Sridhar Kumar Kannam

How fast does water flow in carbon nanotubes

The purpose of this paper is threefold. First, we review the existing literature on flow rates of water in carbon nanotubes. Data for the slip length which characterizes the flow rate are scattered over 5 orders of magnitude for nanotubes of diameter 0.81-10 nm. Second, we precisely compute the slip length using equilibrium molecular dynamics (EMD) simulations, from which the interfacial friction between water and carbon nanotubes can be found, and also via external field driven non-equilibrium molecular dynamics simulations (NEMD). We discuss some of the issues in simulation studies which may be reasons for the large disagreements reported. By using the EMD method friction coefficient to determine the slip length, we overcome the limitations of NEMD simulations. In NEMD simulations, for each tube we apply a range of external fields to check the linear response of the fluid to the field and reliably extrapolate the results for the slip length to values of the field corresponding to experimentally accessible pressure gradients. Finally, we comment on several issues concerning water flow rates in carbon nanotubes which may lead to some future research directions in this area.

Interfacial slip friction at a fluid-solid cylindrical boundary

Recently we proposed a method to calculate the interfacial friction coefficient between fluid and solid at a planar interface. In this work we extend the method to cylindrical systems where the friction coefficient is curvature dependent. We apply the method to methane flow in carbon nanotubes, and find good agreement with non-equilibrium molecular dynamics simulations. The proposed method is robust, general, and can be used to predict the slip for cylindrical nanofluidic systems.

Sensing of protein molecules through nanopores: a molecular dynamics study

Solid-state nanopores have been shown to be suitable for single molecule detection. While numerous modeling investigations exist for DNA within nanopores, there are few simulations of protein translocations. In this paper, we use atomistic molecular dynamics to investigate the translocation of proteins through a silicon nitride nanopore. The nanopore dimensions and profile are representative of experimental systems. We are able to calculate the change in blockade current and friction coefficient for different positions of the protein within the pore. The change in ionic current is found to be negligible until the protein is fully within the pore and the current is lowest when the protein is in the pore center. Using a simple theory that gives good quantitative agreement with the simulation results we are able to show that the variation in current with position is a function of the pore shape. In simulations that guide the protein through the nanopore we identify the effect that confinement has on the friction coefficient of the protein. This integrated view of translocation at the nanoscale provides useful insights that can be used to guide the design of future devices.

Strategies toward enhanced low-pressure volumetric hydrogen storage in nanoporous cryoadsorbents

The volumetric hydrogen capacity remains one of the most challenging criteria for on-board hydrogen storage system requirements. Here a new concept for hydrogen storage of porous aromatic frameworks (PAFs) impregnated with lithium-decorated fullerenes (Li6C60) is described. The loading of Li6C60 and the effect on the adsorption of hydrogen (H2) has been investigated by molecular simulation. It is shown that the incorporation of Li6C60 can enhance the volumetric capacity of H2 from 12 to 44 g L(-1), a 260% increase at 10 bar and 77 K. The impregnation of Li6C60 increases the heat of adsorption and surface area at the cost of the available pore volume. However, the increase in adsorbed hydrogen outweighs any pore volume loss under optimized Li6C60 loading and operating conditions. In addition, the H2 volumetric uptake is shown to correlate with the volumetric surface area at all pressures whereas the H2 gravimetric uptake correlates with the heat of adsorption at low pressures, surface area at moderate pressures, and pore volume at high pressures.

Dynamical Interactions of 5-Fluorouracil Drug with Dendritic Peptide Vectors: The Impact of Dendrimer Generation, Charge, Counterions, and Structured Water

Molecular dynamics simulations are utilized to investigate the interactions between the skin cancer drug 5-fluorouracil (5FU) and peptide-based dendritic carrier systems. We find that these drug-carrier interactions do not conform to the traditional picture of long-time retention of the drug within a hydrophobic core of the dendrimer carrier. Rather, 5FU, which is moderately soluble in its own right, experiences weak, transient chattering interactions all over the dendrimer, mediated through multiple short-lived hydrogen bonding and close contact events. We find that charge on the periphery of the dendrimer actually has a negative effect on the frequency of drug-carrier interactions due to a counterion screening effect that has not previously been observed. However, charge is nevertheless an important feature since neutral dendrimers are shown to have a significant mutual attraction that can lead to clustering or agglomeration. This clustering is prevented due to charge repulsion for the titrated dendrimers, such that they remain independent in solution.

