Large-amplitude Density Fluctuations Research Articles

Large amplitude density fluctuations in a lipid bilayer, induced by picosecond electric pulses, and their possible biotechnological applications

Electroporation refers to changes in permeabilization of lipid membranes induced by external pulsed electric fields. Diverse applications of electroporation in biotechnology include gene transfer, modulation of gene expression, cell proliferation, seed germination, sterilization in the food industry, and extraction of chemical compounds. We argue that the formation of large amplitude, subnanometer density fluctuations in lipid bilayers, induced by high-intensity pulsed electric fields with picosecond durations, may produce lipid nanopores and change transmembrane transport, which trigger cellular signalling pathways with subsequent cellular responses and changes in cell physiology, thus, could represent perspectives regarding the development of electroporation biotechnologies.

Simulating TeV gamma-ray morphologies of shell-type supernova remnants

ABSTRACT Supernova remnant (SNR) shocks provide favourable sites of cosmic ray (CR) proton acceleration if the local magnetic field direction is quasi-parallel to the shock normal. Using the moving-mesh magnetohydrodynamical (MHD) code arepo we present a suite of SNR simulations with CR acceleration in the Sedov–Taylor phase that combine different magnetic field topologies, density distributions with gradients and large-scale fluctuations, and – for our core-collapse SNRs – a multiphase interstellar medium with dense clumps with a contrast of 104. Assuming the hadronic gamma-ray emission model for the TeV gamma-ray emission, we find that large-amplitude density fluctuations of δρ/ρ0 ≳ 75 per cent are required to strongly modulate the gamma-ray emissivity in a straw man’s model in which the acceleration efficiency is independent of magnetic obliquity. However, this causes strong corrugations of the shock surface that are ruled out by gamma-ray observations. By contrast, magnetic obliquity-dependent acceleration can easily explain the observed variance in gamma-ray morphologies ranging from SN1006 (with a homogeneous magnetic field) to Vela Junior and RX J1713 (with a turbulent field) in a single model that derives from plasma particle-in-cell simulations. Our best-fitting model for SN1006 has a large-scale density gradient of ∇n ≃ 0.0034 cm−3 pc−1 pointing from south-west to north-east and a magnetic inclination with the plane of the sky of ≲10°. Our best-fitting model for Vela Junior and RX J1713 adopts a combination of turbulent magnetic field and dense clumps to explain their TeV gamma-ray morphologies and moderate shock corrugations.

Construction of a Universal Gel Model with Volume Phase Transition

The physical principle underlying the familiar condensation transition from vapor to liquid is the competition between the energetic tendency to condense owing to attractive forces among molecules of the fluid and the entropic tendency to disperse toward the maximum volume available as limited only by the walls of the container. Van der Waals incorporated this principle into his equation of state and was thus able to explain the discontinuous nature of condensation as the result of instability of intermediate states. The volume phase transition of gels, also discontinuous in its sharpest manifestation, can be understood similarly, as a competition between net free energy attraction of polymer segments and purely entropic dissolution into a maximum allowed volume. Viewed in this way, the gel phase transition would require nothing more to describe it than van der Waals’ original equation of state (with osmotic pressure replacing pressure P). But the polymer segments in a gel are networked by cross-links, and a consequent restoring force prevents complete dissolution. Like a solid material, and unlike a van der Waals fluid, a fully swollen gel possesses an intrinsic volume of its own. Although all thermodynamic descriptions of gel behavior contain an elastic component, frequently in the form of Flory-style rubber theory, the resulting isotherms usually have the same general appearance as van der Waals isotherms for fluids, so it is not clear whether the solid-like aspect of gels, that is, their intrinsic volume and shape, adds any fundamental physics to the volume phase transition of gels beyond what van der Waals already knew. To address this question, we have constructed a universal chemical potential for gels that captures the volume transition while containing no quantities specific to any particular gel. In this sense, it is analogous to the van der Waals theory of fluids in its universal form, but although it incorporates the van der Waals universal equation of state, it also contains a network elasticity component, not based on Flory theory but instead on a nonlinear Langevin model, that restricts the radius of a fully swollen spherical gel to a solid-like finite universal value of unity, transitioning to a value less than unity when the gel collapses. A new family of isotherms arises, not present in a preponderately van der Waals analysis, namely, profiles of gel density as a function of location in the gel. There is an abrupt onset of large amplitude density fluctuations in the gel at a critical temperature. Then, at a second critical temperature, the entire swollen gel collapses to a high-density phase.

