Numerical investigations of translocation characteristics of nano-silica lunar dust across pulmonary surfactant monolayer.

The interactions between nano-silica lunar dust (NSLD) on the moon surface and pulmonary surfactant (PS) monolayer will pose risks to astronaut health in future manned lunar exploration missions, but the specifics of these interactions are unknown. This study investigates them using the coarse-grained molecular dynamics method considering different sizes (5, 10, and 15 nm) and shapes (sphere, ellipsoid, and cube), with special focus on the unique morphology of NSLDs with bugles. The key findings are as follows: (1) The 10 nm and 15 nm NSLDs embed in the PS monolayer through the major sphere of spherical-type, major ellipsoid of ellipsoidal-type, or one edge of cubic-type NSLDs upon contact the PS monolayer. (2) Adsorbed NSLDs cause a higher Sz value (ASz > 0.84), while embedded NSLDs cause a lower Sz value (0.47 < ASz < 0.83) that decreases with an increase in the number of bulges. (3) The embedding process absorbs 50-342 dipalmitoylphosphatidylcholine (DPPC) molecules, reducing the PS monolayer area by 0.21%-6.05%. NSLDs with bulges absorb approximately 9-126 additional DPPC molecules and cause a 0.05%-3.22% reduction in the PS monolayer area compared to NSLDs without bulges. (4) NSLDs move obliquely or vertically within the PS monolayer, displaying two distinct stages with varying velocities. Their movement direction and speed are influenced by the increasing complexity of NSLD with more bulges on them. In general, larger NSLDs with sharper shapes and increasing complex morphology of more bulges cause more significant damages to the PS monolayer. These findings have implications for safeguarding astronaut health in future manned lunar exploration missions.

Assessing airflow unsteadiness in the human respiratory tract under different expiration conditions.

To enhance the understanding of airflow characteristics in the human respiratory system, the expiratory airflow in a human respiratory tract model was simulated using large eddy simulation and dynamic mesh under different expiration conditions aligned with clinically measured data. The airflow unsteadiness was quantitatively assessed using power spectral density (PSD) and spectral entropy (SE). The following findings were obtained: (1) The airflow is highly turbulent in the mouth-pharynx region during expiration, with its dynamic characteristics being influenced by both the transient expiration flow pattern at mouth piece and the glottis motion. (2) PSD analysis reveals that the expiratory airflow is very unsteady, exhibiting a broad-band attenuation spectrum in the pharynx-trachea region. When only transient expiration or glottis motion is considered, the PSD spectrum changes slightly. When both are ignored, however, the change is significant, with the peak frequency reduced to 10% of the real expiration condition. (3) SE analysis indicates that the airflow transitions into turbulence in the trachea, and there may be multiple transitions in the region of soft palate. The transient expiration or glottis motion alone increases turbulence intensity by 2%-15%, while ignoring both reduces turbulence intensity by 10%-20%. This study implies that turbulence characteristics can be significantly different under different expiratory conditions, and therefore it is necessary to determine the expiratory flow characteristics using clinically measured expiratory data.

Raman Spectrum of the Li2SO4-MgSO4-H2O System: Excess Spectrum and Hydration Shell Spectrum.

Lithium, as a green energy metal used to promote world development, is an important raw material for lithium-ion, lithium-air, and lithium-sulfur batteries. It is challenging to directly extract lithium resources from brine with a high Mg/Li mass ratio. The microstructure study of salt solutions provides an important theoretical basis for the separation of lithium and magnesium. The changes in the hydrogen bond network structure and ion association of the Li2SO4 aqueous solution and Li2SO4-MgSO4-H2O mixed aqueous solution were studied by Raman spectroscopy. The SO42- fully symmetric stretching vibration peak at 940~1020 cm-1 and the O-H stretching vibration peak at 2800~3800 cm-1 of the Li2SO4 aqueous solution at room temperature were studied by Raman spectroscopy and excess spectroscopy. According to the peak of the O-H stretching vibration spectrum, with an increase in the mass fraction of the Li2SO4 solution, the proportion of DAA-type and DDAA-type hydrogen bonds at low wavenumbers decreases gradually, while the proportion of DA-type hydrogen bonds at 3300 cm-1 increases. When the mass fraction is greater than 6.00%, this proportion increases sharply. Although the spectra of hydrated water molecules and bulk water molecules are different, the spectra of the two water molecules seriously overlap. The spectrum of the anion hydration shell in a solution can be extracted via spectrum division. By analyzing the spectra of these hydration shells, the interaction between the solute and water molecules, the structure of the hydration shell and the number of water molecules are obtained. For the same ionic strength solution, different cationic salts have different hydration numbers of anions, indicating that there is a strong interaction between ions in a strong electrolytic solution, which will lead to ion aggregation and the formation of ion pairs. When the concentration of salt solution increases, the hydration number decreases rapidly, indicating that the degree of ion aggregation increases with increasing concentration.

