While molecules that promote the growth of animal cells have been identified, it remains unclear how such signals are orchestrated to determine a characteristic target size for different cell types. ...It is increasingly clear that cell size is determined by size checkpoints—mechanisms that restrict the cell cycle progression of cells that are smaller than their target size. Previously, we described a p38 MAPK-dependent cell size checkpoint mechanism whereby p38 is selectively activated and prevents cell cycle progression in cells that are smaller than a given target size. In this study, we show that the specific target size required for inactivation of p38 and transition through the cell cycle is determined by CDK4 activity. Our data suggest a model whereby p38 and CDK4 cooperate analogously to the function of a thermostat: while p38 senses irregularities in size, CDK4 corresponds to the thermostat dial that sets the target size.
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•Homeostatic mechanisms maintain cells at a given target size•p38 MAPK promotes growth of cells that are smaller than the critical target size•CDK4 determines the target size by regulating both the duration and rate of cell growth
Tan et al. study homeostatic mechanisms that maintain animal cells at their appropriate target size. They find that p38 is part of a sensing mechanism that identifies inappropriately sized cells, while CDK4 is analogous to a thermostat dial that sets the target size set point referenced by p38.
The particle‐size distribution (PSD) of a soil expresses the mass fractions of various sizes of mineral particles which constitute the soil material. It is a fundamental soil property, closely ...related to most physical and chemical soil properties and it affects almost any soil function. The experimental determination of soil texture, i.e., the relative amounts of sand, silt, and clay‐sized particles, is done in the laboratory by a combination of sieving (sand) and gravitational sedimentation (silt and clay). In the latter, Stokes' law is applied to derive the particle size from the settling velocity in an aqueous suspension. Traditionally, there are two methodologies for particle‐size analysis from sedimentation experiments: the pipette method and the hydrometer method. Both techniques rely on measuring the temporal change of the particle concentration or density of the suspension at a certain depth within the suspension. In this paper, we propose a new method which is based on the pressure in the suspension at a selected depth, which is an integral measure of all particles in suspension above the measuring depth. We derive a mathematical model which predicts the pressure decrease due to settling of particles as function of the PSD. The PSD of the analyzed sample is identified by fitting the simulated time series of pressure to the observed one by inverse modeling using global optimization. The new method yields the PSD in very high resolution and its experimental realization completely avoids any disturbance by the measuring process. A sensitivity analysis of different soil textures demonstrates that the method yields unbiased estimates of the PSD with very small estimation variance and an absolute error in the clay and silt fraction of less than 0.5%.
Key Points
New methodology for automated particle‐size analysis from sedimentation experiments
Accurate and precise identification of particle‐size distribution in high resolution
Contrary to pipette and hydrometer method, no manual operation necessary, and no disturbance of measurement process occurs
Gas physisorption is an experimental technique based on equilibrium Van der Waals interactions between gas molecules and solid particles, that quantifies the specific surface area (SSA), pore size ...distribution (PSD), and pore volume of solids and powders. The performance of catalysts, absorbents, chromatography column materials, and polymer resins depends on these morphological properties. Here we introduce the basic principles and procedures of physical adsorption, especially nitrogen physisorption, as a guide to students and researchers unfamiliar with the field. The Brunauer‐Emmett‐Teller theory (BET) is a common approach to estimate SSA that extends the Langmuir monolayer molecular adsorption model to multilayer layers. It relies on an equilibrium adsorption isotherm, measured at the normal boiling point of the adsorbate, eg, 77 K or 87 K for N2 and Ar, respectively. Web of Science indexed 45 400 articles in 2016 and 2017 that mentioned N2 adsorption porosimetry—BET and BJH (Barrett‐Joyner‐Halenda) keywords. The VOSViewer bibliometric tool grouped these articles into four research clusters: adsorption, activated carbon in aqueous solutions for removal of heavy metal ions; synthesis of nanoparticles and composites; catalysts performance in oxidation and reduction processes; and photocatalytic degradation with TiO2. According to the literature, the accuracy of the density function theory (DFT) method is higher than with the BJH theory and it is more reliable.
