Equilibrium Radius Research Articles

Solubility of argon in laser additive manufactured α-titanium under hot isostatic pressing condition

The solubility of Ar in α-Ti under temperature and pressure conditions typical to hot isostatic pressing (HIP) of pore-laden laser additive manufactured parts is investigated. The equilibrium pressure and radius of an Ar-containing pore during HIP, as a function of initial pore radius, are evaluated from a thermodynamics perspective. It is found that a reduction in pore radius by one to two orders of magnitude, as well as extremely high pore pressures on the order of GPa, can result from HIP. Using density functional theory (DFT), the formation energies of Ar solutes in the pure Ti matrix are calculated. Several different types of solutes, including interstitial in single crystal and substitutional in both single crystal and (101¯1) and (101¯2) twin boundaries, are considered in the calculations. The solubility of Ar in Ti is then calculated based on the formation energies as a function of both temperature and pressure. Although the dissolution of Ar in Ti is highly endothermic, substitutional Ar atoms in Ti, especially in crystallographic defects like twin boundaries, have been found to possess significantly lower formation energies than interstitials. The corresponding solubility of Ar within the Ti matrix is found to be very low. However, it increases substantially under the elevated pressure and temperature conditions in the pores during HIP, which further facilitates the removal of Ar.

Dynamics of two gas bubbles in liquid in an ultrasonic traveling wave

The influence of the liquid viscosity and compressibility on the dynamics of two air bubbles (with equilibrium radii of 5 μm) in water at room conditions under the action of a plane ultrasonic wave traveling along the line of the bubble centers (the wavelength is 5000 μm, the amplitude is 0.3 bar) is studied. The initial distance between the centers of the bubbles is six bubble radii. A mathematical model is used, which is fourth-order accurate in terms of the ratio of the radius of the bubbles to the distance between them. It is shown that the spatial displacements of the bubbles are determined mainly by their hydrodynamic interaction. The influence of the liquid viscosity and compressibility is generally significant, and the viscosity affects much more. Without account of the liquid viscosity and compressibility, the bubbles collide with each other after the action of 4.5 running-wave lengths. With taking into account the liquid compressibility, the bubbles under the same action remain remote at a distance on the order of their equilibrium radii, while with additionally allowing for the liquid viscosity, their spacing is kept close to the initial one.

Gyrotropic Skyrmion Modes in Ultrathin Magnetic Circular Dots

We calculate gyrotropic excitation modes of the skyrmion ground state in ultrathin cylindrical magnetic dots. The skyrmion is assumed to be stabilized at room temperature and zero external magnetic field due to an interplay of the isotropic and Dzyaloshinskii-Moriya exchange interactions, perpendicular magnetic anisotropy, and magnetostatic interaction. The gyrotropic frequencies of the Bloch- and Neel-type magnetic skyrmions are calculated in the gigahertz range as a function of the skyrmion equilibrium radius, dot radius, and the dot magnetic parameters. Recent experiments and simulations of magnetic skyrmion gyrotropic dynamics in nanodots are discussed.

On the stability of bardeen thin-shell wormholes

This paper investigates stability of Bardeen thin-shell wormholes constructed by cut and paste technique. We use Israel formalism to evaluate surface energy density as well as pressure and check the behavior of energy conditions. The linearized stability of the respective thin-shell wormholes is investigated by using three choices of variable equation of state; two of them provide stability regions at equilibrium radius. We conclude that stable Bardeen wormhole solutions exist for particular values of equilibrium radius as well as equation of state parameter.

Superselective Adsorption of Multivalent Polymer Chains to a Surface with Receptors

Multivalent polymer chains exhibit excellent prospect in biomedical applications by serving as therapeutic agents. Using three-dimensional (3D) Langevin dynamics simulations, we investigate adsorption behaviors of multivalent polymer chains to a surface with receptors. Multivalent polymer chains display superselective adsorption. Furthermore, the range of density of surface receptors at which a multivalent polymer chain displays a superselective behavior, narrows down for chains with higher density of ligands. Meanwhile, the optimal density of surface receptors where the highest superselectivity is achieved, decreases with increasing the density of ligands. Then, the conformational properties of bound multivalent chains are studied systematically. Interestingly, we find that the equilibrium radius of gyration Rg and its horizontal component have a maximum as a function of the density of surface receptors. The scaling exponents of Rg with the length of chain suggest that with increasing the density of surface receptors., the conformations of a bound multivalent polymer chain first fall in between those of a two-dimensional (2D) and a 3D chain, while it is slightly collapsed subsequently.

