244 research outputs found
The Importance of Boundary Conditions for Fluctuation Induced Forces between Colloids at Interfaces
We calculate the effective fluctuation induced force between spherical or
disk-like colloids trapped at a flat, fluid interface mediated by thermally
excited capillary waves. This Casimir type force is determined by the partition
function of the system which in turn is calculated in a functional integral
approach, where the restrictions on the capillary waves imposed by the colloids
are incorporated by auxiliary fields. In the long-range regime the fluctuation
induced force is shown to depend sensitively on the boundary conditions imposed
at the three-phase contact line between the colloids and the two fluid phases.
The splitting of the fluctuating capillary wave field into a mean-field and a
fluctuation part leads to competing repulsive and attractive contributions,
respectively, which give rise to cancellations of the leading terms. In a
second approach based on multipole expansion of the Casimir interaction, these
cancellations can be understood from the vanishing of certain multipole moments
enforced by the boundary conditions. We also discuss the connection of the
different types of boundary conditions to certain external fields acting on the
colloids which appear to be realizable by experimental techniques such as the
laser tweezer method.Comment: 24 pages, 4 figure
Effective forces between colloids at interfaces induced by capillary wave-like fluctuations
We calculate the effective force mediated by thermally excited capillary
waves between spherical or disklike colloids trapped at a fluid interface. This
Casimir type interaction is shown to depend sensitively on the boundary
conditions imposed at the three-phase contact line. For large distances between
the colloids an unexpected cancellation of attractive and repulsive
contributions is observed leading to a fluctuation force which decays
algebraically very rapidly. For small separations the resulting force is rather
strong and it may play an important role in two-dimensional colloid aggregation
if direct van der Waals forces are weak.Comment: 7 pages, 3 figures, minor revisions, one additional figur
Analysis of stochastic time series in the presence of strong measurement noise
A new approach for the analysis of Langevin-type stochastic processes in the
presence of strong measurement noise is presented. For the case of Gaussian
distributed, exponentially correlated, measurement noise it is possible to
extract the strength and the correlation time of the noise as well as
polynomial approximations of the drift and diffusion functions from the
underlying Langevin equation.Comment: 12 pages, 10 figures; corrected typos and reference
Ellipsoidal particles at fluid interfaces
For partially wetting, ellipsoidal colloids trapped at a fluid interface,
their effective, interface--mediated interactions of capillary and
fluctuation--induced type are analyzed. For contact angles different from
90, static interface deformations arise which lead to anisotropic capillary
forces that are substantial already for micrometer--sized particles. The
capillary problem is solved using an efficient perturbative treatment which
allows a fast determination of the capillary interaction for all distances
between and orientations of two particles. Besides static capillary forces,
fluctuation--induced forces caused by thermally excited capillary waves arise
at fluid interfaces. For the specific choice of a spatially fixed three--phase
contact line, the asymptotic behavior of the fluctuation--induced force is
determined analytically for both the close--distance and the long--distance
regime and compared to numerical solutions.Comment: 10 pages, submitted to EPJE (Special Issue Soft Matter Conference
Aachen'07
The Casimir effect: from quantum to critical fluctuations
The Casimir effect in quantum electrodynamics (QED) is perhaps the best-known
example of fluctuation-induced long-ranged force acting on objects (conducting
plates) immersed in a fluctuating medium (quantum electromagnetic field in
vacuum). A similar effect emerges in statistical physics, where the force
acting, e.g., on colloidal particles immersed in a binary liquid mixture is
affected by the classical thermal fluctuations occurring in the surrounding
medium. The resulting Casimir-like force acquires universal features upon
approaching a critical point of the medium and becomes long-ranged at
criticality. In turn, this universality allows one to investigate theoretically
the temperature dependence of the force via representative models and to
stringently test the corresponding predictions in experiments. In contrast to
QED, the Casimir force resulting from critical fluctuations can be easily tuned
with respect to strength and sign by surface treatments and temperature
control. We present some recent advances in the theoretical study of the
universal properties of the critical Casimir force arising in thin films. The
corresponding predictions compare very well with the experimental results
obtained for wetting layers of various fluids. We discuss how the Casimir force
between a colloidal particle and a planar wall immersed in a binary liquid
mixture has been measured with femto-Newton accuracy, comparing these
experimental results with the corresponding theoretical predictions.Comment: Talk delivered at the International Workshop "60 Years of Casimir
