3,337 research outputs found
Electroneutrality and Phase Behavior of Colloidal Suspensions
Several statistical mechanical theories predict that colloidal suspensions of
highly charged macroions and monovalent microions can exhibit unusual
thermodynamic phase behavior when strongly deionized. Density-functional,
extended Debye-H\"uckel, and response theories, within mean-field and
linearization approximations, predict a spinodal phase instability of charged
colloids below a critical salt concentration. Poisson-Boltzmann cell model
studies of suspensions in Donnan equilibrium with a salt reservoir demonstrate
that effective interactions and osmotic pressures predicted by such theories
can be sensitive to the choice of reference system, e.g., whether the microion
density profiles are expanded about the average potential of the suspension or
about the reservoir potential. By unifying Poisson-Boltzmann and response
theories within a common perturbative framework, it is shown here that the
choice of reference system is dictated by the constraint of global
electroneutrality. On this basis, bulk suspensions are best modeled by
density-dependent effective interactions derived from a closed reference system
in which the counterions are confined to the same volume as the macroions.
Linearized theories then predict bulk phase separation of deionized suspensions
only when expanded about a physically consistent (closed) reference system.
Lower-dimensional systems (e.g., monolayers, small clusters), depending on the
strength of macroion-counterion correlations, may be governed instead by
density-independent effective interactions tied to an open reference system
with counterions dispersed throughout the reservoir, possibly explaining
observed structural crossover in colloidal monolayers and anomalous
metastability of colloidal crystallites.Comment: 12 pages, 5 figures. Discussion clarified, references adde
Charge Renormalization, Effective Interactions, and Thermodynamics of Deionized Colloidal Suspensions
Thermodynamic properties of charge-stabilised colloidal suspensions depend
sensitively on the effective charge of the macroions, which can be
substantially lower than the bare charge in the case of strong
counterion-macroion association. A theory of charge renormalization is
proposed, combining an effective one-component model of charged colloids with a
thermal criterion for distinguishing between free and associated counterions.
The theory predicts, with minimal computational effort, osmotic pressures of
deionized suspensions of highly charged colloids in close agreement with
large-scale simulations of the primitive model.Comment: 15 pages, 7 figure
Poisson-Boltzmann Theory of Charged Colloids: Limits of the Cell Model for Salty Suspensions
Thermodynamic properties of charge-stabilised colloidal suspensions are
commonly modeled by implementing the mean-field Poisson-Boltzmann (PB) theory
within a cell model. This approach models a bulk system by a single macroion,
together with counterions and salt ions, confined to a symmetrically shaped,
electroneutral cell. While easing solution of the nonlinear PB equation, the
cell model neglects microion-induced correlations between macroions, precluding
modeling of macroion ordering phenomena. An alternative approach, avoiding
artificial constraints of cell geometry, maps a macroion-microion mixture onto
a one-component model of pseudo-macroions governed by effective interactions.
In practice, effective-interaction models are usually based on linear screening
approximations, which can accurately describe nonlinear screening only by
incorporating an effective (renormalized) macroion charge. Combining charge
renormalization and linearized PB theories, in both the cell model and an
effective-interaction (cell-free) model, we compute osmotic pressures of highly
charged colloids and monovalent microions over a range of concentrations. By
comparing predictions with primitive model simulation data for salt-free
suspensions, and with predictions of nonlinear PB theory for salty suspensions,
we chart the limits of both the cell model and linear-screening approximations
in modeling bulk thermodynamic properties. Up to moderately strong
electrostatic couplings, the cell model proves accurate in predicting osmotic
pressures of deionized suspensions. With increasing salt concentration,
however, the relative contribution of macroion interactions grows, leading
predictions of the cell and effective-interaction models to deviate. No
evidence is found for a liquid-vapour phase instability driven by monovalent
microions. These results may guide applications of PB theory to soft materials.Comment: 27 pages, 5 figures, special issue of Journal of Physics: Condensed
Matter on "Classical density functional theory methods in soft and hard
matter
Location of Pc 1–2 waves relative to the magnetopause
Spacecraft-borne and ground-based magnetome- ters frequently detect magnetospheric micropulsations in the period range 0.2–10s, termed Pc 1–2, and attributed to electromagnetic ion cyclotron waves driven by temperature anisotropy (T⊥ \u3e T∥). Previous surveys of Pc 1 occur- rence locations have been limited to L ≤ 9. We present AMPTE/IRM observations of the distribution of Pc 1 waves out to the magnetopause, for a limited region of MLT = 10–14. The probability of wave occurrence Pwav is large (\u3e 0.15) between L = 7–12, peaking at L = 8–10 (Pwav ∼ 0.25). When the L-value is normalized to the magnetopause position Lmp, however, the highest probabilities of Pc 1 wave occurrence are close to the magnetopause, with Pwav ∼ 0.25 for Lnorm ≡ L/Lmp = 0.8–1.0. These results are consis- tent with increased convective growth rate at large L and with the greater effect of magnetosphere compression close to the magnetopause. On the other hand, we only directly observe magnetic field compression for at most about 25% of the wave events
