Critical Threshold for Bubble-like Nucleation during Pseudoboiling at Supercritical Pressures.

Supercritical pseudoboiling was proposed in the 1950s-1960s. Recently, evaporation-like and boiling-like heat transfer have been directly observed in macroscopic scales, and the contribution of pseudoboiling to the total heat transfer rate has been quantitatively characterized experimentally. Here, we explore the critical threshold to generate a bubble-like nucleus at supercritical pressure at the atomic scale, characterized by the total energy (Te = Ke + Pe, where Ke and Pe are kinetic energy and potential energy, respectively). Molecular dynamics simulations are performed, including an argon fluid box heated by a solid wall having its temperature above the fluid temperature. The fluid pressure is controlled by a movable piston wall opposite the heating wall. The effects of pressure, nonuniform heating, and surface wettability on pseudoboiling are investigated. It is found that the criterion Te > 0 should be satisfied for subcritical boiling, matching that reported previously. The criterion for supercritical pseudoboiling was newly obtained such that Te > 0.012 eV at 8 MPa for argon, but the threshold increases as pressure increases. Nonuniform heating and surface wettability do not affect the critical threshold of Te for bubble-like nucleation but affect the location of the initially generated bubble-like nucleus and the stabilized pseudofilm or pseudonucleate heat transfer modes, where the former is similar to (vapor) film boiling and the latter is similar to nucleate boiling at subcritical pressure. Because pseudoboiling occurs without surface tension at supercritical pressure, we observe that the bubble-like structure may not display a perfectly smooth gas-liquid interface but may display an irregular pattern instead. Our work explains pseudoboiling from the viewpoint of the competition between kinetic energy and potential energy and presents a link regarding boiling in the two domains of subcritical pressure and supercritical pressure.

Overcoming the Heat Transfer Paradox: Nano Ridges Induce "Pistol Bubbles" and Reverse the Boiling Curve.

To increase the boiling heat transfer limit, we disrupted the previously nonevaporating region and increased the vaporization activity of "inert" liquid molecules by introducing nano ridges on the boiling surface. This solved the paradox of no heat transfer occurring through the thinnest liquid film in boiling bubbles; thus, the internal heat transfer limit of the bubbles was exceeded. We found that vigorous boiling occurred immediately once the nonevaporating region was activated, and the bubble frequency increased by an order of magnitude, reaching 1186 Hz, which has not been previously reported. With an increase in heat flux, the boiling curve exhibited a "return". We achieved an extremely high bubble frequency by experimentally quantifying the major influence of nonevaporating region disruption on boiling heat transfer. The mechanism behind the generation of the ultrahigh-frequency bubbles was discovered. This study also reveals a new mechanism for the reversed boiling curve.

Prolactin is a Key Factor for Nonalcoholic Fatty Liver Disease in Obese Children.

This study investigates whether serum prolactin (PRL) is a key factor for nonalcoholic fatty liver disease (NAFLD) in children. A total of 691 obese childred participated in this study and were divided into a NAFLD group (n=366) and simple obesity (SOB) group (n=325) according to the hepatic ultrasound results. The two groups were matched for gender, age, pubertal development, and body mass index (BMI). All patients underwent an OGTT test, and fasting blood samples were collected to measure prolactin. Stepwise logistic regression was performed to identify significant predictors of NAFLD. Serum prolactin levels were significantly lower in NAFLD subjects than in the SOB subjects [82.4 (56.36, 118.70) vs. 99.78 (63.89, 153.82), p<0.001] (mIU/l). NAFLD was strongly associated with insulin resistance (HOMA-IR) and prolactin, with lower levels of prolactin increasing the risk of NAFLD (adjusted ORs=1.741; 95% CI: 1.059-2.860) across the prolactin concentration tertiles after adjustment for confounders. Low serum prolactin levels are associated with the presence of NAFLD; thus, increased circulating prolactin might be a compensatory response for obesity in children.

Particle Separation from Liquid Marbles by the Viscous Folding of Liquid Films.

