highlighting that the driving force of these processes is bound to the respect (i) of the Au-Si bulk chemical equilibrium and (ii) of the correct balance of surface/interfacial energies. The motion takes place by means of sudden jumps with length of the order of a few µm and by small readjustments of the order of some nm of the droplet front and back edges. With respect to experiments without electric eld, electromigration changes the droplet motion by widening the droplet dissolution front and by facilitating droplet jumps.

In socio-ecosystems, human activities are structured in time and space by interactions between different regulatory systems with different collective goals. These regulatory systems are modeled by institutions and organizations, and the regulatory mechanisms by norms applied to agents in Multi-Agent Systems (MAS). However, little is said about sharing resources, space and time. In particular, temporal and spatial expressivity is often limited in MAS for institutions and norms. This research proposes an institutional MAS model capable of representing multiple institutions and norms in the socio-ecosystem, in order to account for the multiplicity of interactions through agents, resources, space and time. We propose an extension of Descriptive Logic for the description of institutions and norms, and use Allen's algebra and the RCC8 to represent time and space. The resulting model allows us to know the norms applicable to an agent located socially, spatially and temporally.

International audience

An object at the nanoscale has properties different from those of the macroscopic material from which it is extracted. Nanomaterials are therefore increasingly used for many applications. However, they often are in metastable states from the point of view of their crystallographic structure, their shape, or their composition. This article deals with structure, morphology and stability of nano-objects (nanoparticles, two-dimensional materials, thin films, nanowires and nanotubes) regardless how they are obtained. The influence of the substrate (case of nano-objects deposited) or of the neighboring objects (collective effects) are considered. All these effects must beunderstood and controlled to optimize fabrication and use of nanomaterials

Les proprietes d’un materiau de taille nanometrique different de celles du meme materiau a l’echelle micro ou macroscopique. La taille en dessous de laquelle une propriete est modifiee depend de la propriete elle-meme, via une ou des longueurs caracteristiques. L’objet de cet article est de decrire quelques longueurs caracteristiques afin d’explorer les effets de taille et de forme sur les proprietes mecaniques, electroniques, optiques, de transport, magnetiques et thermodynamiques ou chimiques des nano-objets. Ces proprietes intrinseques donnent lieu a des innovations dans de nombreux domaines industriels (medecine, energie, environnement, transport, textile, industrie chimique, electronique…).

International audience; The Burton, Cabrera and Frank (BCF) theory plays a key conceptual role in understanding and modeling the crystal growth of vicinal surfaces. In BCF theory the adatom concentration on a vicinal surface obeys to a diffusion equation, generally solved within quasi-static approximation where the adatom concentration at a given distance x from a step has a steady state value) (x n. Recently, we show that going beyond this approximation [B. Ranguelov and S. Stoyanov, Phys. Rev.B, 76, 035443 (2007)], for fast surface diffusion and slow attachment/detachment kinetics of adatoms at the steps, a train of fast-moving steps is unstable against the formation of steps density waves. More precisely, the step density waves are generated if the step velocity exceeds a critical value related to the strength of the step-step repulsion. This theoretical treatment corresponds to the case when the time to reach a steady state concentration of adatoms on a given terrace is 2 comparable to the time for a non-negligible change of the step configuration leading to a terrace adatom concentration) , (t x n that depends not only on the terrace width, but also on its " past width ". This formation of step density waves originates from the high velocity of step motion and has nothing to do with usual kinetic instabilities of step bunching induced by Ehrlich-Schwoebel effect, surface electromigration and/or the impact of impurities on the step rate. The so-predicted formation of step density waves is illustrated by numerical integration of the equations for step motion. In order to complete our previous theoretical treatment of the non-stationary BCF problem, we perform an in-situ reflection electron microscopy experiment at specific temperature interval and direction of the heating current, in which, for the first time, the step density waves instability is evidenced on Si(111) surface during high Si adatoms deposition rates.