Nucleation of magnetic bubble domains in iron garnet films by means of an electric probe

The possibility of the local nucleation of magnetic bubble domains from a single-domain state in an arbitrary region of a iron garnet film ((210) crystallographic orientation) by means of an electrically charged tip electrode has been experimentally demonstrated. The size of magnetic bubble domains nucleated near the contact point between the probe and sample depends on the magnitude of a DC voltage supplied to the probe. After the removal of the voltage, magnetic bubble domains move away from the probe, decreasing to the equilibrium radius.

Stability and natural frequency of nonspherical mode of an encapsulated microbubble in a viscous liquid

The dynamics of encapsulated microbubbles (EMBs) subject to an ultrasound wave have wide and important medical applications, including sonography, drug delivery, and sonoporation. The nonspherical shape oscillation of an EMB, termed as shape modes, is one of the core mechanisms of these applications and therefore its natural frequency is a fundamentally important parameter. Based on the linear stability theory, we show that shape modes of an EMB in a viscous Newtonian liquid are stable. We derive an explicit expression for the natural frequency of shape modes, in terms of the equilibrium radius of an EMB, and the parameters of the external liquid, coating, and internal gases. The expression is validated by comparing to the numerical results obtained from the dynamic equations of shape modes of an EMB. The natural frequency of shape modes shifts appreciably due to the viscosity of the liquid, and this trend increases with the mode number. The significant viscous effects are due to the no-slip condition for the liquid flow at the surface of an EMB. Our results show that when subject to an acoustic wave, the shape instability for an EMB is prone to appear if 2ωk/ωd = n, where ωk is the natural frequency of shape modes, ωd is the driving frequency of the acoustic wave, and n is a natural number. The effects of viscosity on the natural frequency is thus critical in setting the driving frequency of ultrasound to avoid or activate shape modes of EMBs, which should be considered in the applications of medical ultrasound.

Dark spherical shell solitons in three-dimensional Bose-Einstein condensates: Existence, stability, and dynamics

In this work we study spherical shell dark soliton states in three-dimensional atomic Bose-Einstein condensates. Their symmetry is exploited in order to analyze their existence, as well as that of topologically charged variants of the structures, and, importantly, to identify their linear stability Bogolyubov-de Gennes spectrum. We compare our effective 1D spherical and 2D cylindrical computations with the full 3D numerics. An important conclusion is that such spherical shell solitons can be stable sufficiently close to the linear limit of the isotropic condensates considered herein. We have also identified their instabilities leading to the emergence of vortex line and vortex ring cages. In addition, we generalize effective particle pictures of lower dimensional dark solitons and ring dark solitons to the spherical shell solitons concerning their equilibrium radius and effective dynamics around it. In this case too, we favorably compare the resulting predictions such as the shell equilibrium radius, qualitatively and quantitatively, with full numerical solutions in 3D.

Electron Paramagnetic Resonance Spectroscopic Investigation of the Dynamics of Spin Probe in Room Temperature Ionic Liquid

Rotational motion of the spin probe 4-hydroxyl-2,2,6,6- tetramethyl-1-piperidinyloxy (TEMPOL) in methyl imidazoium based ionic liquid was studied by Electron Paramagnetic Resonance spectroscopy (EPR). The influence of rotational tumbling of the paramagnetic spin probe on the width and shape of EPR transition was studied to calculate rotational correlation time, equilibrium radius, and rotational diffusion coefficient of spin probe in different ionic liquid media; CnmimPF6/NTf2 (n=4, 6, 8). The glass transition temperatures for CnmimNTf2 (n=4, 6) were calculated from the inflection point of τc vs T plots whereas activation energy of the rotation were calculated from the Arrhenius plots of lnD vs 1/ T(K).

Note on linearized stability of Schwarzschild thin-shell wormholes with variable equations of state

We discuss how the assumption of variable equation of state (EoS) allows the elimination of the instability at equilibrium throat radius $a_0=3M$ featured by previous Schwarzschild thin-shell wormhole models. Unobstructed stability regions are found for three choices of variable EoS. Two of these EoS entail linear stability at every equilibrium radius. Particularly, the thin-shell remains stable as $a_0$ approaches the Schwarzschild radius $2M$. A perturbative analysis of the wormhole equation of motion is carried out in the case of variable Chaplygin EoS. The squared proper angular frequency $\omega_0^2$ of small throat oscillations is linked with the second derivative of the thin-shell potential. In various situations $\omega_0^2$ remains positive and bounded in the limit $a_0\rightarrow 2M$.