Particle separation from fluid interfaces is one of the major challenges due to the large capillary energy associated with particle adsorption. Previous approaches rely on physicochemical modification or tuning the electrostatic action. Here, we show experimentally that particle separation can be achieved by fast dynamics of drop impact on soap films. When a droplet wrapped with particles (liquid marble) collides with a soap film, it undergoes bouncing and coalescence, stripping and viscous separation, or tunneling through the film. Despite the violence of splashing events, the process robustly yields the stripping in a tunable range. This viscous separation is supported by the transfer front of dynamic contact among the film, particle crust, and drop and can be well controlled in a deterministic manner by selectable impact parameters. By extensive experiments, together with thermodynamic analysis, we disclose that the separation thresholds depend on the energy competition between the kinetic energy, the increased surface energy, and the viscous dissipation. The mechanical cracking of the particle crust arises from the complex coupling between interfacial stress and viscous forces. This study is of potential benefit in soft matter research and also permits the study of a drop with colloid and surface chemistry.

Effect of particle size on the stripping dynamics during impact of liquid marbles onto a liquid film.

The robust attachment of particles at fluid interfaces is favorable for engineering new materials due to the large capillary energy, but it meets significant challenges when particle removal is a requirement. A previous study has shown that soap films can be utilized to achieve particle separation from liquid marbles. Here, we investigate the effects of particle size on the particle separation from liquid marbles using fast dynamics of drop impact on a soap film. Experimental observations disclose that the fast dynamics of the liquid marble involves coalescence, bouncing, stripping, or tunneling through the film by controlling the falling height and drop volume. More importantly, the active regime of the stripping mode can be selective-controlled by tuning the particle size, and the smaller stabilizing particles make a wider stripping regime. This is attributed to the smaller change of the surface energy resulting from the larger surface tension of LMs wrapped by smaller particles. Theoretical analysis reveals that the stripping thresholds are determined by the energy competition between kinetic energy, the increased surface energy and viscous dissipation, which offers important insights into particle separation by tuning the particle size. The present study provides guidelines for applications that involve phase separation.

Line Tension of Nanodroplets on a Concave Surface.

The contact angle of a nanodroplet on a surface may deviate from that of a macrodroplet on a surface. Even though there are many studies regarding line tension, it is not well understood. In this paper, molecular dynamics simulation is performed for nanodroplets on a concave solid wall. The Lennard-Jones (L-J) potential is directly used or modified to simulate the force interaction between argon atoms and between solid-liquid particles. The initial droplet radius is 4, 5, and 6 nm, respectively. The k coefficient is defined as the ratio of the initial droplet radius with respect to the curvature radius of concave walls, which is in the range of 0-0.9, in which k = 0 refers to a flat surface. We found that indeed the contact angle θ of a nanodroplet on a concave wall deviates from that of a macrodroplet on a flat surface. Contact angles display a two region distribution, in which θ increases with increasing k for k < 0.5 and decreases with increasing k for k > 0.5. The k coefficient influences the droplet morphology. With k in the range of 0-0.9, the vapor-liquid interface is switched from a convex shape to a flat shape and finally reaches a concave shape. The line tension generally behaves in an increasing trend with the increase of k but becomes constant when k is beyond 0.7. The liquid densities, radial distribution functions, and coordination numbers show that the liquid particles are more closely packed with each other with the increase of k. The line tension achieves a positive sign and on the magnitude of 10-11 N, which has a linear increase with respect to the peak density of the first liquid layer.

In Situ Oil Separation and Collection from Water under Surface Wave Condition.

Removal of oil from water is strongly desired due to environmental pollution, and related studies are mainly limited to the material itself. Here, we propose an oil-water separation device, called a floating well, which includes a container for oil collection and a modified mesh screen for oil separation. The mesh screen is superhydrophilic to oil and superhydrophobic to water. The oil removal experiment was performed in a basin. Under the calm surface condition, the oil collection efficiency is shown to be 42-69%. The inadequate oil collection is explained by the breakdown of a thin oil film above water to form an oil-free area, preventing direct contact between the separator and the oil. Sustained by a wave generator, we are surprised to find that with a low-frequency surface wave, the oil collection efficiency is increased to 98%, and the collection speed reaches 2.5 times that under the calm surface condition. The almost complete collection is due to the sustained contact between the separator and the oil, under which the surface wave continuously drives the oil film toward the separator from elsewhere, thus the oil-free area cannot be formed. Our work presents a new clue for large-scale in situ applications, in which the nature wave energy of river/sea can be the driving force for continuous oil separation and collection.

