scholarly journals Finite-size transitions in complex membranes

2020 ◽  
Author(s):  
M. Girard ◽  
T. Bereau
Keyword(s):  
Experimental Evidence ◽  
Critical Point ◽  
Finite Size ◽  
Underlying Mechanism ◽  
Large Temperature ◽  
Strong Argument ◽  
Biological Regulation ◽  
Critical Manifold ◽  
Critical Composition ◽  
Dynamics Simulations

ABSTRACTThe lipid raft hypothesis postulates that cell membranes possess some degree of lateral organization. The last decade has seen a large amount of experimental evidence for rafts. Yet, the underlying mechanism remains elusive. One hypothesis that supports rafts relies on the membrane to lie near a critical point. While supported by experimental evidence, the role of regulation is unclear. Using both a lattice model and molecular dynamics simulations, we show that lipid regulation of a many-component membrane can lead to critical behavior over a large temperature range. Across this range, the membrane displays a critical composition due to finite-size effects. This mechanism provides a rationale as to how cells tune their composition without the need for specific sensing mechanisms. It is robust and reproduces important experimentally verified biological trends: membrane-demixing temperature closely follows cell growth temperature, and the composition evolves along a critical manifold. The simplicity of the mechanism provides a strong argument in favor of the critical membrane hypothesis.SIGNIFICANCEWe show that biological regulation of a large amount of phospholipids in membranes naturally leads to a critical composition for finite-size systems. This suggests that regulating a system near a critical point is trivial for cells. These effects vanish logarithmically and therefore can be present in micron-sized systems.

Crystal to Glass Transition and Melting in Two Dimensions

1993 ◽  
Vol 321 ◽  
Author(s):  
M. Li ◽  
W. L. Johnson ◽  
W. A. Goddard
Keyword(s):  
Glass Transition ◽  
Intermediate Phase ◽  
Orientational Order ◽  
Finite Size ◽  
Two Dimensions ◽  
Size Difference ◽  
Two Dimensional ◽  
Atomic Size ◽  
Bond Orientational Order ◽  
Dynamics Simulations

ABSTRACTThermodynamic properties, structures, defects and their configurations of a two-dimensional Lennard-Jones (LJ) system are investigated close to crystal to glass transition (CGT) via molecular dynamics simulations. The CGT is achieved by saturating the LJ binary arrays below glass transition temperature with one type of the atoms which has different atomic size from that of the host atoms. It was found that for a given atomic size difference larger than a critical value, the CGT proceeds with increasing solute concentrations in three stages, each of which is characterized by distinct behaviors of translational and bond-orientational order correlation functions. An intermediate phase which has a quasi-long range orientational order but short range translational order has been found to exist prior to the formation of the amorphous phase. The destabilization of crystallinity is observed to be directly related to defects. We examine these results in the context of two dimensional (2D) melting theory. Finite size effects on these results, in particular on the intermediate phase formation, are discussed.

scholarly journals Self-Diffusion Coefficients of Methane/n-Hexane Mixtures at High Pressures: An Evaluation of the Finite-Size Effect and a Comparison of Force Fields

2019 ◽  
Author(s):  
Thiago José Pinheiro dos Santos ◽  
Charlles Abreu ◽  
Bruno Horta ◽  
Frederico W. Tavares
Keyword(s):  
Molecular Dynamics ◽  
Diffusion Coefficients ◽  
High Pressures ◽  
Force Fields ◽  
Transport Coefficients ◽  
Finite Size ◽  
Finite Size Effect ◽  
Self Diffusion ◽  
Analytical Correction ◽  
Dynamics Simulations

Mass transport coefficients play an important role in process design and in compositional grading of oil reservoirs. As experimental measurements of these properties can be costly and hazardous, Molecular Dynamics simulations emerge as an alternative approach. In this work, we used Molecular Dynamics to calculate the self-diffusion coefficients of methane/n-hexane mixtures at different conditions, in both liquid and supercritical phases. We evaluated how the finite box size and the choice of the force field affect the calculated properties at high pressures. Results show a strong dependency between self-diffusion and the simulation box size. The Yeh-Hummer analytical correction [J. Phys. Chem. B, 108, 15873 (2004)] can attenuate this effect, but sometimes makes the results depart from experimental data due to issues concerning the force fields. We have also found that different all-atom and united-atom models can produce biased results due to caging effects and to different dihedral configurations of the n-alkane.

