Fluid-solid transitions in photonic crystals of soft, thermoresponsive microgels.

Microgels are often discussed as well-suited model system for soft colloids. In contrast to rigid spheres, the microgel volume and, coupled to this, the volume fraction in dispersion can be manipulated by external stimuli. This behavior is particularly interesting at high packings where phase transitions can be induced by external triggers such as temperature in the case of thermoresponsive microgels. A challenge, however, is the determination of the real volume occupied by these deformable, soft objects and consequently, to determine the boundaries of the phase transitions. Here we propose core-shell microgels with a rigid silica core and a crosslinked, thermoresponsive poly-N-isopropylacrylamide (PNIPAM) shell with a carefully chosen shell-to-core size ratio as ideal model colloids to study fluid-solid transitions that are inducible by millikelvin changes in temperature. Specifically, we identify the temperature ranges where crystallization and melting occur using absorbance spectroscopy in a range of concentrations. Slow annealing from the fluid to the crystalline state leads to photonic crystals with Bragg peaks in the visible wavelength range and very narrow linewidths. Small-angle X-ray scattering is then used to confirm the structure of the fluid phase as well as the long-range order, crystal structure and microgel volume fraction in the solid phase. Thanks to the scattering contrasts and volume ratio of the cores with respect to the shells, the scattering data do allow for form factor analysis revealing osmotic deswelling at volume fractions approaching and also exceeding the hard sphere packing limit.

Phase behavior of ultrasoft spheres show stable bcc lattices.

The phase behavior of supersoft spheres is explored using solutions of ultralow cross-linked poly(N-isopropylacrylamide)-based microgels as a model system. For these microgels, the effects of the electric charges on their surfaces can be neglected and therefore only the role of softness on the phase behavior is investigated. The samples show a liquid-to-crystal transition at higher volume fraction with respect to both hard spheres and stiffer microgels. Furthermore, stable body centered cubic (bcc) crystals are observed in addition to the expected face centered cubic (fcc) crystals. Small-angle x-ray and neutron scattering with contrast variation allow the characterization of both the microgel-to-microgel distance and the architecture of single microgels in crowded solutions. The measurements reveal that the stable bcc crystals depend on the interplay between the collapse and the interpenetration of the external shell of the ultralow cross-linked microgels.

Exploring the colloid-to-polymer transition for ultra-low crosslinked microgels from three to two dimensions.

Microgels are solvent-swollen nano- and microparticles that show prevalent colloidal-like behavior despite their polymeric nature. Here we study ultra-low crosslinked poly(N-isopropylacrylamide) microgels (ULC), which can behave like colloids or flexible polymers depending on dimensionality, compression or other external stimuli. Small-angle neutron scattering shows that the structure of the ULC microgels in bulk aqueous solution is characterized by a density profile that decays smoothly from the center to a fuzzy surface. Their phase behavior and rheological properties are those of soft colloids. However, when these microgels are confined at an oil-water interface, their behavior resembles that of flexible macromolecules. Once monolayers of ultra-low crosslinked microgels are compressed, deposited on solid substrate and studied with atomic-force microscopy, a concentration-dependent topography is observed. Depending on the compression, these microgels can behave as flexible polymers, covering the substrate with a uniform film, or as colloidal microgels leading to a monolayer of particles.

Hollow microgels squeezed in overcrowded environments.

We study how a cavity changes the response of hollow microgels with respect to regular ones in overcrowded environments. The structural changes of hollow poly(N-isopropylacrylamide) microgels embedded within a matrix of regular ones are probed by small-angle neutron scattering with contrast variation. The form factors of the microgels at increasing compressions are directly measured. The decrease of the cavity size with increasing concentration shows that the hollow microgels have an alternative way with respect to regular cross-linked ones to respond to the squeezing due to their neighbors. The structural changes under compression are supported by the radial density profiles obtained with computer simulations. The presence of the cavity offers to the polymer network the possibility to expand toward the center of the microgels in response to the overcrowded environment. Furthermore, upon increasing compression, a two step transition occurs: First the microgels are compressed but the internal structure is unchanged; then, further compression causes the fuzzy shell to collapse completely and reduce the size of the cavity. Computer simulations also allow studying higher compression degrees than in the experiments leading to the microgel's faceting.