Prevalence and associated factors for iron deficiency anemia among pregnant women in Fuyang, China.

Iron deficiency anemia (IDA) in pregnant women was associated with increased risks of both maternal and neonatal adverse outcomes. This study aimed to investigate the prevalence of IDA and identify the associated factors of IDA among pregnancy woman in Fuyang, China. The cross-sectional study was carried out in 789 pregnant women using simple random sampling method living in Fuyang, China from August 2017 to May 2018. Overall, the prevalence of IDA was 39.8%. 1-unit increment in maternal age was associated with a 5% increased risk of IDA, and 1-unit increased in BMI was associated with a 6% decreased risk of IDA (all P < .01). When compared with exercise for <30 minutes, 30-60 minutes of daily exercise was associated with a lower risk of IDA (P = .02). Similarly, when compared with a low frequency intake of iron-rich food, intake≥once/week was associated with a lower risk of IDA (P < .0001). However, vitamin C-rich food intake ≥twice/day was associated with a higher risk of IDA (P = .01) and a normal or bad appetite increased the risk of IDA when compared with a good appetite (P < .01). Our findings suggested that high frequency of iron-rich food intake, good appetite, and moderate physical activity were important for the prevention of IDA during pregnancy.

A New Mechanism of Light-Induced Bubble Growth to Propel Microbubble Piston Engine.

Radiation pressure refers to the momentum transfer of photons during light "particles" impacting a surface. The force is too small to drive microengines. Different from the classical radiation pressure, the indirect radiation pressure (Fm ) is introduced, coming from the momentum change of light-induced bubble expansion. Fm is shown to obey Fm ∼ (I·rb )2 , behaving faster growth of indirect radiation pressure versus light intensity I and bubble radius rb . An effective bubble size range is identified for Fm to suppress other forces for bubble in liquid. The top laser irradiation on nanofluid is used in this experiment. A well-defined bubble pulsating flow, being a new principle of bubble piston engine, is demonstrated. During pulse on (≈ns scale), Fm exceeding other forces generates an extremely large acceleration, which is three to four orders larger than the gravity acceleration, propelling the bubble traveling downward. During pulse off, the bubble is floating upward due to the nonexistence of Fm . In such a way, the piston engine sustains the oscillating ranges of 38-347 µm for bubble diameters and 2.7-457.9 µm for traveling distances of piston. This work is useful to manipulate bubble dynamics in solar energy systems, and can find various applications in optofluidics.

Theoretical Analysis of a Sessile Evaporating Droplet on a Curved Substrate with an Interfacial Cooling Effect.

Sessile droplet evaporation is widely encountered in nature, and it has numerous applications in industrial and scientific communities; therefore, the accurate prediction of droplet evaporation has great significance in practical applications. In this paper, for the first time, a comprehensive theoretical model is built up for diffusion-controlled heat and mass transfer for sessile droplet evaporation on a curved substrate in toroidal coordinates. The evaporative mass transfer is coupled with the heat transfer across the gas-liquid droplet interface, as well as the heat transfer across the solid-liquid interface of the curved substrate. The effects of interfacial cooling and thermal conductivity of the droplet and substrate as well as their initial shapes on the droplet evaporation are provided in details. It is found that the evaporative flux usually increases sharply near the droplet edge due to the short distance for heat conduction from the substrate to the droplet; however, it can be reversed from sharp increasing to decreasing at a low thermal conductivity ratio kR < 0.3 of the substrate over droplet or large initial droplet contact angle θ > 30°. The interfacial evaporative cooling effect can always suppress the droplet evaporation. The lifetime of evaporative droplet can be prolonged with the decreasing thermal conductivity ratio, increasing evaporative cooling number, and increasing initial droplet contact angle or tangential angle of a curved substrate. These findings may be of great significance in the applications of droplet evaporation on the curved substrate.

Multiscale Characteristic in Symmetric/Asymmetric Solar-Driven Nanofluid Droplet Evaporation.

