Crystal growth by precipitation under microgravity

The importance of understanding the mechanisms associated with defect generation during growth and the influence of gravity is stressed. An experiment is described. The advantages of adapting this experiment to the FES are then discussed. A brief survey of the ground based research under way is given

L'hydrodynamique, une composante importante dans la croissance des cristaux en solution

Different aspects of the solution growth of crystals are reviewed in order to examine the role of the hydrodynamic environment.Différents aspects de la croissance des cristaux en solution sont successivement passés en revue pour montrer le rôle de l'environnement hydrodynamique

Satellite reflection study of the incommensurate phase in (N(CH3) 4)2ZnCl4 crystal by means of real-time X-ray synchrotron topography

A real-time X-ray Synchrotron study of (N(CH3)4) 2ZnCl4 single crystals allows one to follow the evolution of a satellite reflection above the lock-in transition. Thermal cycles give evidence for relaxation effects associated with the defect distribution.Une étude de monocristaux de (N(CH3)4)2ZnCl 4 au rayonnement Synchrotron a permis de suivre, en temps réel, l'évolution d'une réflexion satellite au-dessus de la transition de blocage. Des effets de relaxation associés à la distribution des défauts ont été mis en évidence lors de cycles thermiques

Critical viscoelastic study of the gelation of silica particles

We present an analysis of the critical behaviour of viscosity and elastic modulus during the gelation of silica particles. Two systems are studied : gelation in NaCl solutions and in pure water. The experiments show a cross-over between entropic elasticity near the gelation threshold and « mechanical » elasticity farther from it.Nous présentons une analyse de comportement critique de la viscosité et du module élastique lors de la gélification des particules de silice. Deux systèmes sont étudiés : gélification dans une solution de NaCl ou dans l'eau pure. Les expériences montrent une transition d'un régime d'élasticité entropique au voisinage du seuil de gélation vers un régime d'élasticité « mécanique » au-delà du seuil

On positivity and minimality for second-order holonomic sequences

An infinite sequence ⟨u_n⟩_n of real numbers is holonomic (also known as P-recursive or P-finite) if it satisfies a linear recurrence relation with polynomial coefficients. Such a sequence is said to be positive if each u_n ≥ 0, and minimal if, given any other linearly independent sequence ⟨v_n⟩_n satisfying the same recurrence relation, the ratio u_n/v_n → 0 as n → ∞. In this paper we give a Turing reduction of the problem of deciding positivity of second-order holonomic sequences to that of deciding minimality of such sequences. More specifically, we give a procedure for determining positivity of second-order holonomic sequences that terminates in all but an exceptional number of cases, and we show that in these exceptional cases positivity can be determined using an oracle for deciding minimality

The Orbit Problem for parametric linear dynamical systems

We study a parametric version of the Kannan-Lipton Orbit Problem for linear dynamical systems. We show decidability in the case of one parameter and Skolem-hardness with two or more parameters. More precisely, consider a d-dimensional square matrix M whose entries are algebraic functions in one or more real variables. Given initial and target vectors u,v ∈ ℚ^d, the parametric point-to-point orbit problem asks whether there exist values of the parameters giving rise to a concrete matrix N ∈ ℝ^{d× d}, and a positive integer n ∈ ℕ, such that N^{n} u = v. We show decidability for the case in which M depends only upon a single parameter, and we exhibit a reduction from the well-known Skolem Problem for linear recurrence sequences, suggesting intractability in the case of two or more parameters