Charge polarization, local electroneutrality breakdown and eddy formation due to electroosmosis in varying-section channels

We characterize the dynamics of an electrolyte embedded in a varying-section channel under the action of a constant external electrostatic field. By means of molecular dynamics simulations we determine the stationary density, charge and velocity profiles of the electrolyte. Our results show that when the Debye length is comparable to the width of the channel bottlenecks a concentration polarization along with two eddies sets inside the channel. Interestingly, upon increasing the external field, local electroneutrality breaks down and charge polarization sets leading to the onset of net dipolar field. This novel scenario, that cannot be captured by the standard approaches based on local electroneutrality, opens the route for the realization of novel micro and nano-fluidic devices

Driven diffusion against electrostatic or effective energy barrier across Alpha-Hemolysin

We analyze the translocation of a charged particle across an Alpha-Hemolysin (aHL) pore in the framework of a driven diffusion over an extended energy barrier generated by the electrical charges of the aHL. A one-dimensional electrostatic potential is extracted from the full 3D solution of the Poisson's equation. We characterize the particle transport under the action of a constant forcing by studying the statistics of the translocation time. We derive an analytical expression of translocation time average that compares well with the results from Brownian dynamic simulations of driven particles over the electrostatic potential. Moreover, we show that the translocation time distributions can be perfectly described by a simple theory which replaces the true barrier by an equivalent structureless square barrier. Remarkably our approach maintains its accuracy also for low-applied voltage regimes where the usual inverse-Gaussian approximation fails. Finally we discuss how the comparison between the simulated time distributions and their theoretical prediction results to be greatly simplified when using the notion of the empirical Laplace transform technique.Comment: RevTeX 4-1, 11 pages, 6 pdf figures, J. Chem. Phys. 2015 in pres

Geometrically Induced Selectivity and Unidirectional Electroosmosis in Uncharged Nanopores

Selectivity towards positive and negative ions in nanopores is often associated with electroosmotic flow, the control of which is pivotal in several micro-nanofluidic technologies. Selectivity is traditionally understood to be a consequence of surface charges that alter the ion distribution in the pore lumen. Here we present a purely geometrical mechanism to induce ionic selectivity and electroosmotic flow in uncharged nanopores and we tested it via molecular dynamics simulations. Our approach exploits the accumulation of charges, driven by an external electric field, in a coaxial cavity that decorates the membrane close to the pore entrance. The selectivity was shown to depend on the applied voltage and results to be completely inverted when reverting the voltage. The simultaneous inversion of ionic selectivity and electric field direction causes a unidirectional electroosmotic flow. We developed a quantitatively accurate theoretical model for designing pore geometry to achieve the desired electroosmotic velocity. Finally, we demonstrate that unidirectional electroosmosis also occurs for a biological pore whose structure presents a coaxial cavity surrounding the pore constriction. The capability to induce ion selectivity without altering the pore lumen shape or the surface charge paves the way to a more flexible design of selective membranes

On the Mechanism of Chloroquine Resistance in Plasmodium falciparum

Resistance to chloroquine of malaria strains is known to be associated with a parasite protein named PfCRT, the mutated form of which is able to reduce chloroquine accumulation in the digestive vacuole of the pathogen. Whether the protein mediates extrusion of the drug acting as a channel or as a carrier and which is the protonation state of its chloroquine substrate is the subject of a scientific debate. We present here an analytical approach that explores which combination of hypotheses on the mechanism of transport and the protonation state of chloroquine are consistent with available equilibrium experimental data. We show that the available experimental data are not, by themselves, sufficient to conclude whether the protein acts as a channel or as a transporter, which explains the origin of their different interpretation by different authors. Interestingly, though, each of the two models is only consistent with a subset of hypotheses on the protonation state of the transported molecule. The combination of these results with a sequence and structure analysis of PfCRT, which strongly suggests that the molecule is a carrier, indicates that the transported species is either or both the mono and di-protonated forms of chloroquine. We believe that our results, besides shedding light on the mechanism of chloroquine resistance in P. falciparum, have implications for the development of novel therapies against resistant malaria strains and demonstrate the usefulness of an approach combining systems biology strategies with structural bioinformatics and experimental data

Assessment of heat transfer and Mach number effects on high-speed turbulent boundary layers

High-speed vehicles experience a highly challenging environment in which the free-stream Mach number and surface temperature greatly influence aerodynamic drag and heat transfer. The interplay of these two parameters strongly affects the near-wall dynamics of high-speed turbulent boundary layers in a non-trivial way, breaking similarity arguments on velocity and temperature fields, typically derived for adiabatic cases. In this work, we present direct numerical simulations of flat-plate zero-pressure-gradient turbulent boundary layers spanning three free-stream Mach numbers [2,4,6] and four wall temperature conditions (from adiabatic to very cold walls), emphasising the choice of the diabatic parameter $\mathit{\Theta}$ (Zhang, Bi, Hussain & She, J. Fluid Mech., vol. 739, pp. 392-420) to recover a similar flow organisation at different Mach numbers. We link qualitative observations on flow patterns to first- and second-order statistics to explain the strong decoupling of temperature-velocity fluctuations that occurs at reduced wall temperatures and high Mach numbers. For these cases, we find that the mean temperature gradient in the near-wall region can reach such a strong intensity that it promotes the formation of a secondary peak of thermal production in the viscous sublayer, which is in direct contrast with the monotonic behaviour of adiabatic profiles. We propose different physical mechanisms induced by wall-cooling and compressibility that result in apparently similar flow features, such as a higher peak in the streamwise velocity turbulence intensity, and distinct ones, such as the separation of turbulent scales

Hydrophobically gated memristive nanopores for neuromorphic applications

Signal transmission in the brain relies on voltage-gated ion channels, which exhibit the electrical behaviour of memristors, resistors with memory. State-of-the-art technologies currently employ semiconductor-based neuromorphic approaches, which have already demonstrated their efficacy in machine learning systems. However, these approaches still cannot match performance achieved by biological neurons in terms of energy efficiency and size. In this study, we utilise molecular dynamics simulations, continuum models, and electrophysiological experiments to propose and realise a bioinspired hydrophobically gated memristive nanopore. Our findings indicate that hydrophobic gating enables memory through an electrowetting mechanism, and we establish simple design rules accordingly. Through the engineering of a biological nanopore, we successfully replicate the characteristic hysteresis cycles of a memristor and construct a synaptic device capable of learning and forgetting. This advancement offers a promising pathway for the realization of nanoscale, cost- and energy-effective, and adaptable bioinspired memristors.</p

Hydrophobically gated memristive nanopores for neuromorphic applications

Signal transmission in the brain relies on voltage-gated ion channels, which exhibit the electrical behaviour of memristors, resistors with memory. State-of-the-art technologies currently employ semiconductor-based neuromorphic approaches, which have already demonstrated their efficacy in machine learning systems. However, these approaches still cannot match performance achieved by biological neurons in terms of energy efficiency and size. In this study, we utilise molecular dynamics simulations, continuum models, and electrophysiological experiments to propose and realise a bioinspired hydrophobically gated memristive nanopore. Our findings indicate that hydrophobic gating enables memory through an electrowetting mechanism, and we establish simple design rules accordingly. Through the engineering of a biological nanopore, we successfully replicate the characteristic hysteresis cycles of a memristor and construct a synaptic device capable of learning and forgetting. This advancement offers a promising pathway for the realization of nanoscale, cost- and energy-effective, and adaptable bioinspired memristors.</p