Driven by nanoparticle plasmonic heating, sessile droplet evaporation presents challenges on the coupling mechanisms between time-spatial heat source distribution and flow/temperature fields in a droplet. Here, symmetric/asymmetric solar-driven droplet evaporation is investigated. An infrared camera captures droplet surface temperatures in the micrometer scale after correction. An optical three-dimensional profiler quantifies nanoparticle deposition in the nanoscale. We show that droplet surface temperatures do display a nonmonotonic variation trend. Based on measurements, we are able to decouple the droplet into a contact line region (CLR) and a bulk volume region (BVR). The CLR volume is two to three magnitudes smaller than the droplet volume. The temperature gradient is significant in CLR, but flat temperature exists in BVR. Radial flow in BVR transports nanoparticles from the droplet body to the contact line, while Marangoni flow in CLR stabilizes nanoparticles there. Light energy is also decoupled based on its wavelength band. It is found that CLR dominates the visible energy absorption, but BVR has a weak contribution. Top light heating causes symmetry temperatures and a coffee-ring profile along the circumference. However, side heating yields higher temperatures and more nanoparticles deposition on the sunny side than on the night side. The above findings are valid when the initial droplet volume and incident irradiation flux are changed on the hydrophilic wall. The hydrophilic wall and hydrophobic wall maintain the evaporation modes of constant contact diameter and "stick-slip", respectively. The present paper enhances the understanding of light-induced droplet evaporation from the multiscale point of view.

Failure and Recovery of Droplet Nucleation and Growth on Damaged Nanostructures: A Molecular Dynamics Study.

The condensate flooding during dropwise condensation causes serious deterioration in heat transfer performance. In this study, the three-dimensional large-scale molecular dynamics simulation is carried out to investigate the droplet state transition from local flooding mode to Wenzel or from Wenzel to Cassie due to the droplet coalescence under the effect of nanostructure size. In particular, the effect of nanostructure breakage on droplet nucleation and growth is discussed to reveal the mechanism of dropwise condensation heat transfer deterioration. As a potential solution, the lubricant-impregnated surface is proposed to recover the preferred Cassie state by regulating the dynamic wetting characteristics of droplets, and thus the detrimental effect of nanostructure breakage could be effectively avoided.

Impacts of Proteins on Dissolution and Sulfidation of Silver Nanowires in an Aquatic Environment: Importance of Surface Charges.

With increasing utilization of silver nanomaterials, growing concerns are raised on their deleterious effects to the environment. Once discharged in an aquatic environment, the interactions between silver nanowires (AgNWs) and proteins may significantly affect the environmental behaviors, fate, and toxicities of AgNWs. In the present study, three representative model proteins, including ovalbumin (OVA), bovine serum albumin (BSA), and lysozyme (LYZ), were applied to investigate the impacts of the interactions between proteins and AgNWs on the transformations (oxidative dissolution and sulfidation) of AgNWs in an aquatic environment. Fluorescence spectroscopy and isothermal titration calorimetry analyses indicated that there was very weak interaction between OVA or BSA and AgNWs, but there was a strong interaction between the positively charged LYZ and the negatively charged AgNWs. The presence of LYZ not only reversed the surface charge of AgNWs but also resulted in the breakup of the nanowire structure and increased the reactive surface area. The positively charged surface of AgNWs in the presence of LYZ favored the access of sulfide ions. As a consequence, the kinetics of oxidative dissolution and sulfidation of AgNWs were not affected by OVA and BSA but were significantly facilitated by LYZ. The results shed light on the important roles of electrostatic interactions between AgNWs and proteins, which may have important implications for evaluating the fate and effects of silver nanomaterials in complicated environments.

Exergy Analysis of Two-Stage Organic Rankine Cycle Power Generation System.

