1,687 research outputs found
Exotic Electrostatics: Unusual Features of Electrostatic Interactions between Macroions
We present an overview of our understanding of electrostatic interactions
between charged macromolecular surfaces mediated by mobile counter- and coions.
The dichotomy between the weak and the strong coupling regimes is described in
detail and the way they engender repulsive and attractive interactions between
nominally equally charged macroions. We also introduce the concept of dressed
counterions in the case of many-component Coulomb fluids that are partially
weakly and partially strongly coupled to local electrostatic fields leading to
non-monotonic interactions between equally charged macroions. The effect of
quenched surface charge disorder on the counterion-mediated electrostatic
interactions is analyzed within the same conceptual framework and shown to lead
to unexpected and extraordinary electrostatic interactions between randomly
charged surfaces with equal mean surface charge densities or even between
effectively neutral macroion surfaces. As a result, these recent developments
challenge some cherished notions of pop culture.Comment: 18 pages, 5 figure
On the gap-opening criterion of migrating planets in protoplanetary disks
We perform two-dimensional hydrodynamical simulations to quantitatively
explore the torque balance criterion for gap-opening (as formulated by Crida et
al. 2006) in a variety of disks when considering a migrating planet. We find
that even when the criterion is satisfied, there are instances when planets
still do not open gaps. We stress that gap-opening is not only dependent on
whether a planet has the ability to open a gap, but whether it can do so
quickly enough. This can be expressed as an additional condition on the
gap-opening timescale versus the crossing time, i.e. the time it takes the
planet to cross the region which it is carving out. While this point has been
briefly made in the previous literature, our results quantify it for a range of
protoplanetary disk properties and planetary masses, demonstrating how crucial
it is for gap-opening. This additional condition has important implications for
the survival of planets formed by core accretion in low mass disks as well as
giant planets or brown dwarfs formed by gravitational instability in massive
disks. It is particularly important for planets with intermediate masses
susceptible to Type III-like migration. For some observed transition disks or
disks with gaps, we expect that estimates on the potential planet masses based
on the torque balance gap-opening criterion alone may not be sufficient. With
consideration of this additional timescale criterion theoretical studies may
find a reduced planet survivability or that planets may migrate further inwards
before opening a gap.Comment: Accepted by ApJ, 22 pages, 13 figures, 6 table
External Inversion, Internal Inversion, and Reflection Invariance
Having in mind that physical systems have different levels of structure we
develop the concept of external, internal and total improper Lorentz
transformation (space inversion and time reversal). A particle obtained from
the ordinary one by the application of internal space inversion or time
reversal is generally a different particle. From this point of view the
intrinsic parity of a nuclear particle (`elementary particle') is in fact the
external intrinsic parity, if we take into account the internal structure of a
particle. We show that non-conservation of the external parity does not
necessarily imply non-invariance of nature under space inversion. The
conventional theory of beta-decay can be corrected by including the internal
degrees of freedom to become invariant under total space inversion, though not
under the external one.Comment: 15 pages. An early proposal of "mirror matter", published in 1974.
This is an exact copy of the published paper. I am posting it here because of
the increasing interest in the "exact parity models" and its experimental
consequence
Hydration force fluctuations in hydrophilic planar systems
Utilizing all-atom simulations with explicit solvent, the authors model
hydrophilicsurfacesinteracting across water at a fixed chemical potential.
They extract the hydration forces acting between the surfaces and assess force
fluctuations as well as interlamellar water number fluctuations. The trends
obtained from the simulations are captured by a continuum-based description
with effective model parameters. The significance of fluctuations depends on
surfacehydrophilicity and rigidity. The authors show that the force
fluctuations play an important role in kinetic processes in systems with
lateral sizes smaller than several tens of nanometers
Atomistic simulations of wetting properties and water films on hydrophilic surfaces
We use molecular simulations to investigate the wetting behavior of water at
flat polar surfaces. Introducing a computational procedure based on
thermodynamic integration methods, we determine the equilibrium water film
thickness on the surface at given vapor density as well as the corresponding
change of the surface free energy. The wetting film is relevant on polar
surfaces near the wetting transition and significantly alters the surface
contact angle. For thin films, the surface free energy change increases
linearly with the thickness, as predicted by simple thermodynamic arguments.