Spices

2022 ◽  
pp. 760-777
Author(s):  
Nisha ◽  
Deepika
Keyword(s):  
Blood Glucose ◽  
Experimental Evidence ◽  
Bioactive Compounds ◽  
Underlying Mechanism ◽  
Pharmacological Activities ◽  
Glucose Lowering ◽  
Scientific Methods ◽  
Human Trials ◽  
History Of ◽  
Medicinal Properties

The term “spices” has been derived from the word “species,” which was connected to the group of exotic foods in medieval times. Spices and herbs have a long history of culinary use, medicinal properties, and as additives and thus have a distinct place in Ayurveda. Exhibiting the merits of spices by scientific methods still remains a challenge. This review investigates the anti-diabetic properties in preventing and managing diabetics and associated complications with commonly used spices. The bioactive compounds in these spices are additionally discussed. The major aim and object of the present work is to investigate the customary therapeutic usage of basic Indian spices and to corelate their observed pharmacological activities with the presence of explicit bioactive compounds present for the treatment or counteractive action of diabetes. This includes the basic underlying mechanism of their blood glucose lowering property including exploratory experimental evidence from proposed animal and human trials.

Damage-fragmentation transition: Size effect and scaling behavior for impact fragmentation of slender projectiles

2016 ◽  
Vol 27 (2) ◽  
pp. 201-217 ◽  
Author(s):  
Sreten Mastilovic
Keyword(s):  
Size Effect ◽  
Finite Size ◽  
Rigid Wall ◽  
Atomistic Simulations ◽  
Generic Model ◽  
Scaling Analysis ◽  
Impact Fragmentation ◽  
Indispensable Tool ◽  
Dynamics Simulations ◽  
Different Diameters

The focus of the present article is on the size effect of a transition region from the damaged to the fragmented phase in impact-induced breakup of a slender projectile. Molecular dynamics simulations of the classic ballistic Taylor test are performed with a simple generic model to explore an extended low-energy range. In the simulation setup, flat-ended, monocrystalline, nanoscale projectiles, with a fixed aspect ratio but 10 different diameters, collide perpendicularly with a rough rigid wall. With gradually increasing impact energy, a non-negligible projectile disintegration eventually takes place and is identified with the damage-fragmentation phase transition. These atomistic simulations offer an indispensable tool to gain an insight into damage evolution in the neighborhood of the damage-fragmentation transition resulting in the occurrence of fragmentation at the critical point. A finite size scaling analysis of the average fragment mass is carried out to determine critical exponents and dependence of the critical striking velocity upon the slender projectile’s diameter.

Critical Review of Early Age Cycling Effects on the Capacity of Pile to Sleeve Grouted Connections As Treated in ISO 19902

2019 ◽  
Author(s):  
Andi Merxhani ◽  
Jacob Fisker Jensen ◽  
Joao Caetano ◽  
Casper Klintø Christiansen
Keyword(s):  
Compressive Strength ◽  
Experimental Evidence ◽  
Structural Design ◽  
High Performance ◽  
Underlying Mechanism ◽  
Early Age ◽  
Design Code ◽  
Concrete Members ◽  
Jacket Structures ◽  
Shearing Mechanism

