Molecular dynamics simulation of carbon nanotube growth under a tensile strain.

We performed molecular dynamics simulations of carbon nanotube (CNT) to elucidate the growth process in the floating catalyst chemical vapor deposition method (FCCVD). FCCVD has two features: a nanometer-sized cementite (Fe 3 C) particle whose melting point is depressed because of the larger surface-to-volume ratio and tensile strain between the growing CNT and the catalyst. The simulations, including these effects, demonstrated that the number of 6-membered rings of the (6,4) chiral CNT constantly increased at a speed of 1 mm / s at 1273 K , whereas those of the armchair and zigzag CNTs were stopped in the simulations and only reached half of the numbers for chiral CNT. Both the temperature and CNT chirality significantly affected CNT growth under tensile strain.

Artificial intelligence for the prevention and prediction of colorectal neoplasms.

BACKGROUND: Colonoscopy is a useful as a cancer screening test. However, in countries with limited medical resources, there are restrictions on the widespread use of endoscopy. Non-invasive screening methods to determine whether a patient requires a colonoscopy are thus desired. Here, we investigated whether artificial intelligence (AI) can predict colorectal neoplasia. METHODS: We used data from physical exams and blood analyses to determine the incidence of colorectal polyp. However, these features exhibit highly overlapping classes. The use of a kernel density estimator (KDE)-based transformation improved the separability of both classes. RESULTS: Along with an adequate polyp size threshold, the optimal machine learning (ML) models' performance provided 0.37 and 0.39 Matthews correlation coefficient (MCC) for the datasets of men and women, respectively. The models exhibit a higher discrimination than fecal occult blood test with 0.047 and 0.074 MCC for men and women, respectively. CONCLUSION: The ML model can be chosen according to the desired polyp size discrimination threshold, may suggest further colorectal screening, and possible adenoma size. The KDE feature transformation could serve to score each biomarker and background factors (health lifestyles) to suggest measures to be taken against colorectal adenoma growth. All the information that the AI model provides can lower the workload for healthcare providers and be implemented in health care systems with scarce resources. Furthermore, risk stratification may help us to optimize the efficiency of resources for screening colonoscopy.

Viscoelasticity of Low-Molecular-Weight Polyelectrolytes.

Shear-thickening fluids that absorb the impact energy of high-velocity projectiles are of great interest for aerospace and body-armor applications. In such a frame, we investigate transient states of neat and aqueous polyelectrolytes (PE) having low molecular weights and containing poly([2-(methacryloyloxy)ethyl]trimethylammonium) as polycations and poly(acrylamide-co-acrylic acid) as polyanions. We compare results with those of bulk water. We employ nonequilibrium molecular dynamics to simulate oscillatory shear, mainly in the linear viscoelastic regime. We find that neat PE exhibits properties of a viscoelastic solid, whereas water and the aqueous mixture of PE conform to viscoelastic liquids with Maxwellian behavior at low angular frequencies. Terminal relaxation times are ∼0.499 and ∼1.385 ps for water and the aqueous mixture of PE, respectively. At high angular frequencies, storage moduli show anomalous behaviors that correspond to transitions between shear thinning and shear thickening in complex shear viscosities. The change in potential energy with the increase of the angular frequency is mainly driven by intramolecular interactions for neat PE, whereas short-range Coulomb interactions are the major contributions for water and the aqueous mixture of PE. Upon observation of the molecular configurations, only the local polyionic structure in the aqueous mixture of PE shows improvement when increasing the angular frequency, whereas the rest remains barely affected. Thus, the water structure in the aqueous mixture of PE allows the storage of energy elastically through the hydrogen-bond network at large angular frequencies, whereas the mechanical contribution of polyions weakens and fully vanishes at the beginning of shear thinning, explaining the superimposed data with data of bulk water. Our method and findings set the path for future molecular simulations in the nonlinear viscoelastic regime with more complex underlying molecular mechanisms.

Microstructure of the fluid particles around the rigid body at the shear-thickening state toward understanding of the fluid mechanics.

We studied the shear-thickening behavior of systems containing rigid spherical bodies immersed in smaller particles using non-equilibrium molecular dynamics simulations. We generated shear-thickening states through particle mass modulation of the systems. From the microstructures, i.e., two-dimensional pair distribution functions, we found anisotropic structures resulting from shear thickening, that are explained by the difference between the velocities of rigid bodies and fluid particles. The increasing viscosity in our system originated from collisions between fluid particles and rigid bodies. The lubrication forces defined in macroscale physics are then briefly discussed.