Effect", Brasilia, 23-27 June 2008 (17 pages, 7 figures
Free energy of colloidal particles at the surface of sessile drops
The influence of finite system size on the free energy of a spherical
particle floating at the surface of a sessile droplet is studied both
analytically and numerically. In the special case that the contact angle at the
substrate equals a capillary analogue of the method of images is
applied in order to calculate small deformations of the droplet shape if an
external force is applied to the particle. The type of boundary conditions for
the droplet shape at the substrate determines the sign of the capillary
monopole associated with the image particle. Therefore, the free energy of the
particle, which is proportional to the interaction energy of the original
particle with its image, can be of either sign, too. The analytic solutions,
given by the Green's function of the capillary equation, are constructed such
that the condition of the forces acting on the droplet being balanced and of
the volume constraint are fulfilled. Besides the known phenomena of attraction
of a particle to a free contact line and repulsion from a pinned one, we
observe a local free energy minimum for the particle being located at the drop
apex or at an intermediate angle, respectively. This peculiarity can be traced
back to a non-monotonic behavior of the Green's function, which reflects the
interplay between the deformations of the droplet shape and the volume
constraint.Comment: 24 pages, 19 figure
Autosomal Recessive Dilated Cardiomyopathy due to DOLK Mutations Results from Abnormal Dystroglycan O-Mannosylation
Genetic causes for autosomal recessive forms of dilated cardiomyopathy (DCM) are only rarely identified, although they are thought to contribute considerably to sudden cardiac death and heart failure, especially in young children. Here, we describe 11 young patients (5–13 years) with a predominant presentation of dilated cardiomyopathy (DCM). Metabolic investigations showed deficient protein N-glycosylation, leading to a diagnosis of Congenital Disorders of Glycosylation (CDG). Homozygosity mapping in the consanguineous families showed a locus with two known genes in the N-glycosylation pathway. In all individuals, pathogenic mutations were identified in DOLK, encoding the dolichol kinase responsible for formation of dolichol-phosphate. Enzyme analysis in patients' fibroblasts confirmed a dolichol kinase deficiency in all families. In comparison with the generally multisystem presentation in CDG, the nonsyndromic DCM in several individuals was remarkable. Investigation of other dolichol-phosphate dependent glycosylation pathways in biopsied heart tissue indicated reduced O-mannosylation of alpha-dystroglycan with concomitant functional loss of its laminin-binding capacity, which has been linked to DCM. We thus identified a combined deficiency of protein N-glycosylation and alpha-dystroglycan O-mannosylation in patients with nonsyndromic DCM due to autosomal recessive DOLK mutations
The Location and Nature of General Anesthetic Binding Sites on the Active Conformation of Firefly Luciferase; A Time Resolved Photolabeling Study
Firefly luciferase is one of the few soluble proteins that is acted upon by a wide variety of general anesthetics and alcohols; they inhibit the ATP–driven production of light. We have used time–resolved photolabeling to locate the binding sites of alcohols during the initial light output, some 200 ms after adding ATP. The photolabel 3-azioctanol inhibited the initial light output with an IC50 of 200 µM, close to its general anesthetic potency. Photoincorporation of [3H]3-azioctanol into luciferase was saturable but weak. It was enhanced 200 ms after adding ATP but was negligible minutes later. Sequencing of tryptic digests by HPLC–MSMS revealed a similar conformation–dependence for photoincorporation of 3-azioctanol into Glu-313, a residue that lines the bottom of a deep cleft (vestibule) whose outer end binds luciferin. An aromatic diazirine analog of benzyl alcohol with broader side chain reactivity reported two sites. First, it photolabeled two residues in the vestibule, Ser-286 and Ile-288, both of which are implicated with Glu-313 in the conformation change accompanying activation. Second, it photolabeled two residues that contact luciferin, Ser-316 and Ser-349. Thus, time resolved photolabeling supports two mechanisms of action. First, an allosteric one, in which anesthetics bind in the vestibule displacing water molecules that are thought to be involved in light output. Second, a competitive one, in which anesthetics bind isosterically with luciferin. This work provides structural evidence that supports the competitive and allosteric actions previously characterized by kinetic studies
Production in Pichia pastoris of complementary protein-based polymers with heterodimer-forming WW and PPxY domains
РЕЦЕПТОР ЧЕЛОВЕЧЕСКОГО ЭПИДЕРМАЛЬНОГО ФАКТОРА РОСТА HER2 ПРИ РАКЕ ЖЕЛУДКА (РЕЗУЛЬТАТЫ МНОГОЦЕНТРОВОГО КЛИНИЧЕСКОГО ИССЛЕДОВАНИЯ)
In this article results of the multicenter trial are displayed. Data of successful efficacy of trastuzumab treatment in HER2 positive gastric cancer patients has shown.В статье приведены данные многоцентрового исследования. Показаны успешные результаты лечения трастузумабом больных с HER2 позитивным раком желудка
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