Reversibility and hysteresis of the sharp yielding transition of a colloidal glass under oscillatory shear.

The mechanical response of glasses remains challenging to understand. Recent results indicate that the oscillatory rheology of soft glasses is accompanied by a sharp non-equilibrium transition in the microscopic dynamics. Here, we use simultaneous x-ray scattering and rheology to investigate the reversibility and hysteresis of the sharp symmetry change from anisotropic solid to isotropic liquid dynamics observed in the oscillatory shear of colloidal glasses (D. Denisov, M.T. Dang, B. Struth, A. Zaccone, P. Schall, Sci. Rep. 5 14359 (2015)). We use strain sweeps with increasing and decreasing strain amplitude to show that, in analogy with equilibrium transitions, this sharp symmetry change is reversible and exhibits systematic frequency-dependent hysteresis. Using the non-affine response formalism of amorphous solids, we show that these hysteresis effects arise from frequency-dependent non-affine structural cage rearrangements at large strain. These results consolidate the first-order-like nature of the oscillatory shear transition and quantify related hysteresis effects both via measurements and theoretical modelling.

Resolving structural modifications of colloidal glasses by combining x-ray scattering and rheology.

Glasses have liquid-like structure, but exhibit solid-like properties. A central question concerns the relation between the structure and mechanical properties of glasses, but structural changes remain difficult to resolve. We use a novel combination of rheology and x-ray scattering to resolve structural changes in colloidal glasses and link them directly to their mechanical behavior. By combining stress and structure factor measurements, we resolve shear induced changes in the nearest neighbor configuration as a function of applied stress, allowing us to elucidate the structural origin of the genuine shear banding transition of glasses.

Nonlinear behavior of nematic platelet dispersions in shear flow.

Dispersions of platelets in the nematic phase are submitted to large amplitude oscillatory shear flow and probed by high temporal resolution small angle x-ray scattering. The response displays rich dynamic and structural behavior. Under small amplitude deformations we observe an elastic response, while structurally symmetry is broken: a preferential direction of deformation is selected which induces off-plane orientation of the platelets. We associate the elastic responses with the tilting director of the platelets towards the flow direction at all strain amplitudes. At large strain amplitudes there is a yielding transition between elastic and plastic deformation, accompanied by a flipping of the director. At intermediate strain amplitudes the director has a rich dynamic behavior, illustrating the complex motion of platelets in shear flow. These observations are confirmed by steady-shear flow reversal experiments, which underline the unique character of sheared nematic platelet dispersions.

Growth of bimolecular films of three-tailed amphiphiles.

The three-tailed amphiphile ferric stearate molecule, which forms a bimolecular layer on water surface with molecules in the lower and upper layers in different conformations, has been studied to understand transfer and growth of bimolecular films on the surface of hydrophilic silicon substrates. This bimolecular film forms a two-dimensional lattice on water with a slightly distorted hexagonal lattice where both the in-plane and out-of-plane domain sizes are small. The film also showed larger microscopic rigidity compared to its macroscopic mechanical response. This asymmetric bimolecular layer was found to be preserved when the film is transferred on the substrates at different values of surface pressures ranging from 1 mN/m to near-collapse (55 mN/m). Both the upper and lower layers become denser and interfaces between these layers become sharper with increase in deposition pressure but the growths have different natures. The lower layer of transferred film is dense from 1 mN/m and, except for a steplike increase between 20 and 30 mN/m, changes slowly in density. The density of the upper molecular layer grows continuously with surface pressure.

Observation of the desorption of mineral nanoparticles from a hybrid Langmuir monolayer: an in situ grazing incidence X-ray (GIXOS) study.

The kinetics and mechanism of the desorption of mineral clay particles originally adsorbed to a phospholipid monolayer has been studied. The desorption was induced by the injection of citric acid solution into the aqueous subphase. To follow the related kinetic processes, grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) was used with a remarkable time resolution of the order of one minute. From the variations of the electron density profile normal to the surface, the Langmuir desorption isotherm and the related kinetic constants were determined.

Dramatic enhancement of capillary wave fluctuations of a decorated water surface.

We have demonstrated by x-ray diffuse scattering that a bimolecular layer of a preformed three-tailed amphiphile, ferric stearate, drastically enhances capillary wave fluctuations on water surface due to a reduction in surface tension to 1 mN/m . The bimolecular layer is composed of molecules in symmetric configuration, on top of molecules in asymmetric configuration with ferric ions in contact with water. Unlike the usual Langmuir monolayers, this layer of molecules does not rupture under compression, but becomes thicker. This behavior mimics folding of a membrane on a liquid surface and is closely related to the cohesive interaction brought by the ferric ions. The low effective tension of this artificial membrane depends on the available area and reduces as the microscopic excess area increases.

X-ray synchrotron study of liquid-vapor interfaces at short length scales: effect of long-range forces and bending energies.

We have investigated the small-scale structure of the liquid-vapor interface using synchrotron x-ray scattering for liquids with different molecular structures and interactions. The effective momentum-dependent surface energy first decreases from its macroscopic value due to the effect of long-range forces, and then increases with increasing wave vector. The results are analyzed using a recent density functional theory. The large wave-vector increase is attributed to a bending energy for which local and nonlocal contributions are equally important.

Analysis of the cleaved topaz (001) surface.

We report on the first study of the cleaved (001) topaz surface and the characterization of the chemical composition and atomic arrangement of the surface. We conclude that there is strong evidence for a hydroxyl group termination appropriate for further chemical reactions. The surface itself is easily accessible, atomically flat and suitable for potential technological applications.

Rapid roughening in thin film growth of an organic semiconductor (diindenoperylene).

The scaling exponents alpha, beta, and 1/z in thin films of the organic molecule diindenoperylene deposited on SiO2 under UHV conditions are determined. Atomic-force microscopy, x-ray reflectivity, and diffuse x-ray scattering were employed. The surface width displays power law scaling over more than 2 orders of magnitude in film thickness. We obtained alpha = 0.684+/-0.06, beta = 0.748+/-0.05, and 1/zeta = 0.92+/-0.20. The derived exponents point to an unusually rapid growth of vertical roughness and lateral correlations. We suggest that they could be related to lateral inhomogeneities arising from the formation of grain boundaries between tilt domains in the early stages of growth.

[Treatment of undescended testis with IH-RH nasal spray]. / Die Behandlung des Maldescensus testis mit LH-RH-Nasalspray.

23 boys from one to 12 years old with 39 undescended testes were treated with LH-RH. This releasing hormone was administered in the form of a nasal spray six times a day. Irrespective of the body weight, the daily dose was 1.2 mg. The treatment was continued for a total of four weeks, even when success was already visible after two weeks. 16 testes reached the desired scrotal position. In addition, pendulous testes developed four times; five testes showed a partial descensus. Of twelve testes which did not alter their position in the craniocaudal direction, four were significantly better mobilizable than previously. Genital and extragenital side effects were rare. In this novel treatment principle, children are spared intramuscular injection (up to now with chorionic gonadotropin). According to available communications, the descensus rate is between 22 and 68% and is comparable to that after HCG therapy. With the release of LH and FSH, LH-RH imitates a physiological hormone phase and thereby leads to descensus. Accordingly, the side effects are less than after HCG. Taking into account the guidelines for treatment of maldescensus testis worked out by the National Health Foundation, this new therapy principle is to be recommended.