Neat and Aqueous Polyelectrolytes under a Steady-Shear Flow.

Materials enabling impact-energy absorption of high-velocity projectiles are of great interest for applications like aerospace. In such a frame, shear thickening fluids were found very useful. Here, we investigated nanorheological properties of neat and aqueous polyelectrolytes of low molecular weights containing poly([2-(methacryloyloxy) ethyl] trimethyl ammonium) as polycations and poly(acrylamide-co-acrylic acid) as polyanions. Results were compared with pure water. We employed nonequilibrium molecular dynamics with the SLLOD algorithm to compute the viscosity at various shear rates. Systems containing polyelectrolytes exhibit shear thickening. The analysis of molecular configurations revealed a strong disruption of the ionic structure and more clusters with smaller sizes on increasing the shear rate. Potential energies showed that shear thickening originates from an increase in intramolecular and van der Waals interactions resulting from the increasing difficultly of polyelectrolyte-based systems to relax at high shear rates. Our method and findings underscore the importance of accounting for the molecular scale in the design of materials absorbing the impact energy efficiently.

New Insights in the Natural Organic Matter Fouling Mechanism of Polyamide and Nanocomposite Multiwalled Carbon Nanotubes-Polyamide Membranes.

Polyamide (PA) membranes comprise most of the reverse osmosis membranes currently used for desalination and water purification. However, their fouling mechanisms with natural organic matter (NOM) is still not completely understood. In this work, we studied three different types of PA membranes: a laboratory made PA, a commercial PA, and a multiwalled carbon nanotube (CNT-PA nanocomposite membrane during cross-flow measurements by NaCl solutions including NOM, humic acid (HA), or alginate, respectively). Molecular dynamic simulations were also used to understand the fouling process of NOM down to its molecular scale. Low molecular weight humic acid binds to the surface cavities on the PA structures that leads to irreversible adsorption induced by the high surface roughness. In addition, the larger alginate molecules show a different mechanism, due to their larger size and their ability to change shape from the globule type to the uncoiled state. Specifically, alginate molecules either bind through Ca2+ bridges or they uncoil and spread on the surface. This work shows that carbon nanotubes can help to decrease roughness and polymer mobility on the surfaces of the membranes at the molecular scale, which represents a novel method to design antifouling membranes.

Effective Antiscaling Performance of Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposites.

The antiscaling properties of multiwalled carbon nanotube (MWCNT)-polyamide (PA) nanocomposite reverse-osmosis (RO) desalination membranes (MWCNT-PA membranes) were studied. An aqueous solution of calcium chloride (CaCl2) and sodium bicarbonate (NaHCO3) was used to precipitate in situ calcium carbonate (CaCO3) to emulate scaling. The MWCNT contents of the studied nanocomposite membranes prepared by interfacial polymerization ranged from 0 wt % (plain PA) to 25 wt %. The inorganic antiscaling performances were compared for the MWCNT-PA membranes to laboratory-made plain and commercial PA-based RO membranes. The scaling process on the membrane surface was monitored by fluorescence microscopy after labeling the scale with a fluorescent dye. The deposited scale on the MWCNT-PA membrane was less abundant and more easily detached by the shear stress under cross-flow compared to other membranes. Molecular dynamics simulations revealed that the attraction of Ca2+ ions was hindered by the interfacial water layer formed on the surface of the MWCNT-PA membrane. Together, our findings revealed that the observed outstanding antiscaling performance of MWCNT-PA membranes results from (i) a smooth surface morphology, (ii) a low surface charge, and (iii) the formation of an interfacial water layer. The MWCNT-PA membranes described herein are advantageous for water treatment.

Antiorganic Fouling and Low-Protein Adhesion on Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposite.