Abstract The treatment of early age cyclic loading (EAC) on pile-sleeve grouted connections is one of the challenging problems encountered in the design of offshore jacket structures. ISO 19902 appears to be the only offshore structural design code that quantifies the strength de-rating due to EAC. However, the mechanism of EAC considered in the ISO standard is little understood. Main provision is that the strength reduction due to EAC is considered to be linearly proportional to the compressive strength of the grout. This provision is conservatively applicable for medium strength grouts. However, it is over-conservative when modern high-performance grouts are used, and it can be further argued that it actually loses its physical meaning. Thus, a further investigation into the mechanics of the problem is deemed beneficial in order to understand better its underlying mechanism. Using existing experimental evidence and a simple mechanical model, it is shown that the EAC mechanism considered in ISO 19902 triggers the grout matrix failure mechanism of the grouted connections. This behavior is characteristic of segregating grout materials. The benefit of the approach followed is that it links the interpretation of EAC to the grout material properties. The phenomenon is then understood using methods and experimental results that are well established in concrete mechanics. After a comparison with existing experimental evidence on grouted connections and concrete members, it is suggested that the strength drop for the specific shearing mechanism is independent of the uniaxial compressive strength of the grout material.

Frictional Properties of Self-Assembled Alkylsilane Chains on Silica

2001 ◽  
Vol 687 ◽  
Author(s):  
M. Chandross ◽  
B. Park ◽  
M. Stevens ◽  
G.S. Grest
Keyword(s):  
Size Effects ◽  
Finite Size ◽  
Shear Velocity ◽  
Finite Size Effects ◽  
Silica Surfaces ◽  
Frictional Properties ◽  
Covalently Bonded ◽  
Comparable Effect ◽  
System Data ◽  
Dynamics Simulations

AbstractWe present the results of molecular dynamics simulations of pairs of alkylsilane monolayers on silica surfaces under shear. In particular, we investigate the effects of shear velocity on the friction for chains of 6, 8, 12, and 18 carbon atoms covalently bonded to a crystalline surface. Our studies are performed at loads close to 0.2 and 2 GPa for relative velocities of 0.2, 2.0, and 20.0 m/s. We find that for perfect (defect-free) monolayers, the effects of chain length and velocity are weak, indicating that the experimentally measured dependence of friction on these properties is primarily due to defects in the monolayer. We have investigated possible finite size effects by varying our system dimensions from 43 Å ×50 Å Å to 174 Å × 201 Å. We find that increasing the surface area by a factor of N reduces the noise in the shear stress by a factor of , and has a comparable effect to averaging the smaller system data over bins of points. This indicates that finite size effects are negligible in our simulations.

scholarly journals Molecular-dynamics simulations of urea nucleation from aqueous solution

2014 ◽  
Vol 112 (1) ◽  
pp. E6-E14 ◽  
Author(s):  
Matteo Salvalaglio ◽  
Claudio Perego ◽  
Federico Giberti ◽  
Marco Mazzotti ◽  
Michele Parrinello
Keyword(s):  
Aqueous Solution ◽  
Free Energy ◽  
Molecular Simulations ◽  
Finite Size ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Science And Engineering ◽  
Finite Size Effects ◽  
Dynamics Simulations ◽  
Insight Into

Despite its ubiquitous character and relevance in many branches of science and engineering, nucleation from solution remains elusive. In this framework, molecular simulations represent a powerful tool to provide insight into nucleation at the molecular scale. In this work, we combine theory and molecular simulations to describe urea nucleation from aqueous solution. Taking advantage of well-tempered metadynamics, we compute the free-energy change associated to the phase transition. We find that such a free-energy profile is characterized by significant finite-size effects that can, however, be accounted for. The description of the nucleation process emerging from our analysis differs from classical nucleation theory. Nucleation of crystal-like clusters is in fact preceded by large concentration fluctuations, indicating a predominant two-step process, whereby embryonic crystal nuclei emerge from dense, disordered urea clusters. Furthermore, in the early stages of nucleation, two different polymorphs are seen to compete.

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