Effects of face shield on an emitter during a cough process: A large-eddy simulation study.

Face shield is a common personal protection equipment for pandemic. In the present work, three-dimensional computational fluid dynamic (CFD) method is used to simulate a cough jet from an emitter who wears a face shield. A realistic manikin model with a simplified mouth cavity is employed. A large eddy simulation with a dynamic structure subgrid scale model is applied to model the turbulence. An Eulerian-Lagrangian approach is adopted to model the two-phase flows, with which the droplets are represented by a cloud of particles. The droplet breakup, evaporation, dispersion, drag force, and wall impingement are considered in this model. An inlet velocity profile that is based on a variable mouth opening area is considered. Special attentions have been put the vortex structure and droplet re-distribution induced by the face shield. It is found that the multiple vortices are formed when the cough jet impinges on the face shield. Some droplets move backward and others move downward after the impinging. It is also found that a small modification of the face shield significantly modifies the flow field and droplet distribution. We conclude that face shield significantly reduces the risk factor in the front of the emitter, meanwhile the risk factor in the back of the emitter increases. When the receiver standing in front of the emitter is shorter than the emitter, the risk is still very high. More attentions should be paid on the design of the face field, clothes cleaning and floor cleaning of the emitters with face shields. Based on the predicted droplet trajectory, a conceptual model for droplet flux is proposed for the scenario with the face shield.

Raman spectroscopy study for the systems (LiCl-H2O and LiCl-MgCl2-H2O): Excess spectra and hydration shell spectra.

The micro-structure of hydration shell of solute in water is significant for understanding the properties of aqueous solutions. Raman spectroscopy has been employed for studying the hydration shell structure of the solute for decades, however, Raman imaging data is still seriously overlapped, making it challenging to obtain information on the spectrum of hydrated water molecules. In this paper, Raman spectroscopy was employed to study the O-H vibration peaks of LiCl aqueous solution and LiCl-MgCl2-H2O mixed aqueous solution. The changes of stretching vibration peak of 2800 âˆ¼ 3800 cm-1O-H and hydrogen bond network structure in aqueous solution were analyzed at room temperature and ion association. With the increase of magnesium salt ratio, the damage of solute to the bulk water gradually decreases in the mixed solution, which indicated that LiCl has a more significant influence on the bulk water molecules. It is mainly due to the intense hydration of Li+, which can not only affect the water molecules in the first hydration shell but also affect the water molecules in the second hydration shell. The number of water molecules in the first hydration shell were obtained by extracting the spectra of different solute first hydration shells from the solution spectra. Those spectra of the hydration shell were employed to study the micro-structures of the first hydration shells of anions, and the aggregation behavior of ions in the the mixed solution.

Numerical study of the impact of glottis properties on the airflow field in the human trachea using V-LES.

The influences of the profiles and cross-sectional areas of glottal aperture on the upper respiratory airway are investigated using an idealized cast-based mouth-throat model and three dimensional computational fluid dynamics (CFD). The open source CFD code OpenFOAM is employed. The transient flows are modeled using the very-large eddy simulation with the Smagorinsky sub-grid scale (SGS) model. Five different shapes of glottis are considered, including circular glottis with 100 %, 75 % and 50 % cross-sectional area and elliptic glottis with 75 % and 50 % cross-sectional area. Both instantaneous and averaged flow fields are analyzed. It is found that the variations of glottis have great impacts on the properties of downstream flow fields such as the secondary flow, laryngeal jet, recirculation zone, turbulent kinetic energy, and vortex. Evident impacts are observed in the region within 6 tracheal diameters downstream of the glottis. The profile of the glottis has more impacts on the laryngeal shape, while the cross-sectional area has more impacts on velocity of the laryngeal jet and turbulent intensity. It is concluded that both the glottal areas and profiles are critical for an idealized geometrical mouth-throat model.

Molecular Dynamics Simulations of Vapor-Liquid Interface Properties of n-Heptane/Nitrogen at Subcritical and Transcritical Conditions.