We demonstrate efficient antifouling and low protein adhesion of multiwalled carbon nanotubes-polyamide nanocomposite (MWCNT-PA) reverse-osmosis (RO) membranes by combining experimental and theoretical studies using molecular dynamics (MD) simulations. Fluorescein isothiocyanate (FITC)-labeled bovine serum albumin (FITC-BSA) was used for the fouling studies. The fouling was observed in real time by using a crossflow system coupled to a fluorescence microscope. Notably, it was observed that BSA anchoring on the smooth MWCNT-PA membrane was considerably weaker than that of other commercial/laboratory-made plain PA membranes. The permeate flux reduction of the MWCNT-PA nanocomposite membranes by the addition of FITC-BSA was 15% of its original value, whereas those of laboratory-made plain PA and commercial membranes were much larger at 34%-50%. Computational MD simulations indicated that the presence of MWCNT in PA results in weaker interactions between the membrane surface and BSA molecule due to the formation of (i) a stiffer PA structure resulting in lower conformity of the molecular structure against BSA, (ii) a smoother surface morphology, and (iii) an increased hydrophilicity involving the formation of an interfacial water layer. These results are important for the design and development of promising antiorganic fouling RO membranes for water treatment.

Effective NaCl and dye rejection of hybrid graphene oxide/graphene layered membranes.

Carbon nanomaterials are robust and possess fascinating properties useful for separation technology applications, but their scalability and high salt rejection when in a strong cross flow for long periods of time remain challenging. Here, we present a graphene-based membrane that is prepared using a simple and environmentally friendly method by spray coating an aqueous dispersion of graphene oxide/few-layered graphene/deoxycholate. The membranes were robust enough to withstand strong cross-flow shear for a prolonged period (120 h) while maintaining NaCl rejection near 85% and 96% for an anionic dye. Experimental results and molecular dynamic simulations revealed that the presence of deoxycholate enhances NaCl rejection in these graphene-based membranes. In addition, these novel hybrid-layered membranes exhibit better chlorine resistance than pure graphene oxide membranes. The desalination performance and aggressive shear and chlorine resistance of these scalable graphene-based membranes are promising for use in practical water separation applications.

Strengthened PAN-based carbon fibers obtained by slow heating rate carbonization.

Large efforts have been made over the last 40 years to increase the mechanical strength of polyacrylonitrile (PAN)-based carbon fibers (CFs) using a variety of chemical or physical protocols. In this paper, we report a new method to increase CFs mechanical strength using a slow heating rate during the carbonization process. This new approach increases both the carbon sp(3) bonding and the number of nitrogen atoms with quaternary bonding in the hexagonal carbon network. Theoretical calculations support a crosslinking model promoted by the interstitial carbon atoms located in the graphitic interlayer spaces. The improvement in mechanical performance by a controlled crosslinking between the carbon hexagonal layers of the PAN based CFs is a new concept that can contribute further in the tailoring of CFs performance based on the understanding of their microstructure down to the atomic scale.

Molecular Dynamics Study of Carbon Nanotubes/Polyamide Reverse Osmosis Membranes: Polymerization, Structure, and Hydration.

Carbon nanotubes/polyamide (PA) nanocomposite thin films have become very attractive as reverse osmosis (RO) membranes. In this work, we used molecular dynamics to simulate the influence of single walled carbon nanotubes (SWCNTs) in the polyamide molecular structure as a model case of a carbon nanotubes/polyamide nanocomposite RO membrane. It was found that the addition of SWCNTs decreases the pore size of the composite membrane and increases the Na and Cl ion rejection. Analysis of the radial distribution function of water confined in the pores of the membranes shows that SWCNT+PA nanocomposite membranes also exhibit smaller clusters of water molecules within the membrane, thus suggesting a dense membrane structure (SWCNT+PA composite membranes were 3.9% denser than bare PA). The results provide new insights into the fabrication of novel membranes reinforced with tubular structures for enhanced desalination performance.

High-performance multi-functional reverse osmosis membranes obtained by carbon nanotube·polyamide nanocomposite.

Clean water obtained by desalinating sea water or by purifying wastewater, constitutes a major technological objective in the so-called water century. In this work, a high-performance reverse osmosis (RO) composite thin membrane using multi-walled carbon nanotubes (MWCNT) and aromatic polyamide (PA), was successfully prepared by interfacial polymerization. The effect of MWCNT on the chlorine resistance, antifouling and desalination performances of the nanocomposite membranes were studied. We found that a suitable amount of MWCNT in PA, 15.5 wt.%, not only improves the membrane performance in terms of flow and antifouling, but also inhibits the chlorine degradation on these membranes. Therefore, the present results clearly establish a solid foundation towards more efficient large-scale water desalination and other water treatment processes.