Stability of Colloidal Quasicrystals
Freezing of charge-stabilized colloidal suspensions and relative stabilities
of crystals and quasicrystals are studied using thermodynamic perturbation
theory. Macroion interactions are modelled by effective pair potentials
combining electrostatic repulsion with polymer-depletion or van der Waals
attraction. Comparing free energies -- counterion terms included -- for
elementary crystals and rational approximants to icosahedral quasicrystals,
parameters are identified for which one-component quasicrystals are stabilized
by a compromise between packing entropy and cohesive energy.Comment: 6 pages, 4 figure
Classification of Micro-calcification in Mammograms using Scalable Linear Fisher Discriminant Analysis
Breast cancer is one of the major causes of death in women. Computer Aided Diagnosis (CAD) systems are being developed to assist radiologists in early diagnosis. Microcalcifications can be an early symptom of breast cancer. Besides detection, classification of micro-calcification as benign or malignant is essential in a complete CAD system.We have developed a novel method for the classification of benign and malignant microcalcification using an improved Fisher Linear Discriminant Analysis (LDA) approach for the linear transformation of segmented micro-calcification data in combination with a Support Vector Machine (SVM) variant to classify between the two classes. The results indicate an average accuracy equal to 96% which is comparable to state-of-the art methods in the literature.authorsversionPeer reviewe
Effective interactions, structure, and pressure in charge-stabilized colloidal suspensions: Critical assessment of charge renormalization methods
Charge-stabilized colloidal suspensions display a rich variety of
microstructural and thermodynamic properties, which are determined by
electro-steric interactions between all ionic species. The large size asymmetry
between molecular-scale microions and colloidal macroions allows the microion
degrees of freedom to be integrated out, leading to an effective one-component
model of microion-dressed colloidal quasi-particles. For highly-charged
colloids with strong macroion-microion correlations, nonlinear effects can be
incorporated into effective interactions by means of charge renormalization
methods. Here, we compare and partially extend several practical mean-field
methods of calculating renormalized colloidal interaction parameters, including
effective charges and screening constants, as functions of concentration and
ionic strength. Within the one-component description, we compute structural and
thermodynamic properties from the effective interactions and assess the
accuracy of the different methods by comparing predictions with elaborate
primitive-model simulations [P. Linse, J. Chem. Phys. 113, 4359 (2000)]. We
also compare various prescriptions for the osmotic pressure of suspensions in
Donnan equilibrium with a salt ion reservoir, and analyze instances where the
macroion effective charge becomes larger than the bare one. The methods
assessed include single-center cell, jellium, and multi-center mean-field
theories. The strengths and weaknesses of the various methods are critically
assessed, with the aim of guiding optimal and accurate implementations.Comment: 27 pages, 15 figure
A study of Pc-5 ULF oscillations
International audienceA study of Pc-5 magnetic pulsations using data from the Combined Release and Radiation Effects Satellite (CRRES) was carried out. Three-component dynamic magnetic field spectrograms have been used to survey ULF pulsation activity for the approximate fourteen month lifetime of CRRES. Two-hour panels of dynamic spectra were examined to find events which fall into two basic categories: 1) toroidal modes (fundamental and harmonic resonances) and 2) poloidal modes, which include compressional oscillations. The occurence rates were determined as a function of L value and local time. The main result is a comparable probability of occurence of toroidal mode oscillations on the dawn and dusk sides of the magnetosphere inside geosynchronous orbit, while poloidal mode oscillations occur predominantly along the dusk side, consistent with high azimuthal mode number excitation by ring current ions. Pc-5 pulsations following Storm Sudden Commencements (SSCs) were examined separately. The spatial distribution of modes for the SSC events was consistent with the statistical study for the lifetime of CRRES. The toroidal fundamental (and harmonic) resonances are the dominant mode seen on the dawn-side of the magnetosphere following SSCs. Power is mixed in all three components. In the 21 dusk side SSC events there were only a few examples of purely compressional (two) or radial (one) power in the CRRES study, a few more examples of purely toroidal modes (six), with all three components predominant in about half (ten) of the events. Key words. Magnetospheric physics (MHD waves and instabilities; magnetospheric configuration and dynamics) ? Space plasma physics (waves and instabilities
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