Water flow in carbon nanotubes : The effect of tube flexibility and thermostat

Although the importance of temperature control in nonequilibrium molecular dynamics simulations is widely accepted, the consequences of the thermostatting approach in the case of strongly confined fluids are underappreciated. We show the strong influence of the thermostatting method on the water transport in carbon nanotubes (CNTs) by considering simulations in which the system temperature is controlled via the walls or via the fluid. Streaming velocities and mass flow rates are found to depend on the tube flexibility and on the thermostatting algorithm, with flow rates up to 20% larger when the walls are flexible. The larger flow rates in flexible CNTs are explained by a lower friction coefficient between water and the wall. Despite the lower friction, a larger solid-fluid interaction energy is found for flexible CNTs than for rigid ones. Furthermore, a comparison of thermostat schemes has shown that the Berendsen and Nosé-Hoover thermostats result in very similar transport rates, while lower flow rates are found under the influence of the Langevin thermostat. These findings illustrate the significant influence of the thermostatting methods on the simulated confined fluid transport.

Effects of Confinement on the Dielectric Response of Water Extends up to Mesoscale Dimensions

The extent of confinement effects on water is not clear in the literature. While some properties are affected only within a few nanometers from the wall surface, others are affected over long length scales, but the range is not clear. In this work, we have examined the dielectric response of confined water under the influence of external electric fields along with the dipolar fluctuations at equilibrium. The confinement induces a strong anisotropic effect which is evident up to 100 nm channel width, and may extend to macroscopic dimensions. The root-mean-square fluctuations of the total orientational dipole moment in the direction perpendicular to the surfaces is 1 order of magnitude smaller than the value attained in the parallel direction and is independent of the channel width. Consequently, the isotropic condition is unlikely to be recovered until the channel width reaches macroscopic dimensions. Consistent with dipole moment fluctuations, the effect of confinement on the dielectric response also persists up to channel widths considerably beyond 100 nm. When an electric field is applied in the perpendicular direction, the orientational relaxation is 3 orders of magnitude faster than the dipolar relaxation in the parallel direction and independent of temperature.

Nanosensors for next generation drug screening

One promising path for future drug screening technologies is to examine the binding of ligands to target proteins at the single molecule level by passing them through nanometer sized pores and measuring the change in pore current during translocation. With the aim of evaluating such technologies we perform virtual experiments on the translocation of proteins through silicon nitride nanopores. These simulations consist of large scale, fully atomistic models of the translocation process that involve steering a test protein through the nanopore on a timescale of tens of nanoseconds. We make a comparison between theoretically expected and simulated values of the current drop that is seen when the protein occupies the pore. Details of the stability of the protein and the preservation of its function as measured by its secondary and tertiary structure will be presented to validate both the simulation results and the fundamental design of the proposed device. Finally, the results will be placed in the context of experimental work that combines nanofabrication and microuidics to create a high throughput, low cost, drug screening device.

Slip of Alkanes Confined between Surfactant Monolayers Adsorbed on Solid Surfaces

The slip and friction behavior of n-hexadecane, confined between organic friction modifier surfactant films adsorbed on hematite surfaces, has been studied using nonequilibrium molecular dynamics simulations. The influence of the surfactant type and coverage, as well as the applied shear rate and pressure, has been investigated. A measurable slip length is only observed for surfactant films with a high surface coverage, which provide smooth interfaces between well-defined surfactant and hexadecane layers. Slip commences above a critical shear rate, beyond which the slip length first increases with increasing shear rate and then asymptotes toward a constant value. The maximum slip length increases significantly with increasing pressure. Systems and conditions which show a larger slip length typically give a lower friction coefficient. Generally, the friction coefficient increases linearly with logarithmic shear rate; however, it shows a much stronger shear rate dependency at low pressure than at high pressure. Relating slip and friction, slip only occurs above a critical shear stress, after which the slip length first increases linearly with increasing shear stress and then asymptotes. This behavior is well-described using previously proposed slip models. This study provides a more detailed understanding of the slip of alkanes on surfactant monolayers. It also suggests that high coverage surfactant films can significantly reduce friction by promoting slip, even when the surfaces are well-separated by a lubricant.

Translational diffusion of proteins in nanochannels

Hydrodynamic interactions play an important role in the transport of analytes through nanoscale devices. Of particular note is the role that no-slip boundary conditions have on the drag coefficient of confined particles and molecules. In this work, we use a coarse grained molecular dynamics model to measure the diffusion coefficients of proteins confined within cylindrical nanochannels of similar dimension. Finite-size corrected bulk diffusion coefficients are found to agree with experimental data, while in channels, a good match is found between theoretical expressions based on continuum fluid mechanics and the reduction of the translational diffusion coefficient across a range of protein to channel size ratios. These results demonstrate that it is possible to directly use molecular simulation to make quantitative predictions of the effects of hydrodynamics on diffusion at length scales of order 1 nm.