Universal McMillan-Mayer van der Waals Langevin Gel.

We present a theory for a universal gel based on a McMillan–Mayer treatment of a solute–solvent fluid as a generalization of the universal van der Waals equation of state for a pure liquid/vapor system. The elastic resilience of the networked gel is modeled by a universal Langevin function. This combination of van der Waals interactions and nonlinear Langevin elasticity produces an abrupt onset of large-amplitude density fluctuations deep in the interior of the gel at a critical temperature. Then, at a second, lower, critical temperature, the entire swollen gel collapses to a high-density phase. The universal gel has an “upper” critical temperature behavior, meaning that the gel transition to high density occurs on decreasing the temperature. At the cost of loss of universality, the theory is generalized to predict lower critical temperature dependence, whereby an aqueous hydrophobic gel exhibits phase coexistence when the temperature is raised. The theory is consistent with the Gibbs phase rule, suitably generalized to coexisting phases that are not at the same pressure in equilibrium conditions.

X-Ray Diffuse Scattering Measurements of Nucleation Dynamics at Femtosecond Resolution

Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process.

Primary Nucleation in Polymer Crystallization

Isothermal crystallization of a low-molecular-weight poly(ethylene oxide) from both isotropic and structured melts at low undercoolings has been investigated via simultaneous measurements of small angle and wide angle X-ray scatterings (SAXS and WAXS). An evolution of the SAXS pattern was observed prior to the appearance of crystallinity determined through WAXS. A possible interpretation of this experimental observation is provided through the nucleation concept: primary nuclei exist in the melt due to localized large-amplitude density fluctuations in the isotropic medium.

Linear and nonlinear dispersive effects on magnetospheric field line resonances

Shear Alfvén wave dispersion produced by electron inertia, ion gyro-kinetic and electron thermal pressure is modeled using two-fluid MHD and kinetic theory. In a dipolar magnetosphere, dispersion and non-linearity determine the spatial structure, temporal evolution and amplitude of parallel electric fields and large amplitude density fluctuations near to the polar ionosphere. Deep auroral density cavities are found to have a strong influence on auroral electric field generation. Many features of satellite and ground based observations of discrete arcs are predicted using two-fluid MHD, but large parallel electric fields (mV/m) and keV electron precipitation cannot easily be explained. To explain the observed electric fields it is necessary to evaluate the non-local kinetic electron response to standing shear Alfvén waves on dipolar magnetic field lines. It is shown that electron trapping leads to a significant reduction of the collisionless electron conductivity and a large enhancement of parallel electric fields in the 1 – 4 mHz frequency range of observed field line resonances.

Plasma fluctuations driven by a randomized laser beam

Nonthermal fluctuations produced by a randomized laser beam in an underdense plasma have been investigated in the regime of a nonlocal electron transport. A nonlocal closure to linearized fluid equations for fluctuating hydrodynamic variables properly accounts for inverse bremsstrahlung heating and ponderomotive force effects. It has been shown that a typical randomized laser beam as used in the inertial confinement fusion experiments can generate large amplitude density fluctuations well above thermal levels. An expression for the Thomson scattering cross-section from these fluctuations has been derived and discussed. The vortical velocity and magnetic field fluctuations can also be enhanced with the vortical plasma velocity reaching values comparable to the ion acoustic velocity.

The Structure of the Central Disk of NGC 1068: A Clumpy Disk Model

NGC 1068 is one of the best studied Seyfert II galaxies, for which the blackhole mass has been determined from the Doppler velocities of water maser. We show that the standard $\alpha$-disk model of NGC 1068 gives disk mass between the radii of 0.65 pc and 1.1 pc (the region from which water maser emission is detected) to be about 7x10$^7$ M$_\odot$ (for $\alpha=0.1$), more than four times the blackhole mass, and a Toomre Q-parameter for the disk is $\sim$0.001. This disk is therefore highly self-gravitating and is subject to large-amplitude density fluctuations. We conclude that the standard $\alpha$-viscosity description for the structure of the accretion disk is invalid for NGC 1068. In this paper we develop a new model for the accretion disk. The disk is considered to be composed of gravitationally bound clumps; accretion in this clumped disk model arises because of gravitational interaction of clumps with each other and the dynamical frictional drag exerted on clumps from the stars in the central region of the galaxy. The clumped disk model provides a self-consistent description of the observations of NGC 1068. The computed temperature and density are within the allowed parameter range for water maser emission, and the rotational velocity in the disk falls off as $r^{-0.35}$.

