Influence of surfactants on the structure of titanium oxide gels : experiments and simulations

We report here on experimental and numerical studies of the influence of surfactants on mineral gel synthesis. The modification of the gel structure when the ratios water-precursor and water-surfactant vary is brought to the fore by fractal dimension measures. A property of {\em polydispersity of the initial hydrolysis} is proposed to explain these results, and is successfuly tested through numerical experiments of three dimensional chemically limited aggregation.Comment: 12 pages, 4 Postscript figures, uses RevTe

Structure factor of polymers interacting via a short range repulsive potential: application to hairy wormlike micelles

We use the Random Phase Approximation (RPA) to compute the structure factor, S(q), of a solution of chains interacting through a soft and short range repulsive potential V. Above a threshold polymer concentration, whose magnitude is essentially controlled by the range of the potential, S(q) exhibits a peak whose position depends on the concentration. We take advantage of the close analogy between polymers and wormlike micelles and apply our model, using a Gaussian function for V, to quantitatively analyze experimental small angle neutron scattering profiles of semi-dilute solutions of hairy wormlike micelles. These samples, which consist in surfactant self-assembled flexible cylinders decorated by amphiphilic copolymer, provide indeed an appropriate experimental model system to study the structure of sterically interacting polymer solutions

Shear induced instabilities in layered liquids

Motivated by the experimentally observed shear-induced destabilization and reorientation of smectic A like systems, we consider an extended formulation of smectic A hydrodynamics. We include both, the smectic layering (via the layer displacement u and the layer normal p) and the director n of the underlying nematic order in our macroscopic hydrodynamic description and allow both directions to differ in non equilibrium situations. In an homeotropically aligned sample the nematic director does couple to an applied simple shear, whereas the smectic layering stays unchanged. This difference leads to a finite (but usually small) angle between n and p, which we find to be equivalent to an effective dilatation of the layers. This effective dilatation leads, above a certain threshold, to an undulation instability of the layers. We generalize our earlier approach [Rheol. Acta, vol.39(3), 15] and include the cross couplings with the velocity field and the order parameters for orientational and positional order and show how the order parameters interact with the undulation instability. We explore the influence of various material parameters on the instability. Comparing our results to recent experiments and molecular dynamic simulations, we find a good qualitative agreement.Comment: 15 pages, 12 figures, accepted for publication in PR

Structure of giant micelles: a small angle neutron scattering study

The local structure of elongated micelles of cetylpyridinium chlorate in brine is determined using small angle neutron scattering (scattering vectors range from $2 \times 10^{-3}$ to $5 \times 10^{-1} {\rm \AA}^{-1})$. They are found to be long flexible cylinders. The radius of the circular cross section is $20 \pm 1~{\rm \AA}$ and the persistence length characterizing the flexibility is $170 \pm 50~{\rm \AA}$. Both are found to be independent of the nature or of the concentration of the counterion

FOCAL CONICS MOTIONS AND PLASTICITY IN SMECTICS A

Nous présentons les résultats expérimentaux obtenus en appliquant un cisaillement alternatif à un smectique A en géométrie planaire, le vecteur vitesse étant normal en couche. A basse fréquence (10-2 Hz), on observe la création de deux parois de Grandjean. Le déplacement vertical de ces deux parois permet de relaxer la contrainte. Les observations effectuées à l'aide d'un stroboscope à 102 Hz peuvent être interprétées à partir de résultats obtenus à basse fréquence. Ces résultats expérimentaux montrent l'importance de la plasticité dans la fluidité macroscopique des smectiques A.Experimental studies on alternative shear with velocity normal to the layers of a smectic A material with planar boundary conditions are presented. Alternative shear in the 10-2 Hz range shows creation and vertical relative motion of two Grandjean walls in the bulk. Observations by means of stroboscope at 102 Hz frequency can be interpreted using results obtained in the 10-2 Hz range. All these experiments show that plasticity is mainly responsible for the macroscopic fluidity of smectic A

Water/oil interface in L2 phase : a small-angle X-ray scattering study

We have studied the structure of reversed micelles in the ternary system aerosol OT-n heptane-water by small-angle X-ray scattering. A structural model of the interface water/surfactant is proposed At high water content, the film of hydrated surfactants surrounds a free water droplet. The size of the successive layers of the micelle is determined. The measurement of the radius of gyration seems to confirm our model,Nous étudions la structure de micelles inverses du système ternaire aérosol OT-n heptane-eau par diffusion centrale des rayons X. La structure de la partie polaire à l'interface eau huile est précisée. Pour un rapport d'hydratation suffisant, la partie polaire hydratée piège une goutte d'eau « libre ». Les dimensions respectives de chaque zone sont déterminées. Des mesures de rayons de giration confirment ces modèles

