127 research outputs found
Dissipation in ferrofluids: Mesoscopic versus hydrodynamic theory
Part of the field dependent dissipation in ferrofluids occurs due to the
rotational motion of the ferromagnetic grains relative to the viscous flow of
the carrier fluid. The classical theoretical description due to Shliomis uses a
mesoscopic treatment of the particle motion to derive a relaxation equation for
the non-equilibrium part of the magnetization. Complementary, the hydrodynamic
approach of Liu involves only macroscopic quantities and results in dissipative
Maxwell equations for the magnetic fields in the ferrofluid. Different stress
tensors and constitutive equations lead to deviating theoretical predictions in
those situations, where the magnetic relaxation processes cannot be considered
instantaneous on the hydrodynamic time scale. We quantify these differences for
two situations of experimental relevance namely a resting fluid in an
oscillating oblique field and the damping of parametrically excited surface
waves. The possibilities of an experimental differentiation between the two
theoretical approaches is discussed.Comment: 14 pages, 2 figures, to appear in PR
Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal
Defects are believed to play a fundamental role in the supersolid state of
4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at
zero temperature of the properties of solid 4He in presence of many vacancies,
up to 30 in two dimensions (2D). In all studied cases the crystalline order is
stable at least as long as the concentration of vacancies is below 2.5%. In the
2D system for a small number, n_v, of vacancies such defects can be identified
in the crystalline lattice and are strongly correlated with an attractive
interaction. On the contrary when n_v~10 vacancies in the relaxed system
disappear and in their place one finds dislocations and a revival of the
Bose-Einstein condensation. Thus, should zero-point motion defects be present
in solid 4He, such defects would be dislocations and not vacancies, at least in
2D. In order to avoid using periodic boundary conditions we have studied the
exact ground state of solid 4He confined in a circular region by an external
potential. We find that defects tend to be localized in an interfacial region
of width of about 15 A. Our computation allows to put as upper bound limit to
zero--point defects the concentration 0.003 in the 2D system close to melting
density.Comment: 17 pages, accepted for publication in J. Low Temp. Phys., Special
Issue on Supersolid
Jamming at Zero Temperature and Zero Applied Stress: the Epitome of Disorder
We have studied how 2- and 3- dimensional systems made up of particles
interacting with finite range, repulsive potentials jam (i.e., develop a yield
stress in a disordered state) at zero temperature and applied stress. For each
configuration, there is a unique jamming threshold, , at which
particles can no longer avoid each other and the bulk and shear moduli
simultaneously become non-zero. The distribution of values becomes
narrower as the system size increases, so that essentially all configurations
jam at the same in the thermodynamic limit. This packing fraction
corresponds to the previously measured value for random close-packing. In fact,
our results provide a well-defined meaning for "random close-packing" in terms
of the fraction of all phase space with inherent structures that jam. The
jamming threshold, Point J, occurring at zero temperature and applied stress
and at the random close-packing density, has properties reminiscent of an
ordinary critical point. As Point J is approached from higher packing
fractions, power-law scaling is found for many quantities. Moreover, near Point
J, certain quantities no longer self-average, suggesting the existence of a
length scale that diverges at J. However, Point J also differs from an ordinary
critical point: the scaling exponents do not depend on dimension but do depend
on the interparticle potential. Finally, as Point J is approached from high
packing fractions, the density of vibrational states develops a large excess of
low-frequency modes. All of these results suggest that Point J may control
behavior in its vicinity-perhaps even at the glass transition.Comment: 21 pages, 20 figure
Polarizability of Interacting Atoms: Relation to Collision-Induced Light Scattering and Dielectric Models
Delineation of the movement disorders associated with FOXG1 mutations