Organic Rankine cycle (ORC) power generation is an effective way to convert medium and low temperature heat into high-grade electricity. In this paper, the subcritical saturated organic Rankine cycle system with a heat source temperature of 100~150 °C is studied with four different organic working fluids. The variations of the exergy efficiencies for the single-stage/two-stage systems, heaters, and condensers with the heat source temperature are analyzed. Based on the condition when the exergy efficiency is maximized for the two-stage system, the effects of the mass split ratio of the geothermal fluid flowing into the preheaters and the exergy efficiency of the heater are studied. The main conclusions include: The exergy efficiency of the two-stage system is affected by the evaporation temperatures of the organic working fluid in both the high temperature and low temperature cycles and has a maximum value. Under the same heat sink and heat source parameters, the exergy efficiency of the two-stage system is larger than that of the single-stage system. For example, when the heat source temperature is 130 °C, the exergy efficiency of the two-stage system is increased by 9.4% compared with the single-stage system. For the two-stage system, analysis of the four organic working fluids shows that R600a has the highest exergy efficiency, although R600a is only suitable for heat source temperature below 140 °C, while other working fluids can be used in systems with higher heat source temperatures. The mass split ratio of the fluid in the preheaters of the two-stage system depends on the working fluid and the heat source temperature. As the heat source temperature increases, the range of the split ratio becomes narrower, and the curves are in the shape of an isosceles triangle. Therefore, different working fluids are suitable for different heat source temperatures, and appropriate working fluid and split ratio should be determined based on the heat source parameters.

Effects of Temperature and Ionic Concentration on Nanodroplet Electrocoalescence.

Droplet electrocoalescence is of interest for various applications such as petroleum dehydration, electrospray ionization, and surface self-cleaning. Here, the effects of temperature and ionic concentration on nanodroplet electrocoalescence are investigated by molecular dynamics simulation. The results show that low ionic concentration rapidly drives ions towards water clusters and leads to dipole polarization of droplets. With an increase of ionic concentration, the particle-particle interaction is enhanced, but the mobility of free water molecules and salt ions is curbed by hydration and ion pairs, which then slows the electrocoalescence. Low temperature accelerates the rotation of water molecules but does not enhance the mobility of ions. Alternatively, high temperature not only breaks the self-assembly of water molecules along the electric field direction but also helps ions to overcome the electrostatic barrier between particles. The latter effect promotes dipole polarization to compensate for the shortcoming of less orientation polarization. The combined effects of ion concentration and temperature are investigated and unified by droplet conductivity from the microscopic point of view. The conductivity increases with the increase in temperatures and ionic concentrations. We confirm that the accurate control of droplet electrocoalescence can be achieved by a suitable combination of temperature and ionic concentration.

The prevalence of premature thelarche in girls and gynecomastia in boys and the associated factors in children in Southern China.

BACKGROUND: To investigate the prevalence and risk factors of premature thelarche (PT) in girls and gynecomastia (GM) in boys in Southern China. METHODS: We conducted a cross-sectional study of preschool children across 9 cities in Zhejiang province. A total of 6273 children in the age-group of 2-7 years were recruited from January 2014 to March 2015. Relevant information was collected from mothers through face-to-face interviews. Logistic regression models were used to examine the correlates of PT and GM. Odds ratios (ORs) with 95% confidence intervals (CIs) are reported. RESULTS: The prevalence of PT among girls was 4.8% and that of GM among boys was 0.8%. One hundred girls were diagnosed with PT before the age of 2 years; 69 (69.0%) of these girls experienced spontaneous resolution of PT. Twenty-four boys were diagnosed with GM before the age of 2 years; 10 (41.7%) of these experienced spontaneous resolution of GM. Children borne of mothers with early onset of menarche and those belonging to high-income families were at a higher risk of premature breast development. Greater consumption of eggs was associated with premature breast development in early childhood. CONCLUSIONS: Socioeconomic status of family, early onset of menarche in mother, and consumption of eggs were strongly associated with premature breast development in early childhood.

Impacts of sulfidation of silver nanowires on the degradation of bisphenol A in water.

