6,768 research outputs found
Critical Casimir Forces in Colloidal Suspensions
Some time ago, Fisher and de Gennes pointed out that long-ranged correlations
in a fluid close to its critical point Tc cause distinct forces between
immersed colloidal particles which can even lead to flocculation [C. R. Acad.
Sc. Paris B 287, 207 (1978)]. Here we calculate such forces between pairs of
spherical particles as function of both relevant thermodynamic variables, i.e.,
the reduced temperature t = (T-Tc)/Tc and the field h conjugate to the order
parameter. This provides the basis for specific predictions concerning the
phase behavior of a suspension of colloidal particles in a near-critical
solvent.Comment: 29 pages, 14 figure
Polymer depletion effects near mesoscopic particles
The behavior of mesoscopic particles dissolved in a dilute solution of long,
flexible, and nonadsorbing polymer chains is studied by field-theoretic
methods. For spherical and cylindrical particles the solvation free energy for
immersing a single particle in the solution is calculated explicitly. Important
features are qualitatively different for self-avoiding polymer chains as
compared with ideal chains. The results corroborate the validity of the
Helfrich-type curvature expansion for general particle shapes and allow for
quantitative experimental tests. For the effective interactions between a small
sphere and a wall, between a thin rod and a wall, and between two small spheres
quantitative results are presented. A systematic approach for studying
effective many-body interactions is provided. The common Asakura-Oosawa
approximation modelling the polymer coils as hard spheres turns out to fail
completely for small particles and still fails by about 10% for large
particles.Comment: 68 pages, 6 figure
Understanding Collective Dynamics of Soft Active Colloids by Binary Scattering
Collective motion in actively propelled particle systems is triggered on the
very local scale by nucleation of coherently moving units consisting of just a
handful of particles. These units grow and merge over time, ending up in a
long-range ordered, coherently-moving state. So far, there exists no bottom-up
understanding of how the microscopic dynamics and interactions between the
constituents are related to the system's ordering instability. In this paper,
we study a class of models for propelled colloids allowing an explicit
treatment of the microscopic details of the collision process. Specifically,
the model equations are Newtonian equations of motion with separate force terms
for particles' driving, dissipation and interaction forces. Focusing on dilute
particle systems, we analyze the binary scattering behavior for these models,
and determine-based on the microscopic dynamics-the corresponding
collision-rule, i.e., the mapping of pre-collisional velocities and impact
parameter on post-collisional velocities. By studying binary scattering we also
find that the considered models for active colloids share the same principle
for parallel alignment: the first incoming particle (with respect to the center
of collision) is aligned to the second particle as a result of the encounter.
This behavior is distinctively different to alignment in non-driven dissipative
gases. Moreover, the obtained collision rule lends itself as a starting point
to apply kinetic theory for propelled particle systems in order to determine
the phase boundary to a long-range ordered, coherently-moving state. The
microscopic origin of the collision rule offers the opportunity to
quantitatively scrutinize the predictions of kinetic theory for propelled
particle systems through direct comparison with multi-particle simulations.Comment: 19 pages, 12 figure
The strong Novikov conjecture for low degree cohomology
We show that for each discrete group G, the rational assembly map
K_*(BG) \otimes Q \to K_*(C*_{max} G) \otimes \Q is injective on classes dual
to the subring generated by cohomology classes of degree at most 2 (identifying
rational K-homology and homology via the Chern character). Our result implies
homotopy invariance of higher signatures associated to these cohomology
classes. This consequence was first established by Connes-Gromov-Moscovici and
Mathai.
Our approach is based on the construction of flat twisting bundles out of
sequences of almost flat bundles as first described in our previous work. In
contrast to the argument of Mathai, our approach is independent of (and indeed
gives a new proof of) the result of Hilsum-Skandalis on the homotopy invariance
of the index of the signature operator twisted with bundles of small curvature.Comment: 11 page
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Expression of natural killer receptor alleles at different Ly49 loci occurs independently and is regulated by major histocompatibility complex class I molecules.
Ly49 receptor genes are expressed by subsets of natural killer (NK) cells in an overlapping fashion, accounting for the capacity of NK subsets to attack host cells that have selectively downregulated self-major histocompatibility complex (MHC) class I molecules. It was shown previously that most NK cells express only one or the other allele of a given Ly49 gene, while a smaller population expresses both alleles. However, the methods used to detect monoallelic and biallelic cells were nonquantitative. Here, new allele-specific antibodies were used to provide the first quantitative examination of biallelic and monoallelic expression of Ly49A and Ly49G2. The results demonstrate conclusively that most Ly49A(+) and Ly49G2(+) NK cells express the corresponding gene in a monoallelic fashion, with a smaller subset expressing both alleles. Unexpectedly, biallelic Ly49A(+) NK cells were more numerous than predicted by completely independent allelic expression, suggesting some heterogeneity among NK progenitors in the potential to express a given Ly49 gene. The data also show that cells expressing one allele of Ly49G2 may express Ly49A from the same or opposite chromosome with equal likelihood, indicating that the expressed allele is chosen independently for different Ly49 genes. Finally, the data demonstrate that biallelic expression of Ly49A or Ly49G2 occurs least frequently in mice that express ligands for these receptors (H-2(d) mice), and most frequently in class I-deficient mice. Thus, biallelic expression of Ly49 genes is regulated by interactions of NK cell progenitors with MHC class I molecules
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