37 research outputs found
Heavy Particle Modes and Signature of the I-Regime
The recently discovered properties of the I-confinement Regime are explained as resulting from the excitation of a heavy particle mode. The theoretically predicted mode phase velocity in the direction of the electron diamagnetic velocity and the induced confinement of impurities at the edge of the plasma column have been confirmed by the experiments. The direction of the mode phase velocity is consistent with that (opposite) of the spontaneous rotation in the plasma core. The mode is of the âion-mixingâ type, in that it does not produce any electron transport across the fields and it involves significant poloidal magnetic field fluctuations.United States. Department of Energ
Freezing point depression and freeze-thaw damage by nano-fuidic salt trapping
A remarkable variety of organisms and wet materials are able to endure
temperatures far below the freezing point of bulk water. Cryo-tolerance in
biology is usually attributed to "anti-freeze" proteins, and yet massive
supercooling (C) is also possible in porous media containing only
simple aqueous electrolytes. For concrete pavements, the common wisdom is that
freeze-thaw damage results from the expansion of water upon freezing, but this
cannot explain the large pressures (~MPa) required to damage concrete,
the observed correlation between pavement damage and de-icing salts, or the
damage of cement paste loaded with benzene (which contracts upon freezing). In
this Letter, we propose a different mechanism -- nanofluidic salt trapping --
which can explain the observations, using simple mathematical models of
dissolved ions confined to thin liquid films between growing ice and charged
surfaces. Although trapped salt lowers the freezing point, ice nucleation in
charged pores causes enormous disjoining pressures via the rejected ions, until
their removal by precipitation or surface adsorption at a lower temperatures
releases the pressure and allows complete freezing. The theory is able to
predict the non-monotonic salt-concentration dependence of freeze-thaw damage
in concreter and provides a general framework to understand the origins of
cryo-tolerance.Comment: 5 figure
Activity-induced propulsion of a vesicle
Modern biomedical applications such as targeted drug delivery require a
delivery system capable of enhanced transport beyond that of passive Brownian
diffusion. In this work an osmotic mechanism for the propulsion of a vesicle
immersed in a viscous fluid is proposed. By maintaining a steady-state solute
gradient inside the vesicle, a seepage flow of the solvent (e.g., water) across
the semipermeable membrane is generated which in turn propels the vesicle. We
develop a theoretical model for this vesicle-solute system in which the seepage
flow is described by a Darcy flow. Using the reciprocal theorem for Stokes flow
it is shown that the seepage velocity at the exterior surface of the vesicle
generates a thrust force which is balanced by the hydrodynamic drag such that
there is no net force on the vesicle. We characterize the motility of the
vesicle in relation to the concentration distribution of the solute confined
inside the vesicle. Any osmotic solute is able to propel the vesicle so long as
a concentration gradient is present. In the present work, we propose active
Brownian particles (ABPs) as a solute. To maintain a symmetry-breaking
concentration gradient, we consider ABPs with spatially varying swim speed and
ABPs with constant properties but under the influence of an orienting field. In
particular, it is shown that at high activity the vesicle velocity is
, where is the swim pressure just
outside the thin accumulation boundary layer on the interior vesicle surface,
is the unit normal vector of the vesicle boundary,
is the membrane permeability, is the viscosity of the solvent, and
is the membrane thickness
On the IMF in a Triggered Star Formation Context
The origin of the stellar initial mass function (IMF) is a fundamental issue
in the theory of star formation. It is generally fit with a composite power
law. Some clues on the progenitors can be found in dense starless cores that
have a core mass function (CMF) with a similar shape. In the low-mass end,
these mass functions increase with mass, albeit the sample may be somewhat
incomplete; in the high-mass end, the mass functions decrease with mass. There
is an offset in the turn-over mass between the two mass distributions. The
stellar mass for the IMF peak is lower than the corresponding core mass for the
CMF peak in the Pipe Nebula by about a factor of three. Smaller offsets are
found between the IMF and the CMFs in other nebulae. We suggest that the offset
is likely induced during a starburst episode of global star formation which is
triggered by the formation of a few O/B stars in the multi-phase media, which
naturally emerged through the onset of thermal instability in the cloud-core
formation process. We consider the scenario that the ignition of a few massive
stars photoionizes the warm medium between the cores, increases the external
pressure, reduces their Bonnor?Ebert mass, and triggers the collapse of some
previously stable cores. We quantitatively reproduce the IMF in the low-mass
end with the assumption of additional rotational fragmentation.Comment: 3 figure
Multiscale Poromechanics of Wet Cement Paste
Capillary effects such as imbibition-drying cycles impact the mechanics of
granular systems over time. A multiscale poromechanics framework was applied to
cement paste, that is the most common building material, experiencing broad
humidity variations over the lifetime of infrastructure. First, the liquid
density distribution at intermediate to high relative humidities is obtained
using a lattice gas density functional method together with a realistic
nano-granular model of cement hydrates. The calculated adsorption/desorption
isotherms and pore size distributions are discussed and compare well to
nitrogen and water experiments. The standard method for pore size distribution
determination from desorption data is evaluated. Then, the integration of the
Korteweg liquid stress field around each cement hydrate particle provided the
capillary forces at the nanoscale. The cement mesoscale structure was relaxed
under the action of the capillary forces. Local irreversible deformations of
the cement nano-grains assembly were identified due to liquid-solid
interactions. The spatial correlations of the nonaffine displacements extend to
a few tens of nm. Finally, the Love-Weber method provided the homogenized
liquid stress at the micronscale. The homogenization length coincided with the
spatial correlation length nonaffine displacements. Our results on the solid
response to capillary stress field suggest that the micronscale texture is not
affected by mild drying, while local irreversible deformations still occur.
