99 research outputs found
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 (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
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