Coal samples for low-pressure nitrogen (N2) adsorption measurement in previous work cover a large particle size range (from 0.075 to 4.75 mm). However, minimal attention has been paid to the effect ...of coal particle size on pore structure using gas adsorption methods. Anthracite coal collected from the Zhina Coalfield, China, was crushed, subsampled, and sieved to eight particle size ranges: 1–2 mesh (8000–25400 µm), 40–50 mesh (270–380 µm), 50–70 mesh (212–270 µm), 70–90 mesh (160–212 µm), 90–160 mesh (96–160 µm), 160–200 mesh (75–96 µm), 200–300 mesh (48–75 µm), and >300 mesh (<48 µm). The adsorption–desorption isotherms of each subsample were measured using N2 at 77.35 K to compare differences in pore structure characteristics. The results of the N2 adsorption tests show that particle size has a significant effect on pore volume, specific surface area, and pore size distribution of coal. Specifically, decreasing coal particle size results in continuous increase in macro- and mesopore volumes and specific surface areas. This can be attributed to the fact that smaller-sized coal particles open more of the previously closed pores, which are then accessible to adsorping gas. The contribution of closed pores to the total pore volume is 94.94% in the pore aperture range of 3.1–370 nm. The volume of closed macropores varies from 48.96 to 84.69% of the total closed pore volume. According to optical microscope and SEM observations of the Zhina Coalfield subsamples, massive gas pores exist in an isolated form with poor connectivity; some plant tissue pores are filled by pyrites and clay minerals, and may be totally occluded. Thus, gas pores contribute the dominant amount of the closed pore volume. In addition, different Zhina Coalfield subsamples show varied hysteresis loop shapes, indicating that closed pores in coal possess a variety of pore morphologies and sizes. To improve the accuracy and comparability of the pore structure of coal, we propose >300 mesh as the preferred particle size of coal for all low-pressure N2 adsorption measurement in future work. Furthermore, caution must be used in evaluating coal bed methane resource recovery potential as coal possesses high closed porosity; failure to account for this will result in an overestimation of the amount of gas that can be recovered from coal seams during production.
Abstract
The strong metal-support interaction (SMSI) has long been studied in heterogonous catalysis on account of its importance in stabilizing active metals and tuning catalytic performance. As a ...dynamic process taking place at the metal-support interface, the SMSI is closely related to the metal surface properties which are usually affected by the size of metal nanoparticles (NPs). In this work we report the discovery of a size effect on classical SMSI in Au/TiO
2
catalyst where larger Au particles are more prone to be encapsulated than smaller ones. A thermodynamic equilibrium model was established to describe this phenomenon. According to this finding, the catalytic performance of Au/TiO
2
catalyst with uneven size distribution can be improved by selectively encapsulating the large Au NPs in a hydrogenation reaction. This work not only brings in-depth understanding of the SMSI phenomenon and its formation mechanism, but also provides an alternative approach to refine catalyst performance.
We extend the basic Schumpeterian endogenous growth model by allowing incumbents to undertake innovations to improve their products, while entrants engage in more “radical” innovations to replace ...incumbents. Our model provides a tractable framework for the analysis of growth driven by both entry of new firms and productivity improvements by continuing firms. The model generates a non-degenerate equilibrium firm size distribution driven by entry of new firms and expansion exit of existing firms. When there is also costly imitation preventing any sector from falling too far below the average, the stationary firm size distribution is Pareto with an exponent approximately equal to one (the so-called “Zipf distribution”).
Herein we report a simple dual‐soft‐template approach to prepare walnut‐shaped macro‐/mesoporous polydopamine particles with diameter of ca. 270 nm, highly accessible bicontinuous channels and wide ...pore size distribution from ca. 20 nm to ca. 95 nm. This approach provides great opportunities to tailor the soft template‐directed assembly processes and generate various polydopamine particles with controllable mesophase curvature. Walnut‐shaped mesoporous carbon particles with large open mesochannels in the range of ca. 13 nm to ca. 50 nm can be fabricated by subsequent thermal treatment under nitrogen atmosphere. Lastly, we demonstrate that the as‐derived walnut‐shaped carbon particles manifest enhanced electrocatalytic performance for oxygen reduction reaction in alkaline electrolyte.
Hole nuts: Novel walnut‐shaped macro‐/mesoporous particles are prepared through a dual‐soft‐template strategy. This approach provides great opportunities to tailor the polymer‐directed assembly processes and generate various polydopamine particles with controllable mesophase curvature. Their dervied carbon particles can be used in the oxygen reduction reaction.
