1,721,033 research outputs found
Charge Transport in Nanochannels: A Molecular Theory
No full textWe introduce a theoretical and numerical method to investigate the flow of charged fluid mixtures under extreme confinement. We model the electrolyte solution as a ternary mixture, comprising two ionic species of opposite charge and a third uncharged component. The microscopic approach is based on kinetic theory and is fully self- consistent. It allows to determine configurational properties, such as layering near the confining walls, and the flow properties. We show that, under appropriate as- sumptions, the approach reproduces the phenomenological equations used to describe electrokinetic phenomena, without requiring the introduction of constitutive equations to determine the fluxes. Moreover, we model channels of arbitrary shape and nanometric roughness, features that have important repercussions on the transport properties of these system
Kinetic Density Functional Theory: A Microscopic Approach to Fluid Mechanics
No full textIn the present paper we give a brief summary of some recent theoretical advances in the treatment of inhomogeneous fluids and methods which have applications in the study of dynamical properties of liquids in situations of extreme confinement, such as nanopores, nanodevices, etc. The approach obtained by combining kinetic and density functional methods is microscopic, fully self-consistent and allows to determine both configurational and flow properties of dense fluids. The theory predicts the correct hydrodynamic behavior and provides a practical and numerical tool to determine how the transport properties are modified when the length scales of the confining channels are comparable with the size of the molecules. The applications range from the dynamics of simple fluids under confinement, to that of neutral binary mixtures and electrolytes where the theory in the limit of slow gradients reproduces the known phenomenological equations such as the Planck—Nernst—Poisson and the Smolochowski equations. The approach here illustrated allows for fast numerical solution of the evolution equations for the one-particle phase-space distributions by means of the weighted density lattice Boltzmann method and is particularly useful when one considers flows in complex geometries
Pressure induced core packing and interfacial dehydration in nonionic C12E6 micelle in aqueous solution
No full textSteric Modulation of Ionic Currents in DNA Translocation Through Nanopores
No full textIonic currents accompanying DNA translocation strongly depend on molarity of the electrolyte solution and the shape and surface charge of the nanopore. By means of the Poisson–Nernst–Planck equations it is shown how conductance is modulated by the presence of the DNA intruder and as a result of competing electrostatic and confinement factors. The theoretical results reproduce quantitatively the experimental ones and are summarized in a conductance diagram that allows distinguishing the region of reduced conductivity from the region of enhanced conductivity as a function of molarity and the pore dimension
Water confined in nanopores: Spontaneous formation of microcavities
No full textMolecular dynamics simulations of water confined in nanometer sized, hydrophobic channels show that water forms localized cavities for pore diameter greater than or similar to 2.0 nm. The cavities present nonspherical shape and lay preferentially adjacent to the confining wall inducing a peculiar form to the liquid exposed surface. The regime of localized cavitation appears to be correlated with the formation of a vapor layer, as predicted by the Lum-Chandler-Weeks theory [J. Phys. Chem. B 103, 4570 (1999)], implying partial filling of the pore
Pressure and surface tension of an active simple liquid: a comparison between kinetic, mechanical and free-energy based approaches
Full text linkWe discuss different definitions of pressure for a system of active spherical particles driven by a non-thermal coloured noise. We show that mechanical, kinetic and free-energy based approaches lead to the same result up to first order in the non-equilibrium expansion parameter. The first prescription is based on a generalisation of the kinetic mesoscopic virial equation and expresses the pressure exerted on the walls in terms of the average of the virial of the inter-particle forces. In the second approach, the pressure and the surface tension are identified with the volume and area derivatives, respectively, of the partition function associated with the known stationary non-equilibrium distribution of the model. The third method is a mechanical approach and is related to the work necessary to deform the system. The pressure is obtained by comparing the expression of the work in terms of local stress and strain with the corresponding expression in terms of microscopic distribution. This is determined from the force balance encoded in the Born-Green-Yvon equation. Such a method has the advantage of giving a formula for the local pressure tensor and the surface tension even in inhomogeneous situations. By direct inspection, we show that the three procedures lead to the same values of the pressure, and give support to the idea that the partition function, obtained via the unified coloured noise approximation, is more than a formal property of the system, but determines the stationary non-equilibrium thermodynamics of the model
Key Role of Proximal Water in Regulating Thermostable Proteins
No full textThree homologous proteins with mesophilic, thermophilic and hyperthermophilic character have been studied via molecular dynamics simulations at four different temperatures in order to investigate how water controls thermostability. The water-exposed surface of the protein is shown to increase with the degree of thermophilicity, and the role of water in enhancing the protein internal flexibility and structural robustness is elucidated. The presence of water-water hydrogen bond clusters enveloping the macromolecule is shown to correlate with thermal robustness when going from the mesophilic to the hyperthermophilic variants. Our analysis indicates that essential contributions to thermostability stem from protein-water surface effects whereas the protein internal packing plays a minor role
A laboratory model of the aortic root flow including the coronary arteries
No full textCardiovascular flows have been extensively investigated by means of in vitro models to assess the prosthetic valve performances and to provide insight into the fluid dynamics of the heart and proximal aorta. In particular, the models for the study of the flow past the aortic valve have been continuously improved by including, among other things, the compliance of the vessel and more realistic geometries. The flow within the sinuses of Valsalva is known to play a fundamental role in the dynamics of the aortic valve since they host a recirculation region that interacts with the leaflets. The coronary arteries originate from the ostia located within two of the three sinuses, and their presence may significantly affect the fluid dynamics of the aortic root. In spite of their importance, to the extent of the authors’ knowledge, coronary arteries were not included so far when modeling in vitro the transvalvular aortic flow. We present a pulse duplicator consisting of a passively pulsing ventricle, a compliant proximal aorta, and coronary arteries connected to the sinuses of Valsalva. The coronary flow is modulated by a self-regulating device mimicking the physiological mechanism, which is based on the contraction and relaxation of the heart muscle during the cardiac cycle. Results show that the model reproduces satisfyingly the coronary flow. The analysis of the time evolution of the velocity and vorticity fields within the aortic root reveals the main characteristics of the backflow generated through the aorta in order to feed the coronaries during the diastole. Experiments without coronary flow have been run for comparison. Interestingly, the lifetime of the vortex forming in the sinus of Valsalva during the systole is reduced by the presence of the coronaries. As a matter of fact, at the end of the systole, that vortex is washed out because of the suction generated by the coronary flow. Correspondingly, the valve closure is delayed and faster compared to the case with no coronary flow
Water around thermophilic proteins: the role of charged and apolar atoms
No full textThe thermal response of three proteins with mesophilic, thermophilic and hyperthermophilic character hints at the essential role played in thermostability by the protein-water interface. The formation of spanning water clusters enveloping the macromolecule and their resistance to thermal stress is shown to correlate with the charge distribution at the protein surface; in particular our findings suggest an effective role of the superficial charge distribution in stabilizing the global connectivity of the hydration water
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