Vapor-liquid interfacial properties of n-heptane/nitrogen at various temperatures and pressures in the ranges of 270-648 K and 10-60 bar were investigated using molecular dynamics (MD) simulations. Additionally, density gradient theory (DGT) was used to predict the binary system interface for qualitative comparison purposes. Results show that surface tension decreases with the increases in both temperature and pressure, but the rate of decrease against the latter is smaller. The interface thickness, which is of the order of several nanometers, was also shown to have a much stronger dependence on temperature and almost invariant with respect to pressure. MD also reveals the phenomenon of nitrogen enrichment in the outer interfacial region at low subcritical temperatures. A sensitivity analysis of influence parameters on DGT results revealed that surface tension values are a stronger function of n-heptane influence parameter and almost invariant with respect to nitrogen influence parameter. The velocity distribution function (VDF) shows that the molecules in and away from the interface follow the Maxwellian distribution of kinetic theory at subcritical conditions. The VDF of the velocity component normal to the interface of the evaporated n-heptane molecules adjacent to the interface exhibits a deviation from the Maxwellian distribution. The VDF of all three components of velocity of n-heptane molecules at supercritical conditions exhibits a departure from the Maxwellian distribution as well.

Investigation of airflow field in the upper airway under unsteady respiration pattern using large eddy simulation method.

In this paper, the airflow field in the upper airway under unsteady respiration process is predicted using large eddy simulation. The geometrical model is created by combining a popular cast-based mouth-throat model with tracheo-bronchial airways modeled with a trumpet-shaped conduit. The respiration process is simulated by sinusoidal displacing the bottom surface of the geometrical model. Large eddy simulation with dynamic sub-grid scale model is adopted for modeling the turbulent flow via a commercial CFD software, Converge. This study has found that (1) the secondary vortices in the mouth cavity are much more complex considering the lung expansion than setting the quasi-steady inspiration flow at the mouth-inlet; (2) the properties of secondary vortices in the trachea are not evidently different at the same Reynolds number at the accelerating and decelerating inspiration phases; (3) the reversed pharynx jet as well as recirculation zone is much unsteadier at the accelerating expiration phase than decelerating expiration phase for the same Reynolds number. We conclude that the properties of airflow structures are highly impacted by the respiration pattern and more investigations should be conducted, particularly, on the airflow structures during expiration phase for further understanding the properties of flow field.

Raman spectroscopy of diesel and gasoline engine-out soot using different laser power.

We studied engine-out soot samples collected from a heavy-duty direct-injection diesel engine and port-fuel injection gasoline spark-ignition engine. The two types of soot samples were characterized using Raman spectroscopy with different laser powers. A Matlab program using least-square-method with trust-region-reflective algorithm was developed for curve fitting. A DOE (design of experiments) method was used to avoid local convergence. The method was used for two-band fitting and three-band fitting. The fitting results were used to determine the intensity ratio of D (for "Defect" or "Disorder") and G (for "Graphite") Raman bands. It is found that high laser power may cause oxidation of soot sample, which gives higher D/G intensity ratio. Diesel soot has consistently higher amorphous/graphitic carbon ratio, and thus higher oxidation reactivity, compared to gasoline soot, which is reflected by the higher D/G intensity ratio in Raman spectra measured under the same laser power.

Solution structure of energy stored system I: aqua-B(OH)4(-): a DFT, Car-Parrinello molecular dynamics, and Raman study.

A systematic study on the structure, stability, and Raman spectra of the metaborate anion hydrated clusters, B(OH)4(-)(H2O)n, (n = 1-15) was carried out by DFT in both gaseous and aqueous phase at the B3LYP/aug-cc-pVDZ level; all of these stable configurations were described, and the most stable hydrated clusters were chosen. The hydrogen bonds in those hydrated clusters were described in three different items: symmetrical double hydrogen bonding (DHB), single hydrogen bonding (SHB), and interwater hydrogen bonding (WHB). The distance of SHB is shorter than that of DHB, and multiple SHBs are more stable than a single DHB. In small size clusters (n ≤ 5), a structure with more DHBs is more stable than other arrangements. With continued increase in size, more SHBs were found in the first hydration sphere: when n ≥ 9, only SHBs can be found, and when n ≥ 12, a full hydration structure is formed with 12 SHBs and a hydration number of 10-12. The Car-Parrinello molecular dynamics simulation shows that only the first hydration sphere can be found, and the hydration number of B(OH)4(-) is 9.2 and the hydration distance is 3.68. The total symmetrical stretching vibration of B(OH)4(-) in hydrated B(OH)4(-)(H2O)n is blue shifted with increasing cluster size. After consideration of hydration, the calculated characteristic frequencies are in accord with the experiment characteristic frequency of B(OH)4(-).