For thick films we observe deviations from linearity, which we rationalize by
the formation of hydrogen bonds between water molecules in the film. Our
approach provides an efficient and accurate technique to calculate the wetting
properties of surface layers, which we verify by simulating water droplets on
the surfaces
From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces
Using all-atom molecular dynamics (MD) simulations at constant water chemical
potential in combination with basic theoretical arguments, we study hydration-
induced interactions between two overall charge-neutral yet polar planar
surfaces with different wetting properties. Whether the water film between the
two surfaces becomes unstable below a threshold separation and cavitation
gives rise to long-range attraction, depends on the sum of the two individual
surface contact angles. Consequently, cavitation-induced attraction also
occurs for a mildly hydrophilic surface interacting with a very hydrophobic
surface. If both surfaces are very hydrophilic, hydration repulsion dominates
at small separations and direct attractive force contribution can—if strong
enough—give rise to wet adhesion in this case. In between the regimes of
cavitation-induced attraction and hydration repulsion we find a narrow range
of contact angle combinations where the surfaces adhere at contact in the
absence of cavitation. This dry adhesion regime is driven by direct
surface–surface interactions. We derive simple laws for the cavitation
transition as well as for the transition between hydration repulsion and dry
adhesion, which favorably compare with simulation results in a generic
adhesion state diagram as a function of the two surface contact angles
Adaptive Resolution Molecular Dynamics Simulation: Changing the Degrees of Freedom on the Fly
We present a new adaptive resolution technique for efficient particle-based
multiscale molecular dynamics (MD) simulations. The presented approach is
tailor-made for molecular systems where atomistic resolution is required only
in spatially localized domains whereas a lower mesoscopic level of detail is
sufficient for the rest of the system. Our method allows an on-the-fly
interchange between a given molecule's atomic and coarse-grained level of
description, enabling us to reach large length and time scales while spatially
retaining atomistic details of the system. The new approach is tested on a
model system of a liquid of tetrahedral molecules. The simulation box is
divided into two regions: one containing only atomistically resolved
tetrahedral molecules, the other containing only one particle coarse-grained
spherical molecules. The molecules can freely move between the two regions
while changing their level of resolution accordingly. The coarse-grained and
the atomistically resolved systems have the same statistical properties at the
same physical conditions.Comment: 17 pages, 11 figures, 5 table
Counterion-Mediated Weak and Strong Coupling Electrostatic Interaction between Like-Charged Cylindrical Dielectrics
We examine the effective counterion-mediated electrostatic interaction
between two like-charged dielectric cylinders immersed in a continuous
dielectric medium containing neutralizing mobile counterions. We focus on the
effects of image charges induced as a result of the dielectric mismatch between
the cylindrical cores and the surrounding dielectric medium and investigate the
counterion-mediated electrostatic interaction between the cylinders in both
limits of weak and strong electrostatic couplings (corresponding, e.g., to
systems with monovalent and multivalent counterions, respectively). The results
are compared with extensive Monte-Carlo simulations exhibiting good agreement
with the limiting weak and strong coupling results in their respective regime
of validity.Comment: 19 pages, 10 figure
Evolutionary models of cold and low-mass planets: Cooling curves, magnitudes, and detectability
Future instruments like NIRCam and MIRI on JWST or METIS at the ELT will be
able to image exoplanets that are too faint for current direct imaging
instruments. Evolutionary models predicting the planetary intrinsic luminosity
as a function of time have traditionally concentrated on gas-dominated giant
planets. We extend these cooling curves to Saturnian and Neptunian planets. We
simulate the cooling of isolated core-dominated and gas giant planets with
masses of 5 Earthmasses to 2 Jupitermasses. The luminosity includes the
contribution from the cooling and contraction of the core and of the H/He
envelope, as well as radiogenic decay. For the atmosphere we use grey,
AMES-Cond, petitCODE, and HELIOS models. We consider solar and non-solar
metallicities as well as cloud-free and cloudy atmospheres. The most important
initial conditions, namely the core-to-envelope ratio and the initial
luminosity are taken from planet formation simulations based on the core
accretion paradigm. We first compare our cooling curves for Uranus, Neptune,
Jupiter, Saturn, GJ 436b, and a 5 Earthmass-planet with a 1% H/He envelope with
other evolutionary models. We then present the temporal evolution of planets
with masses between 5 Earthmasses and 2 Jupitermasses in terms of their
luminosity, effective temperature, radius, and entropy. We discuss the impact
of different post formation entropies. For the different atmosphere types and
initial conditions magnitudes in various filter bands between 0.9 and 30
micrometer wavelength are provided. Using black body fluxes and non-grey
spectra, we estimate the detectability of such planets with JWST. It is found
that a 20 (100) Earthmass-planet can be detected with JWST in the background
limit up to an age of about 10 (100) Myr with NIRCam and MIRI, respectively.Comment: Language corrected version and improved arrangements of figures,
online data at:
http://www.space.unibe.ch/research/research_groups/planets_in_time/numerical_data/index_eng.htm
Understanding the “Berg limit”: the 65° contact angle as the universal adhesion threshold of biomatter
Surface phenomena in aqueous environments such as long-range hydrophobic attraction, macromolecular adhesion, and even biofouling are predominantly influenced by a fundamental parameter—the water contact angle. The minimal contact angle required for these and related phenomena to occur has been repeatedly reported to be around 65° and is commonly referred to as the “Berg limit.” However, the universality of this specific threshold across diverse contexts has remained puzzling. In this perspective article, we aim to rationalize the reoccurrence of this enigmatic contact angle. We show that the relevant scenarios can be effectively conceptualized as three-phase problems involving the surface of interest, water, and a generic oil-like material that is representative of the nonpolar constituents within interacting entities. Our analysis reveals that attraction and adhesion emerge when substrates display an underwater oleophilic character, corresponding to a “hydrophobicity under oil”, which occurs for contact angles above approximately 65°. This streamlined view provides valuable insights into macromolecular interactions and holds implications for technological applications
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