Cannabis use and psychosis: re-visiting the role of childhood trauma.

BACKGROUND: Cannabis consumption continues to be identified as a causal agent in the onset and development of psychosis. However, recent findings have shown that the effect of cannabis on psychosis may be moderated by childhood traumatic experiences. METHOD: Using hierarchical multivariate logistic analyses the current study examined both the independent effect of cannabis consumption on psychosis diagnosis and the combined effect of cannabis consumption and childhood sexual abuse on psychosis diagnosis using data from the Adult Psychiatric Morbidity Survey 2007 (n=7403). RESULTS: Findings suggested that cannabis consumption was predictive of psychosis diagnosis in a bivariate model; however, when estimated within a multivariate model that included childhood sexual abuse, the effect of cannabis use was attenuated and was not statistically significant. The multivariate analysis revealed that those who had experienced non-consensual sex in childhood were over six times [odds ratio (OR) 6.10] more likely to have had a diagnosis of psychosis compared with those who had not experienced this trauma. There was also a significant interaction. Individuals with a history of non-consensual sexual experience and cannabis consumption were over seven times more likely (OR 7.84) to have been diagnosed with psychosis compared with those without these experiences; however, this finding must be interpreted with caution as it emerged within an overall analytical step which was non-significant. CONCLUSIONS: Future studies examining the effect of cannabis consumption on psychosis should adjust analyses for childhood trauma. Childhood trauma may advance existing gene-environment conceptualisations of the cannabis-psychosis link.

Nanomechanics of biocompatible TiO(2) nanotubes by Interfacial Force Microscopy (IFM).

Titanium dioxide (TiO(2)) coatings exhibit desirable properties as biocompatible coatings. In this paper we report on mechanical properties and deformation behavior of (TiO(2)) nanotubes grown on pure titanium substrates through anodic oxidation. Characterization of the as-processed coatings was conducted using scanning electron microscopy (SEM). Nanoindentation, using Interfacial Force Microscopy (IFM), was employed to probe the Young's modulus of the nanotubes. Using the IFM technique, the modulus of the nanotube coating may be measured with minimal contribution from the underlying Ti substrate. The modulus of the (TiO(2)) nanotube coating was estimated at 4-8 GPa. This technique was also used to study the inelastic deformation behavior of the nanotubes. (TiO(2)) nanotubes were found to inelastically deform by "tube crushing" in the immediate vicinity of indenter tip, increasing the local density. This increase in local density caused an increase in the Young's modulus from roughly 4 GPa to 30 GPa in the first 30 nm of indentation. Densification and the resulting increase in elastic modulus are related to the total work of inelastic deformation, irrespective of the loading history.

Exploring the liquid-like layer on the ice surface.

Using interfacial force microscopy and a spherical glass probe, we investigate the adhesive and mechanical properties of the so-called liquid-like layer (L-LL) on the surface of ice at various temperatures over the range from -10 to -30 degrees C. We find that the layer thickness closely follows that predicted on thermodynamic grounds, while the adhesive interaction has the behavior of a "frustrated capillary", strongly suggesting that the layer is viscoelastic. This viscoelasticity is directly probed using a lateral-dither technique to obtain information on the layer's viscous response as a function of both temperature and interfacial separation.

Nanomechanical contrasts of gel and fluid phase supported lipid bilayers.

Lipid bilayers exhibit structural diversity that contributes to the complex properties of the cell membrane. We use interfacial force microscopy to correlate mechanical properties with the two-dimensional phase behavior of supported lipid bilayers (SLBs). Upon indentation by a 500 nm tungsten tip, a contrast in the mechanical response is observed for gel vs fluid phase SLBs. We measure the yield force and time scale for recovery for these films. Consistent with a gel phase, a DSPC SLB has a relatively high yield force and slow recovery. In the higher mobility fluid phase, a DLPC SLB has a lower yield force and completely recovers within the experimental time scale. Friction measurements offer further contrast between the two SLBs.

Superplastic nanowires pulled from the surface of common salt.

Superplastic nanowires were formed by touching the NaCl(100) surface with a Au tip in a transmission electron microscope. The nanowires were stretched < or =2.2 microm, or 280%, and bent >90 degrees upon compression, when showered with the electron beam. More surprisingly, no dislocations were observable during the elongation due to fast diffusion. Mechanical measurements in humid atmospheres suggest that salt nanowires also form in ambient environments.

