Numerical analysis of Pickering emulsion stability: insights from ABMD simulations

The issue of the stability of Pickering emulsions is tackled at a mesoscopic level using dissipative particle dynamics simulations within the Adiabatic Biased Molecular Dynamics framework. We consider the early stage of the coalescence process between two spherical water droplets in a decane solvent. The droplets are stabilized by Janus nanoparticles of different shapes (spherical and ellipsoidal) with different three-phase contact angles. Given a sufficiently dense layer of particles on the droplets, we show that the stabilization mechanism strongly depends on the collision speed. This is consistent with a coalescence mechanism governed by the rheology of the interfacial region. When the system is forced to coalesce sufficiently slowly, we investigate at a mesoscopic level how the ability of the nanoparticles to stabilize Pickering emulsions is discriminated by nanoparticle mobility and the associated caging effect. These properties are both related to the interparticle interaction and the hydrodynamic resistance in the liquid film between the approaching interfaces

Buckling in armored droplets

The buckling mechanism in droplets stabilized by solid particles (armored droplets) is tackled at a mesoscopic level using dissipative particle dynamics simulations. We consider one spherical water droplet in a decane solvent coated with nanoparticle monolayers of two different types: Janus (particles whose surface shows two regions with different wetting properties) and homogeneous. The chosen particles yield comparable initial three-phase contact angles, selected to maximize the adsorption energy at the interface. We study the interplay between the evolution of droplet shape, layering of the particles, and their distribution at the interface when the volume of the droplets is reduced. We show that Janus particles affect strongly the shape of the droplet with the formation of a crater-like depression. This evolution is actively controlled by a close-packed particle monolayer at the curved interface. In contrast, homogeneous particles follow passively the volume reduction of the droplet, whose shape does not deviate too much from spherical, even when a nanoparticle monolayer/bilayer transition is detected at the interface. We discuss how these buckled armored droplets might be of relevance in various applications including potential drug delivery systems and biomimetic design of functional surfaces

Relations entre les variations saisonnières du métabolisme hydrique, l'estivation et la reproduction chez Gerbillus nigeriae et Taterillus petteri (Rodentia, Gerbillidae)

Les variations saisonnières de la réserve d'eau corporelle (REC), de la vitesse de renouvellement de l'eau (VRE), et du bilan hydrique (BH) ont été suivies pendant près de huit ans (1984-1991) par la technique à l'eau tritiée chez 2 espèces de rongeurs sahéliens (#Gerbillus nigeriae et #Taterillus petteri) qui vivent au Burkina Faso dans les dunes de la région d'Oursi (14°N). Chez ces 2 espèces sympatriques : (a) la VRE est minimale de mars à mai, maximale de juin à octobre et moyenne de novembre à février; (b) le BH reste équilibré toute l'année sauf en janvier-février où un léger déficit hydrique apparaît. Comparés aux connaissances acquises antérieurement sur l'estivation et la reproduction des 2 espèces, ces résultats nous conduisent à 3 hypothèses : (1) l'arrêt de la reproduction résulterait d'une réduction de la VRE liée à un changement d'alimentation; (2) la forte diminution de l'activité ambulatoire résulterait d'un déficit passager de la balance hydrique qui constituerait le signal physiologique de l'entrée en estivation; et (3) l'effet gonadostimulant de la photopériode, que nous avons mis en évidence précédemment, ne serait possible qu'après rétablissement de l'équilibre hydrique au cours de la période d'estivation. (Résumé d'auteur

Role of structural rigidity and collective behaviour in the molecular design of gas hydrate anti-agglomerants

Anti-agglomerants (AAs) are surface active molecules widely used in the petroleum industry, among others. It is believed that AAs strongly adsorb onto the surface of hydrate particles to prevent the growth of clathrate hydrate within oil pipelines. Small changes in their molecular structures can strongly affect the thermodynamic and kinetic stability of the system as a whole. Here we employ molecular dynamics simulations to study the interplay between the modification of the molecular structure, rigidity and collective effects of AAs designed to prevent hydrate agglomeration under the conditions encountered in rocking cell experiments. The AAs are surface-active compounds with a complex hydrophilic head and three hydrophobic tails whose structural rigidity is enhanced with the attachment of an aromatic group. Extrapolating from our simulation results, we predict that the aromatic group can positively or negatively affect the performance of the AAs, depending on its location along the hydrophobic tail. Our approach is based on first quantifying the molecular mechanisms responsible for the macroscopic performance and then altering the AA molecular structure to amplify said molecular mechanisms. Although the mechanisms at play depend on the application, the methodology implemented could be applicable to other high-tech industries, where the agglomeration of small particles must be controlled

Armored Droplets as Soft Nanocarriers for Encapsulation and Release under Flow Conditions

Technical challenges in precision medicine and environmental remediation create an increasing demand for smart materials that can select and deliver a probe load to targets with high precision. In this context, soft nanomaterials have attracted considerable attention due to their ability to simultaneously adapt their morphology and functionality to complex ambients. Two major challenges are to precisely control this adaptability under dynamic conditions and provide predesigned functionalities that can be manipulated by external stimuli. Here, we report on the computational design of a distinctive class of soft nanocarriers, built from armored nanodroplets, able to selectively encapsulate or release a probe load under specific flow conditions. First, we describe in detail the mechanisms at play in the formation of pocket-like structures in armored nanodroplets and their stability under external flow. Then we use that knowledge to test the capacity of these pockets to yield flow-assisted encapsulation or expulsion of a probe load. Finally, the rheological properties of these nanocarriers are put into perspective with those of delivery systems employed in pharmaceutical and cosmetic technology

1-d gravity in infinite point distributions

The dynamics of infinite, asymptotically uniform, distributions of self-gravitating particles in one spatial dimension provides a simple toy model for the analogous three dimensional problem. We focus here on a limitation of such models as treated so far in the literature: the force, as it has been specified, is well defined in infinite point distributions only if there is a centre of symmetry (i.e. the definition requires explicitly the breaking of statistical translational invariance). The problem arises because naive background subtraction (due to expansion, or by "Jeans' swindle" for the static case), applied as in three dimensions, leaves an unregulated contribution to the force due to surface mass fluctuations. Following a discussion by Kiessling, we show that the problem may be resolved by defining the force in infinite point distributions as the limit of an exponentially screened pair interaction. We show that this prescription gives a well defined (finite) force acting on particles in a class of perturbed infinite lattices, which are the point processes relevant to cosmological N-body simulations. For identical particles the dynamics of the simplest toy model is equivalent to that of an infinite set of points with inverted harmonic oscillator potentials which bounce elastically when they collide. We discuss previous results in the literature, and present new results for the specific case of this simplest (static) model starting from "shuffled lattice" initial conditions. These show qualitative properties (notably its "self-similarity") of the evolution very similar to those in the analogous simulations in three dimensions, which in turn resemble those in the expanding universe.Comment: 20 pages, 8 figures, small changes (section II shortened, added discussion in section IV), matches final version to appear in PR