Auroral density fluctuations on dispersive field line resonances

A model is presented that describes the excitation of density perturbations and parallel electric fields by standing shear Alfvén waves on dipole fields in Earth's magnetosphere. The model includes the effects of electron inertia and gyro‐kinetic dispersion, accounting for field‐aligned variations of the electron and ion temperatures and the ambient plasma density. In a model dipole magnetosphere, it is found that dispersion and nonlinearity determine the depth, spatial structure, and temporal evolution of large‐amplitude density fluctuations near to the polar ionospheres. The characteristics of magnetospheric density cavities and their relationship to auroral luminosity, or field‐aligned currents, is discussed in the context of recent satellite and ground based observations.

One-dimensional analytical model of the phase shift due to Bragg backscattering of an ordinary wave by large amplitude density fluctuations

The phase shift due to the resonant Bragg scattering of an ordinary wave in a fluctuating plasma is numerically computed for large amplitude density fluctuations, i.e. well beyond the Born approximation. The phase response (the phase shift against the perturbation position in the density gradient) is strongly distorted with respect to the small amplitude case, and phase jumps occur. For localized quasi-monochromatic fluctuations, the results are explained by an analytical model where the Helmholtz equation is approximated by a Mathieu equation. The maximum phase shift and the phase jumps are well predicted by the model. It turns out that the theoretical predictions also apply to moderate amplitudes. For an actual phase measurement where the number of different frequencies (i.e. the number of perturbation positions) is finite, most phase jumps are missed, due to the steepness of the phase jumps, and the measured phase shift increases continuously.

Relativistic instabilities of large amplitude waves in plasmas

A critical review of relativistic instabilities of large amplitude electromagnetic waves in plasmas is presented. For illustrative purposes, we first summarize the small amplitude results. We then continue to take into account fully relativistic electron quiver velocity and large amplitude density fluctuations driven by a fully relativistic ponderomotive force and investigate modulational and filamentation instabilities of an intense circularly polarized electromagnetic wave. Thus, the nonlinearities arising from the relativistic electron mass variation and the relativistic ponderomotive force are treated exactly, so that our theory is valid for arbitrarily large values of the radiation intensity. Novel relativistic instabilities are shown to exist at large fluxes of the electromagnetic radiation. The relevance of our investigation to inertial confinement fusion and plasma based beat wave accelerators has been pointed out.

Fully relativistic filamentation instability of strong electromagnetic radiation in unmagnetized plasmas

Accounting for fully relativistic electron quiver velocity and large-amplitude density fluctuations driven by a fully relativistic ponderomotive force, filamentation instability of an intense circularly polarized electromagnetic wave has been investigated. Thus, the nonlinearities arising from the relativistic electron mass variation and the relativistic ponderomotive force are treated exactly, so that our theory is valid for arbitrarily large values of the radiation intensity. A novel filamentation instability is shown to exist at high pump power.

Radiative instabilities in a sheared magnetic field

The structure and growth rate of the radiative instability in a sheared magnetic field B have been calculated analytically using the Braginskii fluid equations. In a shear layer, temperature and density perturbations are linked by the propagation of sound waves parallel to the local magnetic field. As a consequence, density clumping or condensation plays an important role in driving the instability. Parallel thermal conduction localizes the mode to a narrow layer where k∥ =k⋅B/‖B‖ is small and stabilizes short wavelengths k>kc, where kc depends on the local radiation and conduction rates. Thermal coupling to ions also limits the width of the unstable spectrum. It is shown that a broad spectrum of modes is typically unstable in tokamak edge plasmas and it is argued that this instability is sufficiently robust to drive the large-amplitude density fluctuations often measured there.

Observation of classical diffusion in the presence of large amplitude density fluctuations

Density fluctuations, δn n ≲ 20%, were observed to have no effect on the classical diffusion of a collisional helium plasma across a poloidal octupole field with an additional weak toroidal field. The implication of these results on tokamak scattering experiments is discussed.

Observation of a stationary shock-like structure and enhanced diffusion in a toroidal plasma

Electrons are injected into an afterglow plasma confined in an ℓ = 3 stellarator in order to raise the velocity of poloidal rotation. A steady shock-like structure characterized by a steep density gradient is observed near the critical rotational speed. Strong enhancement of the diffusion rate occurs at higher radial electric fields, accompanied by large-amplitude density fluctuations. Vortices are generated at the three corners of the triangular magnetic surfaces when the radial electric field is reversed inside the plasma.