Bubble formation in water with addition of a hydrophobic solute.

We show that phase separation can occur in a one-component liquid outside its coexistence curve (CX) with addition of a small amount of a solute. The solute concentration at the transition decreases with increasing the difference of the solvation chemical potential between liquid and gas. As a typical bubble-forming solute, we consider O2 in ambient liquid water, which exhibits mild hydrophobicity and its critical temperature is lower than that of water. Such a solute can be expelled from the liquid to form gaseous domains while the surrounding liquid pressure is higher than the saturated vapor pressure p cx. This solute-induced bubble formation is a first-order transition in bulk and on a partially dried wall, while a gas film grows continuously on a completely dried wall. We set up a bubble free energy ΔG for bulk and surface bubbles with a small volume fraction ϕ. It becomes a function of the bubble radius R under the Laplace pressure balance. Then, for sufficiently large solute densities above a threshold, ΔG exhibits a local maximum at a critical radius and a minimum at an equilibrium radius. We also examine solute-induced nucleation taking place outside CX, where bubbles larger than the critical radius grow until attainment of equilibrium.

Exotic Self-trapped States of an Electron in Superfluid Helium

We explore the possibility that the fast and exotic negative ions in superfluid helium are electrons bound to quantized vortex structures, the simplest being a ring. In the states we consider, the electron energy is only slightly below the conduction band minimum of bulk helium. To support our proposal, we present two calculations. In the first, we show that the electron pressure on the vortex core is insufficient to cavitate the helium and form an electron bubble. In the second, we estimate the equilibrium radius of the vortex ring that would bind an electron and find it is much smaller than the electron bubble, about 7 A. The many exotic ions reported in experiments might be bound states of an electron with more complex vortex structures.

Computational study of the dynamics of two interacting bubbles in a megasonic field

Clarification of the mechanism of particle removal by megasonic cleaning and control of cavitation bubbles in the megasonic field are essential for cleaning of nanodevices without pattern damage. Multiple bubble interactions complicate the mechanism of particle removal. Therefore, it is important to understand multiple bubble dynamics to clarify the mechanism of particle removal by megasonic cleaning. In the present study, the dynamics of two bubbles in a megasonic field with several initial radii and initial separation distances were simulated by numerical analysis using a compressible locally homogeneous model of a gas–liquid two-phase medium. The present numerical method simulated the various complex behaviors of two bubbles, which are repulsive motion, coalescence, periodic and stable motion of the separation distance, and bubble breakup. The initial separation distance strongly affected the behavior of the two bubbles because the effect of the secondary pressure induced by the oscillation of one bubble on the other bubble depends on the separation distance. In particular, when the equilibrium radii are larger than the resonant radius and the radius of one or both bubbles is close to the resonant radius, the bubbles can show characteristic behaviors, such as periodic and stable motion of the separation distance.

Microbubble dynamics monitoring using a dual modulation method.

An experimental method for characterizing microbubbles' oscillations is presented. With a Dual Frequency ultrasound excitation method, both relative and absolute microbubble size variations can be measured. Using the same experimental setup, a simple signal processing step applied to both the amplitude and the frequency modulations yields a two-fold picture of microbubbles' dynamics. In addition, assuming the occurrence of small radial oscillations, the equilibrium radius of the microbubbles can be accurately estimated.

Bubble dynamics in a viscoelastic medium with nonlinear elasticity

AbstractIn a variety of recently developed medical procedures, bubbles are formed directly in soft tissue and may cause damage. While cavitation in Newtonian liquids has received significant attention, bubble dynamics in tissue, a viscoelastic medium, remains poorly understood. To model tissue, most previous studies have focused on Maxwell-based viscoelastic fluids. However, soft tissue generally possesses an original configuration to which it relaxes after deformation. Thus, a Kelvin–Voigt-based viscoelastic model is expected to be a more appropriate representation. Furthermore, large oscillations may occur, thus violating the infinitesimal strain assumption and requiring a nonlinear/finite-strain elasticity description. In this article, we develop a theoretical framework to simulate spherical bubble dynamics in a viscoelastic medium with nonlinear elasticity. Following modern continuum mechanics formalism, we derive the form of the elastic forces acting on a bubble for common strain-energy functions (e.g. neo-Hookean, Mooney–Rivlin) and incorporate them into Rayleigh–Plesset-like equations. The main effects of nonlinear elasticity are to reduce the violence of the collapse and rebound for large departures from the equilibrium radius, and increase the oscillation frequency. The present approach can readily be extended to other strain-energy functions and used to compute the stress/deformation fields in the surrounding medium.

