Experimental and simulation study of the high-pressure behavior of squalane and poly-α-olefins.

The equation of state, dynamical properties, and molecular-scale structure of squalane and mixtures of poly-α-olefins at room temperature are studied with a combination of state-of-the-art, high-pressure experiments and molecular-dynamics simulations. Diamond-anvil cell experiments indicate that both materials are non-hydrostatic media at pressures above ∼1 GPa. The equation of state does not exhibit any sign of a first-order phase transition. High-pressure x-ray diffraction experiments on squalane show that there are no Bragg peaks, and hence, the apparent solidification occurs without crystallization. These observations are complemented by a survey of the equation of state and dynamical properties using simulations. The results show that molecular diffusion is essentially arrested above about 1 GPa, which supports the hypothesis that the samples are kinetically trapped in metastable amorphous-solid states. The shear viscosity becomes extremely large at very high pressures, and the coefficient governing its increase from ambient pressure is in good agreement with the available literature data. Finally, simulated radial distribution functions are used to explore the evolution of the molecular-scale structure with increasing pressure. Subtle changes in the short-range real-space correlations are related to a collapse of the molecular conformations with increasing pressure, while the evolution of the static structure factor shows excellent correlation with the available x-ray diffraction data. These results are of indirect relevance to oil-based lubricants, as the pressures involved are comparable to those found in engines, and hence, the ability of lubricating thin films to act as load-bearing media can be linked to the solidification phenomena studied in this work.

Determination of the bending rigidity of graphene via electrostatic actuation of buckled membranes.

Classical continuum mechanics is used extensively to predict the properties of nanoscale materials such as graphene. The bending rigidity, κ, is an important parameter that is used, for example, to predict the performance of graphene nanoelectromechanical devices and also ripple formation. Despite its importance, there is a large spread in the theoretical predictions of κ for few-layer graphene. We have used the snap-through behavior of convex buckled graphene membranes under the application of electrostatic pressure to determine experimentally values of κ for double-layer graphene membranes. We demonstrate how to prepare convex-buckled suspended graphene ribbons and fully clamped suspended membranes and show how the determination of the curvature of the membranes and the critical snap-through voltage, using AFM, allows us to extract κ. The bending rigidity of bilayer graphene membranes under ambient conditions was determined to be 35.5−15.0 +20.0 eV. Monolayers are shown to have significantly lower κ than bilayers.

Carbon nanotube field effect transistors with suspended graphene gates.

Novel field effect transistors with suspended graphene gates are demonstrated. By incorporating mechanical motion of the gate electrode, it is possible to improve the switching characteristics compared to a static gate, as shown by a combination of experimental measurements and numerical simulations. The mechanical motion of the graphene gate is confirmed by using atomic force microscopy to directly measure the electrostatic deflection. The device geometry investigated here can also provide a sensitive measurement technique for detecting high-frequency motion of suspended membranes as required, e.g., for mass sensing.

Dielectrophoresis-induced separation of metallic and semiconducting single-wall carbon nanotubes in a continuous flow microfluidic system.

The AC dielectrophoresis-induced separation of metallic and semiconducting single-wall carbon nanotubes has been carried out in a continuous flow microfluidic system with isolated electrodes. The separation has been studied for single-wall carbon nanotube aqueous suspensions with ionic (sodium dodecylsulphate) and non-ionic (TritonX-100) surfactants. The efficiency of separation has been determined with the help of resonant Raman spectroscopy using various excitation energies. The prototype microfluidic cell presently shows somewhat inferior separation efficiency with respect to static dielectrophoretic filtering on arrays of microelectrodes but has potential for improvements. Factors influencing the separation efficiency and scaling up of the process are discussed.

Tracing the origin of the HSC hierarchy reveals an SCF-dependent, IL-3-independent CD43(-) embryonic precursor.

Definitive hematopoietic stem cells (HSCs) develop in the aorta gonad mesonephros (AGM) region in a stepwise manner. Type I pre-HSCs express CD41 but lack CD45 expression, which is subsequently upregulated in type II pre-HSCs prior to their maturation into definitive HSCs. Here, using ex vivo modeling of HSC development, we identify precursors of definitive HSCs in the trunk of the embryonic day 9.5 (E9.5) mouse embryo. These precursors, termed here pro-HSCs, are less mature than type I and II pre-HSCs. Although pro-HSCs are CD41(+), they lack the CD43 marker, which is gradually upregulated in the developing HSC lineage. We show that stem cell factor (SCF), but not interleukin-3 (IL-3), is a major effector of HSC maturation during E9-E10. This study extends further the previously established hierarchical organization of the developing HSC lineage and presents it as a differentially regulated four-step process and identifies additional targets that could facilitate the generation of transplantable HSCs from pluripotent cells for clinical needs.