Particle impact on a cohesive granular media.

We investigate numerically the impact process of a particle of diameter d and velocity V_{i} onto a cohesive granular packing made of similar particles via two-dimensional discrete element method simulations. The cohesion is ensured by liquid bridges between neighboring particles and described by short range attraction force based on capillary modeling. The outcome of the impact is analyzed through the production of ejected particles from the packing, referred to as the splash process. We quantify this production as a function of the impact velocity for various capillary strength Γ and liquid content Ω. The numerical data indicate that the splash process is modified when the dimensionless cohesion number Co=6Γ/ρ_{p}gd^{2} (where ρ_{p} is the particle density, d its diameter, and g the gravitational acceleration) exceeds a critical value of the order of the unity. Above this value, we highlight that the ejection process is triggered above a threshold impact Froude number, Fr=V_{i}/sqrt[gd], which depends both on Γ and Ω and scales as Γ^{ß}Ω^{δ}, where the values of the exponents are found close to 1/2 and 1/6, respectively, and can be derived from rational physical arguments. Importantly, we show that, above the threshold, the number of splashed particles follows a linear law with the impact Froude number as in the cohesionless case.

Transition from Saltation to Collisional Regime in Windblown Sand.

We report experiments on windblown sand that highlight a transition from saltation to collisional regime above a critical dimensionless mass flux or Shields number. The transition is first seen through the mass flow rate Q, which deviates from a linear trend with the Shields number and seems to follow a quadratic law. Other physical evidences confirm the change of the transport properties. In particular, the particle velocity and the height of the transport layer increases with increasing Shields number in the collisional regime while the latter are invariant with the wind strength in the saltation regime. Discrete numerical simulations support the experimental findings and ascertain that mid-air collisions are responsible for the change of transport regime.

Particle velocity distribution in saltation transport.

We report on wind-tunnel measurements of particle velocity distribution in aeolian transport. By performing extended statistics, we show that for saltation occurring over an erodible bed the vertical lift-off velocity distributions deviate significantly from a Gaussian law and exhibit a long tail accurately described by a lognormal law. In contrast, saltation over a rigid bed produces Gaussian velocity distributions. These results strongly suggest that the deviation from Gaussian distributions is a consequence of the splash process which is exclusively present in saltation transport over an erodible bed. We further suggest that the non-Gaussian statistics is intimately related to the statistical properties of a single splash event which produces ejection of particles with lift-off velocities distributed according to a lognormal law. This lognormal behavior can be simply inferred from the propagation process of the impact energy through the granular bed which can be viewed as the analog of a fragmentation process. These findings emphasize the crucial role of the splash process in saltation transport.

Scaling laws in aeolian sand transport.

We report on wind tunnel measurements on saltating particles in a turbulent boundary layer and provide evidence that over an erodible bed the particle velocity in the saltation layer and the saltation length are almost invariant with the wind strength, whereas over a nonerodible bed these quantities vary significantly with the air friction speed. It results that the particle transport rate over an erodible bed does not exhibit a cubic dependence with the air friction speed, as predicted by Bagnold, but a quadratic one. This contrasts with saltation over a nonerodible bed where the cubic Bagnold scaling holds. Our findings emphasize the crucial role of the boundary conditions at the bed and may have important practical consequences for aeolian sand transport in a natural environment.

Developpement d'une matrice ethylcellulose/eudragitr pour la liberation controlee et continue de l'insuline

But : Le diabete de type I est une maladie chronique necessitant des prises repetees d'insuline toute une vie durant par voie parenterale. Ce mode d'administration en plus d'etre traumatisant peut poser un probleme d'observance du traitement chez le patient. Dans le souci de pallier ces difficultes; nous avons envisage le developpement d'une matrice ethylcellulose/eudragitr susceptible de faciliter la mise au point d'un systeme therapeutique transdermique de liberation controlee (STTLC) de l'insuline. Materiel et methode : Comme principe actif nous avons utilise de l'insuline humaine anhydre Actrapidr HM des laboratoires Novo Nordisk; les excipients sont l'ethylcellulose; les Eudragitr RS 100 et le butylphtalate. Nous avons elabore deux matrices Ethylcellulose/Eudragit dans les rapports 1 : 1 et 2 : 1 dans lesquels sont incorporees differentes proportions d'insuline. Resultats : L'etude de la liberation de l'insuline en milieu tampon phosphate a pH 7;4 a montre une liberation continue avec des profils fortement dependants du rapport Ethylcellulose/Eudragit et de la charge initiale en insuline. Conclusion : Cette etude a montre que la matrice Ethylcellulose/Eudragit se prete a la mise au point d'un systeme a liberation controlee d'insuline. Ceci nous permet d'envisager pour la poursuite de notre travail l'association de cette matrice avec d'autres elements pour la realisation d'un STTLC de l'insuline