Instabilities in smectics A submitted to an alternating shear flow. II. Experimental results

We present and discuss experimental results on the behaviour of a smectic A placed in a parallel configuration and submitted to an alternating shear flow with velocity normal to the layers. The smectic A compound is 4-cyano-4n-octylbiphenyl (8CB). These results are compared with previous theoretical predictions. One then obtains the critical exponent for the smectic relaxation time. The nematic to smectic A transition appears to be weakly first order. A quantitative analysis of the stress relaxation mechanism above the instability threshold is presented.Nous présentons et discutons les résultats expérimentaux sur le comportement d'un smectique A en géométrie planaire soumis à un cisaillement alternati( le vecteur vitesse étant normal aux couches. Ces études portent sur le 4-cyano-4n-octylbiphenyl (8CB). La comparaison des résultats expérimentaux avec nos prédictions théoriques déjà publiées nous permet d'obtenir en particulier l'exposant critique du temps de relaxation du paramètre d'ordre smectique. La transition nématique-smectique apparaît alors être faiblement du premier ordre. On présente une analyse quantitative du mécanisme de relaxation des contraintes au-dessus du seuil d'instabilité

Transition from L1 to L2 in a ternary system (OBS-pentanol-water): a borderline case between continuous and discontinuous topological inversion

The phase diagram of the ternary system OBS-pentanol-water presents an apparently continuous thin pathway connecting the L1 and L 2 domains. It also shows a small triphasic region which, usually, is rather typical of disconnected L1 and L2 domains. Related to this borderline character, unique phenomena are observed and studied in the thin pathway. A plausible scenario for the topological inversion of structure is proposed for this particular situation.Le diagramme de phase du système ternaire OBS-pentanol-eau présente un étroit passage apparemment continu reliant les domaines d'existence de L1 et de L2. Il présente aussi une petite région triphasique qui est habituellement plutôt caractéristique d'une situation où L1 et L2 sont séparées. Associés à ce caractère limite, des phénomènes physiques originaux sont observés et étudiés dans le passage étroit. Un scénario plausible pour l'inversion topologique de structure est proposé pour cette situation particulière

Dynamics of the director alignment in cholesterics under applied magnetic field

By means of static and hydrodynamic theories, we study the rotary motion of the molecules in a cholesteric slab (of given thickness and strong anchored surfaces), subject to a weak magnetic field. The tilt angle ϕ( z, t) of the molecules is a simple sine wave with a time dependent amplitude characterized by a relaxation time of the order of 10-1 s.Nous étudions, au moyen des théories élastiques et hydrodynamiques, le mouvement de rotation des molécules, provoqué par l'action d'un champ magnétique relativement faible sur un échantillon cholestérique d'épaisseur déterminée avec ancrage fort aux parois. La solution est une fonction sinusoidale simple, dont l'amplitude dépendant du temps est caractérisée par un temps de relaxation de l'ordre de 10-1 s

Dynamics of the first single Grandjean-Cano line in cholesterics under weak applied magnetic fields

Experimental results and theoretical analysis of the dynamics of the first Grandjean-Cano line under a weak applied magnetic field are presented here. The dissipative process is essentially due to the director reorientation following the line motion. We therefore give a detailed study of the director configuration around the line, starting from de Gennes planar model. The dynamics of the line is then analysed and compares well with the experimental results. A macroscopic disclination core radius (≃ 0,5 μ) is deduced from this comparison.Nous présentons ici des résultats expérimentaux et une analyse théorique portant sur la dynamique de la première ligne de Grandjean-Cano sous champ magnétique faible. Le processus dissipatif est essentiellement dû à la réorientation du directeur lors du mouvement de la ligne ce qui nous a conduit à étudier de façon très détaillée la configuration autour de la ligne à partir du modèle planaire de de Gennes. Nous analysons ensuite la dynamique de la ligne. La comparaison entre les résultats théoriques et expérimentaux permet d'atteindre, entre autres paramètres physiques, le rayon du cœur de la disclinaison dont l'ordre de grandeur est du domaine macroscopique