Properties of the organic matrix of bone as well as its function in the microstructure could be the key to the remarkable mechanical properties of bone. Previously, it was found that on the molecular level, calcium-mediated sacrificial bonds increased stiffness and enhanced energy dissipation in bone constituent molecules. Here we present evidence for how this sacrificial bond and hidden length mechanism contributes to the mechanical properties of the bone composite, by investigating the nanoscale arrangement of the bone constituents and their interactions. We find evidence that bone consists of mineralized collagen fibrils and a non-fibrillar organic matrix, which acts as a 'glue' that holds the mineralized fibrils together. We believe that this glue may resist the separation of mineralized collagen fibrils. As in the case of the sacrificial bonds in single molecules, the effectiveness of this mechanism increases with the presence of Ca2+ ions.

We compare the ''long-range hydrophobic forces'' measured (i) in the ''symmetric'' system between two mica surfaces that had been rendered hydrophobic by the adsorption of a double-chained cationic surfactant, and (ii) between one such hydrophobic surface and a hydrophilic surface of bare mica (''asymmetric'' case). In both cases, the forces were purely attractive, stronger than van der Waals, and of long-range, as previously reported, with those of the asymmetric, hydrophobic-hydrophilic system being even stronger and of longer range. Atomic force microscopy images of these surfaces show that the monolayers transform into patchy bilayers when the surfaces are immersed in water, and that the resulting surfaces contain large micrometer-sized regions of positive charges (bilayer) and negative charges (bare mica) while remaining overall neutral. The natural alignment of oppositely charged domains as two such surfaces approach would result in a long-range electrostatic attraction in water, but the short-range, ''truly hydrophobic'' interaction is not explained by these results.forces ͉ hydrophobic ͉ Langmuir-Blodgett films ͉ surfactant monolayers T he hydrophobic interaction is among the most important nonspecific interactions in biological and many colloidal systems. The significant role of the hydrophobic interaction has led to a great deal of study and yet, over 20 years since the first direct measurement of the attraction between two nominally hydrophobic surfaces (1, 2), no single theory is able to account for all observed experimental behavior. One source of confusion in determining the origins of the long-range hydrophobic interaction is the apparent existence of two different force regimes. It has been suggested (3-6) that the measured force between hydrophobic surfaces is in fact a combination of a ''truly hydrophobic'' short-range force (D Ͻ 10 nm) and a longerranged force (D Ͼ 10 nm) due to a mechanism unrelated or only indirectly related to the hydrophobicity of the surfaces. Suggested mechanisms for the long-range attraction include electrostatic charge or correlated dipole-dipole interactions (7-13), water structure (2, 14), phase metastability (15, 16), and preexisting submicroscopic bubbles that bridge the surfaces (17-19). Although there is convincing evidence of bridging nanobubbles between some types of surfaces (18,20), it has become clear that none of these models can explain all of the forces observed between the many different surfaces studied so far.Langmuir-Blodgett (LB)-deposited monolayers of cationic surfactants such as dimethyl-dioctadecyl-ammonium bromide (DODAB) have been used often in the past 25 years to study the hydrophobic interaction (3,15,16,(21)(22)(23), but the data presented in this article indicate that the long-range attraction between such surfaces may not be directly related to their hydrophobicity. In this article, surface forces apparatus (SFA) force measurements and atomic force microscopy (AFM) imaging are combined. Both symmetric (hydrophobic-hydrophobic) and asymmet...

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On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days. Concerns also arose about health risks from fallout, because ash can transport acids as well as toxic compounds, such as fluoride, aluminum, and arsenic. Studies on ash are usually made on material collected far from the source, where it could have mixed with other atmospheric particles, or after exposure to water as rain or fog, which would alter surface composition. For this study, a unique set of dry ash samples was collected immediately after the explosive event and compared with fresh ash from a later, more typical eruption. Using nanotechniques, custom-designed for studying natural materials, we explored the physical and chemical nature of the ash to determine if fears about health and safety were justified and we developed a protocol that will serve for assessing risks during a future event. On single particles, we identified the composition of nanometer scale salt coatings and measured the mass of adsorbed salts with picogram resolution. The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers. Edges remained sharp even after a couple of weeks of abrasion in stirred water suspensions.

The interaction of OH-containing compounds with calcite, CaCO(3), such as is required for the processes that control biomineralization, has been investigated in a low-water solution. We used ethanol (EtOH) as a simple, model, OH-containing organic compound, and observed the strength of its adsorption on calcite relative to OH from water and the consequences of the differences in interaction on crystal growth and dissolution. A combination of atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that EtOH attachment on calcite is stronger than HOH binding and that the first adsorbed layer of ethanol is highly ordered. The strong ordering of the ethanol molecules has important implications for mineral growth and dissolution because it produces a hydrophobic layer. Ethanol ordering is disturbed along steps and at defect sites, providing a bridge from the bulk solution to the surface. The strong influence of calcite in structuring ethanol extends further into the liquid than expected from electrical double-layer theory. This suggests that in fluids where water activity is low, such as in biological systems optimized for biomineralization, organic molecules can control ion transport to and from the mineral surface, confining it to specific locations, thus providing the organism with control for biomineral morphology.

Pore surface properties control oil recovery. This is especially true for chalk reservoirs, where pores are particularly small. Wettability, the tendency for a surface to cover itself with fluid, is traditionally defined by the angle a droplet makes with a surface, but this macroscopic definition is meaningless when the particles are smaller than even the smallest droplet. Understanding surface wetting, at the pore scale, will provide clues for more effective oil recovery. We used a special mode of atomic force microscopy and a hydrophobic tip to collect matrices of 10,000 force curves over 5-؋ 5-m 2 areas on internal pore surfaces and constructed maps of topography, adhesion, and elasticity. We investigated chalk samples from a water-bearing formation in the Danish North Sea oil fields that had never seen oil. Wettability and elasticity were inhomogeneous over scales of 10s of nanometers, smaller than individual chalk particles. Some areas were soft and hydrophobic, whereas others showed no correlation between hardness and adhesion. We conclude that the macroscopic parameter, ''wetting,'' averages the nanoscopic behavior along fluid pathways, and ''mixed-wet'' samples have patches with vastly different properties. Development of reservoir hydrophobicity has been attributed to infiltrating oil, but these new results prove that wettability and elasticity are inherent properties of chalk. Their variability, even on single particles, must result from material originally present during sedimentation or material sorbed from the pore fluid some time later.AFM ͉ force maps ͉ wettability ͉ oil recovery ͉ adhesion

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In marine mussels (Mytilus), byssal threads are made in minutes from prefabricated smectic polymer liquid crystals by a process resembling reaction injection molding. The mesogens in these arrays are known to be natural block copolymers with rodlike collagen cores. Using atomic force microscopy, it was shown that these collagenous mesogens are bent-core or banana-shaped in a manner that is consistent with and predictable from their amino acid sequence. The overall bend angle in preCOL-NG in Mytilus galloprovincialis is about 130 degrees. The mesogens have a center-to-center separation of approximately 22 nm and a length of 200 nm. It is evident that the smectic structure of the prefabricated mesophases remains largely intact over 1-3 microm distances in the molded fibers and is presumably locked in place during molding by cross-linking. Like the smectic liquid crystals of many synthetic banana mesogens, the collagenous mesogens of the byssal threads exhibit SmC(2) symmetry with a characteristic tilt of 24.6 degrees. At about 100% extension, this tilt is considerably reduced and the globular end domains are no longer visible presumably because they have been unraveled.