Allometry refers to the size-related changes of morphological traits and remains an essential concept for the study of evolution and development. This review is the first systematic comparison of ...allometric methods in the context of geometric morphometrics that considers the structure of morphological spaces and their implications for characterizing allometry and performing size correction. The distinction of two main schools of thought is useful for understanding the differences and relationships between alternative methods for studying allometry. The Gould–Mosimann school defines allometry as the covariation of shape with size. This concept of allometry is implemented in geometric morphometrics through the multivariate regression of shape variables on a measure of size. In the Huxley–Jolicoeur school, allometry is the covariation among morphological features that all contain size information. In this framework, allometric trajectories are characterized by the first principal component, which is a line of best fit to the data points. In geometric morphometrics, this concept is implemented in analyses using either Procrustes form space or conformation space (the latter also known as size-and-shape space). Whereas these spaces differ substantially in their global structure, there are also close connections in their localized geometry. For the model of small isotropic variation of landmark positions, they are equivalent up to scaling. The methods differ in their emphasis and thus provide investigators with flexible tools to address specific questions concerning evolution and development, but all frameworks are logically compatible with each other and therefore unlikely to yield contradictory results.
A comprehensive understanding of the deposition mechanisms and morphology of debris flows is necessary to delineate the extent of a debris flow hazard. However, due to the wide range of debris flow ...compositions and the complex topography in the field, there remains a deficiency of fundamental understanding on how the effects of grain-size distribution, water content, and channel slope influence the deposition mechanisms and morphology of debris flow. In this study, a series of experimental tests were carried out using a flume with a horizontal outflow plane to discern the effects of particle size, water content, and slope on the deposition morphology and grain size segregation on the deposition fan. Results reveal that the experimental debris flows are under either viscous or collisional flow regimes. Most experimental debris flow fronts lack high pore fluid pressures, emphasizing the formation of deposits via grain-grain and grain-bed friction and collisions; also high excess pore fluid pressure (positive) behind the front head is measured and it is beneficial for the mobility of debris flows. Both the deposit area and runout-width ratio are positively correlated to the Bagnold and Savage numbers and the initial water contents. Furthermore, an increase of fines content reduces the runout distance. However, this feature is not as obvious for high water content flows (
w
= 28.5% in this study). Moreover, smoother transition topography between the transportation and deposition zone leads to longer runout distances. For debris flows with a high solid fraction (
C
s
> 0.52 in this study), particle sorting is quite inhibited in the deposit fan.
•Gas adsorption (N2 and CO2) was applied to analyze the pore size distribution.•Fractal dimension was used to determine the complexity of pore structures.•Deconvolution method was used to ...characterize the distinct pore size families.•Partial linear least squares were used to describe the correlations between the pore structures and compositions.
Understanding the pore structures of unconventional reservoirs such as shale can assist in estimating their elastic transport and storage properties, thus enhancing the hydrocarbon recovery from such massive resources. Bakken Shale Formation is one of the largest shale oil reserves worldwide located in the Williston Basin, North America. In this paper, we collected a few samples from the Bakken and characterized their properties by using complementary methods including X-ray diffraction (XRD), N2 and CO2 adsorption, and Rock-Eval pyrolysis. The results showed that all range of pore sizes: micro (<2nm), meso (2–50nm) and macro-pores (>50nm) exist in the Bakken shale samples. Meso-pores and macro-pores are the main contributors to the porosity for these samples. Compared with the Middle Bakken, samples from Upper and Lower Bakken own more micro pore volumes. Fractal dimension analysis was performed on the pore size distribution data, and the results indicated more complex pore structures for samples taken from the Upper and Lower Bakken shales than the Middle Bakken. Furthermore, the deconvolution of the pore distribution function from the combination of N2 and CO2 adsorption results proved that five typical pore size families exist in the Bakken shale samples: one micro-pore, one macro-pore and three meso-pore size families. The studies on the correlations between the compositions and the pore structures showed that mostly feldspar and pyrite affect the total pore volume of samples from Middle Bakken Formation whereas clay dominates the total pore volume of samples from Upper/Lower Bakken Formation. TOC and clay content are the major contributors to the micro-pore size family in the Upper/Lower Bakken. Also, it was observed that the increase of hard minerals could increase the percentage of macro-pore family in the Middle Bakken Formation.
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