A self-assembled phospholipid monolayer at an air–water interface is a well-defined model system for studying surface thermodynamics, membrane biophysics, thin-film materials, and colloidal soft matter. Here we report a study of two-dimensional phase transitions in the dipalmitoylphosphatidylcholine (DPPC) monolayer at the air–water interface using a newly developed methodology called constrained drop surfactometry (CDS). CDS is superior to the classical Langmuir balance in its capacity for rigorous temperature control and leak-proof environments, thus making it an ideal alternative to the Langmuir balance for studying lipid polymorphism. In addition, we have developed a novel Langmuir–Blodgett (LB) transfer technique that allows the direct transfer of lipid monolayers from the droplet surface under well-controlled conditions. This LB transfer technique permits the direct visualization of phase coexistence in the DPPC monolayer. With these technological advances, we found that the two-dimensional phase behavior of the DPPC monolayer is analogous to the three-dimensional phase transition of a pure substance. This study has implications in the fundamental understanding of surface thermodynamics as well as applications such as self-assembled monolayers and pulmonary surfactant biophysics.

Background:The contribution of long-chain acyl-CoA dehydrogenase (LCAD) to human fatty acid oxidation is not understood. Results: LCAD localizes to lung alveolar type II cells, which produce pulmonary surfactant; LCAD-deficient mice have surfactant dysfunction. Conclusion: LCAD is important for lung energy metabolism and lung function. Significance: LCAD may play a role in human lung disease and unexplained sudden infant death.

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Mo- and S-based lubricant additives reduce friction in boundary lubrication through the formation of molybdenum disulfide (MoS2) during operation. However, the fundamental mechanisms of MoS2 formation are still not fully understood, in part because direct experimental measurement is challenging during the crystallization process. Previously, reactive molecular dynamics simulations were used to model the formation of crystalline MoS2 by compressing and heating amorphous material consisting of Mo and S. Here, the authors test the robustness of these models to capture the crystallization process under different simulation conditions and with different reactive force fields. Lastly, a reactive force field that contains parameters for Mo, S, and O was modified to enable it to capture MoS2 crystallization in the presence of oxygen.

Many emerging devices and technologies rely on contacts between nanoscale bodies. Recent analytical theories, experiments, and simulations of nanocontacts have made conflicting predictions about the mechanical response as these contacts are loaded and separated. The present investigation combined in situ transmission electron microscopy (TEM) and molecular dynamics (MD) simulation to study the contact between a flat diamond indenter and a nanoscale silicon tip. The TEM was used to pre-characterize the materials, such that an atomistic model tip could be created with identically matched materials, geometry, crystallographic orientation, loading conditions, and degree of amorphization. A large work of adhesion was measured in the experiment and attributed to unpassivated surfaces and a large compressive stress applied before separation, resulting in covalent bonding across the interface. The simulations modeled atomic interactions across the interface using a Buckingham potential in order to reproduce the experimental work of adhesion without explicitly modeling covalent bonds, thereby enabling larger time-and length-scale simulations than would be achievable with a reactive potential. Then, the experimental and simulation tips were loaded under similar conditions with real-time measurement of contact area and deformation, yielding three primary findings. First, the results demonstrated that significant variation in the value of contact area can be obtained from simulations, depending on the technique used to determine it. Therefore, care is required in comparing measured values of contact area between simulations and experiments. Second, the contact area and deformation demonstrated significant hysteresis, with larger values measured upon unloading as compared to loading. Therefore, continuum predictions, in the form of a Maugis-Dugdale contact model, could not be fit to full loading/unloading curves. Third, the loaddependent contact area could be accurately fit by allowing the work of adhesion in the continuum model to increase with applied force from 1.3 to 4.3 J/m 2. The most common mechanisms for hysteretic behavior-which are viscoelasticity, capillary interactions, and plasticity-can be ruled out using the TEM and atomistic characterization. Stress-dependent formation of covalent bonds is suggested as a physical mechanism to describe these findings, which is qualitatively consistent with trends in the areal density of in-contact atoms as measured in the simulation. The implications of these results for real-world nanoscale contacts are that significant hysteresis may cause significant and unexpected deviations in contact size, even for nominally elastic contacts.