These results pave the way towards understanding capillary phenomena induced
stresses in heterogeneous porous media ranging from construction materials,
hydrogels to living systems.Comment: 6 figures in main text, 4 figures in the SI appendi
On the Coagulation and Size Distribution of Pressure Confined Cores
Observations of the Pipe Nebula have led to the discovery of dense starless
cores. The mass of most cores is too small for their self gravity to hold them
together. Instead, they are thought to be pressure confined. The observed dense
cores' mass function (CMF) matches well with the initial mass function (IMF) of
stars in young clusters. Similar CMF's are observed in other star forming
regions such as the Aquila Nebula, albeit with some dispersion. The shape of
these CMF provides important clues to the competing physical processes which
lead to star formation and its feedback on the interstellar media. In this
paper, we investigate the dynamical origin of the the mass function of starless
cores which are confined by a warm, less dense medium. We consider the
coagulation between the cold cores and their ablation due to Kelvin-Helmholtz
instability induced by their relative motion through the warm medium. We are
able to reproduce the observed CMF among the starless cores in the Pipe nebula.
Our results indicate that in environment similar to the Pipe nebula: 1) before
the onset of their gravitational collapse, the mass distribution of the
progenitor cores is similar to that of the young stars, 2) the observed CMF is
a robust consequence of dynamical equilibrium between the coagulation and
ablation of cores, and 3) a break in the slope of the CMF is due to the
enhancement of collisional cross section and suppression of ablation for cores
with masses larger than the cores' Bonnor-Ebert mass.Comment: Accepted by Ap
Dielectric breakdown by electric-field induced phase separation
The control of the dielectric and conductive properties of device-level
systems is important for increasing the efficiency of energy- and
information-related technologies. In some cases, such as neuromorphic
computing, it is desirable to increase the conductivity of an initially
insulating medium by several orders of magnitude, resulting in effective
dielectric breakdown. Here, we show that by tuning the value of the applied
electric field in systems { with variable permittivity and electric
conductivity}, e.g. ion intercalation materials, we can vary the device-level
electrical conductivity by orders of magnitude. We attribute this behavior to
the formation of filament-like conductive domains that percolate throughout the
system, { which form only when the electric conductivity depends on the
concentration}. We conclude by discussing the applicability of our results in
neuromorphic computing devices and Li-ion batteries.Comment: 12 pages, 5 figure
Freezing point depression and freeze-thaw damage by nanofluidic salt trapping
A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryotolerance in biology is usually attributed to âantifreezeâ proteins, and yet massive supercooling (10 MPa) required to damage concrete, the observed correlation between pavement damage and deicing salts, or the FT damage of cement paste loaded with benzene (which contracts upon freezing). In this work, we propose a different mechanismânanofluidic salt trappingâwhich can explain the observations, using simple mathematical models of dissolved ions confined between growing ice and charged pore surfaces. When the transport time scale for ions through charged pore space is prolonged, ice formation in confined pores causes enormous disjoining pressures via the ions rejected from the ice core, until their removal by precipitation or surface adsorption at lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the nonmonotonic salt-concentration dependence of FT damage in concrete and provides some hint to better understand the origins of cryotolerance from a physical chemistry perspective