22,004 research outputs found
Rod-like Polyelectrolytes in Presence of Monovalent Salt
We investigate the properties of rigid polyelectrolyte solutions in presence
of monovalent salt. The free energy within the Debye-H\"uckel-Bjerrum (DHBj)
theory [M. E.
Fisher and Y. Levin, {\it Phys. Rev. Lett.} 71, 3826 (1993)] is constructed.
It is found that at thermodynamic equilibrium the polyelectrolyte solution
consists of clusters composed of one polyion and various counterions. The
distribution of the cluster densities is determined by finding the minimum of
the Helmholtz free energy. The osmotic pressure and the average charge of the
cluster are found and their dependence on Manning parameter is
elucidated. A good agreement with the experimental results is obtained.Comment: 11 pages, Revtex (using twocolumn style), 15 figures and postscript
file. Submitted to Macromomelecule
Memory texts and memory work: Performances of memory in and with visual media
The online version of this article can be found at: http://mss.sagepub.com/content/early/2010/05/24/175069801037003
Observations and scaling of travelling bubble cavitation
Recent observations of growing and collapsing bubbles in flows over axisymmetric headforms have revealed the complexity of the ‘micro-fluid-mechanics’ associated with these bubbles (van der Meulen & van Renesse 1989; Briancon-Marjollet et al. 1990; Ceccio & Brennen 1991). Among the complex features observed were the bubble-to-bubble and bubble-to-boundary-layer interactions which leads to the shearing of the underside of the bubble and alters the collapsing process. All of these previous tests, though, were performed on small headform sizes. The focus of this research is to analyse the scaling effects of these phenomena due to variations in model size, Reynolds number and cavitation number. For this purpose, cavitating flows over Schiebe headforms of different sizes (5.08, 25.4 and 50.8 cm in diameter) were studied in the David Taylor Large Cavitation Channel (LCC). The bubble dynamics captured using high-speed film and electrode sensors are presented along with the noise signals generated during the collapse of the cavities.
In the light of the complexity of the dynamics of the travelling bubbles and the important bubble/bubble interactions, it is clear that the spherical Rayleigh-Plesset analysis cannot reproduce many of the phenomena observed. For this purpose an unsteady numerical code was developed which uses travelling sources to model the interactions between the bubble (or bubbles) and the pressure gradients in the irrotational flow outside the boundary layer on the headform. The paper compares the results of this numerical code with the present experimental results and demonstrates good qualitative agreement between the two
Flexible Polyelectrolytes with Monovalent Salt
We present a model for describing flexible polyelectrolytes in a good solvent
a nd in the presence of monovalent salt . The molecule composed by monomers
is characterized by the end to end distanc e and the
number of associated counterions . At high tem peratures the polyelectrolyte
behaves as a neutral polymer (). Dec reasing the temperature, the
macromolecule changes from this extended configurat ion() to a
stretched form (). At even lower temp eratures, above the
Manning condensation threshold, the polyelectrolyte collapse s (). Our results show good agreement with simulations
Amphiphile Adsorption on Rigid Polyelectrolytes
A theory is presented which quantitatively accounts for the cooperative
adsorption of cationic surfactants to anionic polyelectrolytes. For high salt
concentration we find that the critical adsorption concentration (CAC) is a
bilinear function of the polyion monomer and salt concentrations, with the
coefficients dependent only on the type of surfactant used. The results
presented in the paper might be useful for designing more efficient gene
delivery systems
Polyelectrolyte Solutions with Multivalent Salts
We investigate the thermodynamic properties of a polyelectrolyte solution in
a presence of {\it multivalent} salts. The polyions are modeled as rigid
cylinders with the charge distributed uniformly along the major axis. The
solution, besides the polyions, contain monovalent and divalent counterions as
well as monovalent coions. The strong electrostatic attraction existing between
the polyions and the counterions results in formation of clusters consisting of
one polyion and a number of associated monovalent and divalent counterions. The
theory presented in the paper allows us to explicitly construct the Helmholtz
free energy of a polyelectrolyte solution. The characteristic cluster size, as
well as any other thermodynamic property can then be determined by an
appropriate operation on the free energy
Universality class of quantum criticality for strongly repulsive spin-1 bosons with antiferromagnetic spin-exchange interaction
Using the thermodynamic Bethe ansatz equations we study the quantum phase
diagram, thermodynamics and criticality of one-dimensional spin-1 bosons with
strongly repulsive density-density and antiferromagnetic spin-exchange
interactions. We analytically derive a high precision equation of state from
which the Tomonaga-Luttinger liquid physics and quantum critical behavior of
the system are computed. We obtain explicit forms for the scaling functions
near the critical points yielding the dynamical exponent and correlation
length exponent for the quantum phase transitions driven by either
the chemical potential or the magnetic field. Consequently, we further
demonstrate that quantum criticality of the system can be mapped out from the
finite temperature density and magnetization profiles of the 1D trapped gas.
Our results provide the physical origin of quantum criticality in a 1D
many-body system beyond the Tomonaga-Luttinger liquid description.Comment: 12 pages, 11 figure
Polarization-controlled single photons
Vacuum-stimulated Raman transitions are driven between two magnetic substates
of a rubidium-87 atom strongly coupled to an optical cavity. A magnetic field
lifts the degeneracy of these states, and the atom is alternately exposed to
laser pulses of two different frequencies. This produces a stream of single
photons with alternating circular polarization in a predetermined
spatio-temporal mode. MHz repetition rates are possible as no recycling of the
atom between photon generations is required. Photon indistinguishability is
tested by time-resolved two-photon interference.Comment: 4 pages, 3 figure
Contact and sum-rules in a near-uniform Fermi gas at unitarity
We present an experimental study of the high-energy excitation spectra of
unitary Fermi gases. Using focussed beam Bragg spectroscopy, we locally probe
atoms in the central region of a harmonically trapped cloud where the density
is nearly uniform, enabling measurements of the dynamic structure factor for a
range of temperatures both below and above the superfluid transition. Applying
sum-rules to the measured Bragg spectra, we resolve the characteristic
behaviour of the universal contact parameter, , across the superfluid
transition. We also employ a recent theoretical result for the kinetic
(second-moment) sum-rule to obtain the internal energy of gases at unitarity.Comment: 5 pages, 4 figure
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