Corinne Chevallard

Crystalline amyloid structures at interfaces.

Laying the groundwork: The interfacial self-assembly properties of an amyloid peptide were used to develop crystalline nanostructures at air-water interfaces, which were studied by both AFM microscopy and X-ray diffraction (see image). These structures generate regular arrays of functional groups and pave the way to controlled deposition of inorganic materials like that observed in biomineralization.

Long-Range Nanometer-Scale Organization of Semifluorinated Alkane Monolayers at the Air/Water Interface

We have determined the structure formed at the air-water interface by semifluorinated alkanes (C(8)F(17)C(m)H(2m+1) diblocks, F8Hm for short) for different lengths of the molecule (m = 14, 16, 18, 20) by using surface pressure versus area per molecule isotherms, Brewster angle microscopy (BAM), and grazing incidence x-ray experiments (GISAXS and GIXD). The behavior of the monolayers of diblocks under compression is mainly characterized by a phase transition from a low-density phase to a condensed phase. The nonzero surface pressure phase is crystalline and exhibits two hexagonal lattices at two different scales: a long-range-order lattice of a few tens of nanometers lateral parameter and a molecular array of about 0.6 nm parameter. The extent of this organization is sufficiently large to impact larger scale behavior. Analysis of the various compressibilities evidences the presence of non organized molecules in the monolayer for all 2D pressures. At room temperature, the self-assembled structure appears generic for all the F8Hm investigated.

Interface dynamics in liquid crystals

Abstract:We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns which have been both experimentally and theoretically pointed out.

CaCO3 nanostructured crystals induced by nacreous organic extracts

Nanostructured CaCO3 crystals are produced in vitro by adding the acid-soluble organic matrix (SOM) extracted from the nacreous layer of Pinctada margaritifera pearl oyster shell to a supersaturated solution of calcium carbonate. For SOM concentrations above 25 ppm, the produced calcite crystals show two structural features of nacre and, more generally, of calcareous biocrystals: structuring at the nanometer scale and a hierarchical structure at higher scales. More specifically, the produced polycrystals exhibit hierarchical structures with stacks of mineral layers, the thickness of which is about 200–400 nm. AFM and SEM microscopies supplemented by FTIR spectrometry allowed us to conclude that these nanostructured layers correspond to packed ‘nanograins’ embedded in an organic matrix. Besides, each layering plane exhibits a single crystal character under TEM and SAED characterizations and can be indexed to the {110} planes of calcite. The reported work sheds light on the crucial role of SOM in the formation of oriented nanostructures, which are generally observed in biological calcareous tissues, like nacre.

Limpet Shells from the Aterian Level 8 of El Harhoura 2 Cave (Témara, Morocco): Preservation State of Crossed-Foliated Layers

The exploitation of mollusks by the first anatomically modern humans is a central question for archaeologists. This paper focuses on level 8 (dated around ∼ 100 ka BP) of El Harhoura 2 Cave, located along the coastline in the Rabat-Témara region (Morocco). The large quantity of Patella sp. shells found in this level highlights questions regarding their origin and preservation. This study presents an estimation of the preservation status of these shells. We focus here on the diagenetic evolution of both the microstructural patterns and organic components of crossed-foliated shell layers, in order to assess the viability of further investigations based on shell layer minor elements, isotopic or biochemical compositions. The results show that the shells seem to be well conserved, with microstructural patterns preserved down to sub-micrometric scales, and that some organic components are still present in situ. But faint taphonomic degradations affecting both mineral and organic components are nonetheless evidenced, such as the disappearance of organic envelopes surrounding crossed-foliated lamellae, combined with a partial recrystallization of the lamellae. Our results provide a solid case-study of the early stages of the diagenetic evolution of crossed-foliated shell layers. Moreover, they highlight the fact that extreme caution must be taken before using fossil shells for palaeoenvironmental or geochronological reconstructions. Without thorough investigation, the alteration patterns illustrated here would easily have gone unnoticed. However, these degradations are liable to bias any proxy based on the elemental, isotopic or biochemical composition of the shells. This study also provides significant data concerning human subsistence behavior: the presence of notches and the good preservation state of limpet shells (no dissolution/recrystallization, no bioerosion and no abrasion/fragmentation aspects) would attest that limpets were gathered alive with tools by Middle Palaeolithic (Aterian) populations in North Africa for consumption.

Monovalent cations trigger inverted bilayer formation of surfactant films.

We monitored single-layer Langmuir-Blodgett films of behenic acid deposited on silanized glass or silicon substrates by atomic force microscopy (AFM) in liquid. We observed the in situ transformation of the monolayer to a bilayer when the surrounding solution was NaOH or KOH with pH > 8.3. The final state is that of an inverted bilayer, in which both the hydrophobic OTS (octadecyltrichlorosilane) and the alkane chains are exposed to the surrounding solution, defying common intuition based on hydrophobic-hydrophilic energy considerations. Strong sodium-containing carboxylic dimers formed between the headgroups are shown to be responsible for the stabilization of this configuration; calcium ions slow down/inhibit the transformation.

“Magic Spiral” Submitted to a Torque: Topological Flows Driven by Ericksen Stresses in Sm C Films

Abstract We will show here that, in a two-dimensional liquid crystal system where disclinations are present, the elastic torque cannot balance alone an external torque; a hydrodynamic flow must indeed occur so that the system can reach a stationary state. We suggest calling such flows topological flows. We will indicate how they are produced by the Ericksen forces.

Unusual Micrometric Calcite-Aragonite Interface in the Abalone Shell Haliotis ~Mollusca, Gastropoda!

Species of Haliotis (abalone) show high variety in structure and mineralogy of the shell. One of the European species (Haliotis tuberculata) in particular has an unusual shell structure in which calcite and aragonite coexist at a microscale with small patches of aragonite embedded in larger calcitic zones. A detailed examination of the boundary between calcite and aragonite using analytical microscopies shows that the organic contents of calcite and aragonite differ. Moreover, changes in the chemical composition of the two minerals seem to be gradual and define a micrometric zone of transition between the two main layers. A similar transition zone has been observed between the layers in more classical and regularly structured mollusk shells. The imbrication of microscopic patches of aragonite within a calcitic zone suggests the occurrence of very fast physiological changes in these taxa.

Unusual Micrometric Calcite- Aragonite Interface in the Abalone Shell Haliotis similar to Mollusca, Gastropoda!

Species of Haliotis (abalone) show high variety in structure and mineralogy of the shell. One of the European species (Haliotis tuberculata) in particular has an unusual shell structure in which calcite and aragonite coexist at a microscale with small patches of aragonite embedded in larger calcitic zones. A detailed examination of the boundary between calcite and aragonite using analytical microscopies shows that the organic contents of calcite and aragonite differ. Moreover, changes in the chemical composition of the two minerals seem to be gradual and define a micrometric zone of transition between the two main layers. A similar transition zone has been observed between the layers in more classical and regularly structured mollusk shells. The imbrication of microscopic patches of aragonite within a calcitic zone suggests the occurrence of very fast physiological changes in these taxa.

Cladis' orbiting disclinations in smectic films submitted to a torque

Abstract We study here the behaviour of S=+1 and S=-1 disclinations in SmC films submitted to a rotating electric field. In agreement with Cladis et al. experiments we observe that a S=+1 disclination submitted to large enough torque leaves the centre of the film and is orbiting around a target-like pattern. A single S=-1 disclination submitted to a torque behaves differently: independently of the applied torque, it stays in the centre of the film. We explain this difference as due to the presence (S=+1) and the absence (S=-1) of energy barriers with respect to the phase winding.