Density dependent friction of lipid monolayers.

We measure frictional properties of liquid-expanded and liquid-condensed phases of lipid Langmuir-Blodgett monolayers by interfacial force microscopy. We find that over a reasonably broad surface-density range, the friction shear strength of the lipid monolayer film is proportional to the surface area (42-74 A2/molecule) occupied by each molecule. The increase in frictional force (i.e., friction shear strength with molecular area can be attributed to the increased conformational freedom and the resulting increase in the number of available modes for energy dissipation.

Hydrophilicity and the viscosity of interfacial water.

We measure the viscosity of nanometer-thick water films at the interface with an amorphous silica surface. We obtain viscosity values from three different measurements: friction force in a water meniscus formed between an oxide-terminated W tip and the silica surface under ambient conditions; similar measurements for these interfaces under water; and the repulsive "drainage" force as the two surfaces approach at various speeds in water. In all three cases, we obtain effective viscosities that are approximately 10(6) times greater than that of bulk water for nanometer-scale interfacial separations. This enhanced viscosity is not observed when we degrade the hydrophilicity of the surface by terminating it with -H or -CH3. In view of recent results from other interfaces, we conclude that the criterion for the formation of a viscous interphase is the degree of hydrophilicity of the interfacial pair.

Viscous water meniscus under nanoconfinement.

A dramatic transition in the mechanical properties of water is observed at the nanometer scale. For a water meniscus formed between two hydrophilic surfaces in the attractive region, with < or = 1 nm interfacial separation, the measured viscosity is 7 orders of magnitude greater than that of bulk water at room temperature. Grand canonical Monte Carlo simulations reveal enhancement in the tetrahedral structure and in the number of hydrogen bonds to the surfaces as a source for the high viscosity; this results from a cooperative effect of hydrogen bonding of water molecules to both hydrophilic surfaces.

Role of stress on charge transfer through self-assembled alkanethiol monolayers on Au.

We have studied charge transfer through alkanethiol molecules self-assembled on Au(111) substrates using interfacial force microscopy. Simultaneous measurement of the tip-substrate current and the normal interfacial force reveals the critical role of tip-film contact. Measurable currents are seen only for tip-applied stresses above about 20 MPa, after which the current rises exponential with stress. We suggest that charge transfer results from stress-induced band-gap states near the Fermi level in these normally highly insulating molecular films.

The mechanical response of gold substrates passivated by self-assembling monolayer films.

Interfacial force microscopy has been used to show that a single layer of self-assembling molecules adsorbed on a gold substrate can prevent adhesion between gold and a tungsten probe. The passivated gold is able to elastically support large repulsive loads, with plots of load versus deformation closely following the Hertzian model. The gold shear-stress threshold for plastic deformation is determined to be approximately 1 gigapascal, which is in agreement with the theoretical value for the intrinsic gold-lattice stability.

Catalysis: new perspectives from surface science.

One-sixth of the value of all goods manufactured in the United States involves catalytic processes. However, in spite of this dramatic economic impact, little is known about this broad subject at the molecular level. In the last two decades a variety of techniques have been developed for studying at the atomic level the structure, composition, and chemical bonding at surfaces. These techniques have been used to study adsorption and reaction on metal single crystals in an ultrahigh vacuum environment or to analyze catalysts before and after reaction. An important new development has been the coupling of an apparatus for the measurement of reaction kinetics at elevated pressures with an ultrahigh vacuum system for surface analysis. This approach has demonstrated that metal single crystals can be used to successfully model many important catalytic reactions and has established a direct link between the results of ultrahigh vacuum surface measurements and the chemistry that occurs under typical catalytic-processing conditions.

Surface electronic properties of tungsten, tungsten carbide, and platinum.

The local electronic structures of the surface regions of tungsten, tungsten carbide, and platinum have been compared. Contrary to the hypothesis that the platinum-like catalytic activity of tungsten carbide results from the contribution of carbon valence electrons to the 5d band of tungsten, the width of the unfilled portion of the d band increases on going from tungsten to tungsten carbide.