Silver nanowires (AgNWs) are widely produced in many electronic and optical products, and could be inevitably discharged into the aquatic environments. Sulfidation is one of the most important transformation processes of AgNWs, and could significantly affect their fate and interactions with other pollutants in aquatic environment. In the present study, the sulfidation products of AgNWs with different atomic ratio of Ag and S were prepared under environmentally relevant conditions. The crystal structure, elemental composition, morphology and size of the sulfidation products were comprehensively characterized by powder X-ray diffraction, UV-vis spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscope. The products were heterostructured nanowires and the Ag2S/Ag molar ratio increased with extension of the reaction time. The produced Ag2S-Ag nanowires displayed a good photocatalytic activity and facilitated the degradation of the copresent organic pollutant bisphenol A (BPA) under simulated sunlight irradiation. As sulfidation time increased, more Ag2S was generated and the Ag2S-Ag composites displayed high promotion effect on BPA degradation. This effect could be ascribed to the favorable synergistic effects between Ag2S and AgNWs, such as high electron-hole separation efficiency and low charge transfer resistance. The chemical scavenger experiments demonstrated that superoxide anion radicals and photogenerated holes in the sulfidation products of AgNWs could be the main reactive species for photocatalytic degradation.

Hierarchical Discriminant Analysis.

The Internet of Things (IoT) generates lots of high-dimensional sensor intelligent data. The processing of high-dimensional data (e.g., data visualization and data classification) is very difficult, so it requires excellent subspace learning algorithms to learn a latent subspace to preserve the intrinsic structure of the high-dimensional data, and abandon the least useful information in the subsequent processing. In this context, many subspace learning algorithms have been presented. However, in the process of transforming the high-dimensional data into the low-dimensional space, the huge difference between the sum of inter-class distance and the sum of intra-class distance for distinct data may cause a bias problem. That means that the impact of intra-class distance is overwhelmed. To address this problem, we propose a novel algorithm called Hierarchical Discriminant Analysis (HDA). It minimizes the sum of intra-class distance first, and then maximizes the sum of inter-class distance. This proposed method balances the bias from the inter-class and that from the intra-class to achieve better performance. Extensive experiments are conducted on several benchmark face datasets. The results reveal that HDA obtains better performance than other dimensionality reduction algorithms.

How to Construct a Combined S-CO2 Cycle for Coal Fired Power Plant?

It is difficult to recover the residual heat from flue gas when supercritical carbon dioxide (S-CO2) cycle is used for a coal fired power plant, due to the higher CO2 temperature in tail flue and the limited air temperature in air preheater. The combined cycle is helpful for residual heat recovery. Thus, it is important to build an efficient bottom cycle. In this paper, we proposed a novel exergy destruction control strategy during residual heat recovery to equal and minimize the exergy destruction for different bottom cycles. Five bottom cycles are analyzed to identify their differences in thermal efficiencies (ηth,b), and the CO2 temperature entering the bottom cycle heater (T4b) etc. We show that the exergy destruction can be minimized by a suitable pinch temperature between flue gas and CO2 in the heater via adjusting T4b. Among the five bottom cycles, either the recompression cycle (RC) or the partial cooling cycle (PACC) exhibits good performance. The power generation efficiency is 47.04% when the vapor parameters of CO2 are 620/30 MPa, with the double-reheating-recompression cycle as the top cycle, and RC as the bottom cycle. Such efficiency is higher than that of the supercritical water cycle power plant.

Peptide-TLR7/8a-Coordinated DNA Vaccines Elicit Enhanced Immune Responses against Infectious Diseases.

DNA vaccines represent an innovative approach for the immunization of diverse diseases. However, their clinical trial outcomes are constrained by suboptimal transfection efficiency and immunogenicity. In this work, we present a universal methodology involving the codelivery of Toll-like receptor 7/8 agonists (TLR7/8a) and antigen gene using TLR7/8a-conjugated peptide-coated poly(ß-amino ester) (PBAE) nanoparticles (NPs) to augment delivery efficiency and immune response. Peptide-TLR7/8a-coated PBAE NPs exhibit advantageous biophysical attributes, encompassing diminutive particle dimensions, nearly neutral ζ potential, and stability in the physiological environment. This synergistic approach not only ameliorates the stability of plasmid DNA (pDNA) and gene delivery efficacy but also facilitates subsequent antigen production. Furthermore, under optimal formulation conditions, the TLR7/8a-conjugated peptide coated PBAE NPs exhibit a potent capacity to induce robust immune responses. Collectively, this nanoparticulate gene delivery system demonstrates heightened transfection efficacy, stability, biodegradability, immunostimulatory effect, and low toxicity, making it a promising platform for the clinical advancement of DNA vaccines.