Experimental Method for Microbubbles Dynamics Monitoring and Radius Sizing

Rationale and aim: Within the context of divers’ decompression illness prevention, ultrasonic detection and sizing of circulating microbubbles in blood is of great interest. In order to be representative of the divers gas tension level (supersaturation) and thus, to optimize decompression stages, the measurements (made in the right ventricle region) should be performed during a short period of time (ventricle filling <20ms), efficient to detect a broad range of bubbles’ radii population (radius from 20 to 200 _m) and harmless (Mechanical Index MI<0.3). Materials and methodsBased on a bi-frequency ultrasound excitation, the purpose of our method is to measure the relative and the absolute microbubble size variations. Because of our research interests, the experimental investigations were conducted on natural microbubbles, with radius ranging between 20 and 200μm, excited around their resonance frequencies by a low frequency transducer. Different types of excitation were tested (sweep, burst, pulse). A pair of high frequency transducers were arranged to focus at a common point. One of the transducers was used to transmit a 2ms duration high-frequency (1MHz) pulse while the other was used to passively receive backscattered signals. The scattered signal was acquired and visualized on a digital oscilloscope and transferred for offline calculations. Signal treatment were conducted in order to recover the amplitude and frequency modulations. ResultsUsing the same experimental setup, simple signal processing applied on both the amplitude and the frequency modulations leads to a double characterization of the microbubble dynamics. Moreover, under the assumption of small radial oscillations, the equilibrium radius of the microbubble can be accurately estimated.

Charged Thin Shell Wormholes with Variable Equations of State

Using the Darmois-Israel formalism the dynamical analysis of Reissner Nordstrom (RN) thin shell wormholes, at the wormhole throat, are determined by considering linearized radial perturbations around static solutions. Linearized stability of thin-shell wormholes with barotropic equation of state (EoS) and with two different EoS is derived. In the first case of variable EoS, with regular coefficients, a sequence of semi-infinite stability regions is found such that every throat in equilibrium becomes stable for a particular subsequence. In the second case, a singular EoS (in such variable EoS the coefficients is explicitly dependent on throat radius), the second derivative of the effective potential is positive definite, so linearized stability is guaranteed for every equilibrium radius.

Charged thin shell wormholes with variable equations of state

Using the Darmois-Israel formalism the dynamical analysis of Reissner Nordstrom (RN) thin shell wormholes, at the wormhole throat, are determined by considering linearized radial perturbations around static solutions. Linearized stability of thin-shell wormholes with barotropic equation of state (EoS) and with two different EoS is derived. In the first case of variable EoS, with regular coefficients, a sequence of semi-infinite stability regions is found such that every throat in equilibrium becomes stable for a particular subsequence. In the second case, a singular EoS (in such variable EoS the coefficients is explicitly dependent on throat radius), the second derivative of the effective potential is positive definite, so linearized stability is guaranteed for every equilibrium radius.

Lateral Exchange Smooths the Way for Vimentin Filaments

Lateral Exchange Smooths the Way for Vimentin Filaments

Enhancement of response speed of viscous fluids using overdrive voltage

Fast, identical response speeds over a full operating range are critical for electrowetting-driven devices such as liquid lenses, microswitches, and reflective displays. In this study, electrowetting actuation with overdrive signals is employed to reduce response time. We explore spreading behaviors and response time (i.e., the time required to reach equilibrium radius) of electrowetted droplets under various DC and overdrive electrowetting actuation conditions. When the droplet is actuated by an overdrive electrical signal, response time is reduced to as low as 1/25 of the response time using a DC signal. We present a theoretical model that considers contact angle saturation and predicts spreading dynamics of droplets under DC and overdrive electrowetting actuation to within 5% error. Using only signal control, our proposed method can be used for multiple practical electrowetting-based applications without any modification of the current platform of electrowetting systems.