Objective: The primary objective of this research was to characterize the movement disorders associated with FOXG1 mutations. Methods: We identified patients with FOXG1 mutations who were referred to either a tertiary movement disorder clinic or tertiary epilepsy service and retrospectively reviewed medical records, clinical investigations, neuroimaging, and available video footage. We administered a telephone-based questionnaire regarding the functional impact of the movement disorders and perceived efficacy of treatment to the caregivers of one cohort of participants. Results: We identified 28 patients with FOXG1 mutations, of whom 6 had previously unreported mutations. A wide variety of movement disorders were identified, with dystonia, choreoathetosis, and orolingual/facial dyskinesias most commonly present. Ninety-three percent of patients had a mixed movement disorder phenotype. In contrast to the phenotype classically described with FOXG1 mutations, 4 patients with missense mutations had a milder phenotype, with independent ambulation, spoken language, and normocephaly. Hyperkinetic involuntary movements were a major clinical feature in these patients. Of the symptomatic treatments targeted to control abnormal involuntary movements, most did not emerge as clearly beneficial, although 4 patients had a caregiver-reported response to levodopa. Conclusions: Abnormal involuntary movements are a major feature of FOXG1 mutations. Our study delineates the spectrum of movement disorders and confirms an expanding clinical phenotype. Symptomatic treatment may be considered for severe or disabling cases, although further research regarding potential treatment strategies is necessary
SLC25A22 is a novel gene for migrating partial seizures in infancy
Objective To identify a genetic cause for migrating partial seizures in infancy (MPSI). Methods We characterized a consanguineous pedigree with MPSI and obtained DNA from affected and unaffected family members. We analyzed single nucleotide polymorphism 500K data to identify regions with evidence of linkage. We performed whole exome sequencing and analyzed homozygous variants in regions of linkage to identify a candidate gene and performed functional studies of the candidate gene SLC25A22. Results In a consanguineous pedigree with 2 individuals with MPSI, we identified 2 regions of linkage, chromosome 4p16.1-p16.3 and chromosome 11p15.4-pter. Using whole exome sequencing, we identified 8 novel homozygous variants in genes in these regions. Only 1 variant, SLC25A22 c.G328C, results in a change of a highly conserved amino acid (p.G110R) and was not present in control samples. SLC25A22 encodes a glutamate transporter with strong expression in the developing brain. We show that the specific G110R mutation, located in a transmembrane domain of the protein, disrupts mitochondrial glutamate transport. Interpretation We have shown that MPSI can be inherited and have identified a novel homozygous mutation in SLC25A22 in the affected individuals. Our data strongly suggest that SLC25A22 is responsible for MPSI, a severe condition with few known etiologies. We have demonstrated that a combination of linkage analysis and whole exome sequencing can be used for disease gene discovery. Finally, as SLC25A22 had been implicated in the distinct syndrome of neonatal epilepsy with suppression bursts on electroencephalogram, we have expanded the phenotypic spectrum associated with SLC25A22. Ann Neurol 2013;74:873-882 © 2013 American Neurological Association
The broad phenotypic spectrum of PPP2R1A-related neurodevelopmental disorders correlates with the degree of biochemical dysfunction
Purpose: Neurodevelopmental disorders (NDD) caused by protein
phosphatase 2A (PP2A) dysfunction have mainly been associated
with de novo variants in PPP2R5D and PPP2CA, and more rarely in
PPP2R1A. Here, we aimed to better understand the latter by
characterizing 30 individuals with de novo and often recurrent
variants in this PP2A scaffolding Aα subunit.
Methods: Most cases were identified through routine clinical
diagnostics. Variants were biochemically characterized for phosphatase activity and interaction with other PP2A subunits.
Results: We describe 30 individuals with 16 different variants in
PPP2R1A, 21 of whom had variants not previously reported. The severity
of developmental delay ranged from mild learning problems to severe
intellectual disability (ID) with or without epilepsy. Common features
were language delay, hypotonia, and hypermobile joints. Macrocephaly
was only seen in individuals without B55α subunit-binding deficit, and
these patients had less severe ID and no seizures. Biochemically more
disruptive variants with impaired B55α but increased striatin binding
were associated with profound ID, epilepsy, corpus callosum hypoplasia,
and sometimes microcephaly.
Conclusion: We significantly expand the phenotypic spectrum of
PPP2R1A-related NDD, revealing a broader clinical presentation of the
patients and that the functional consequences of the variants are more
diverse than previously reported
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