508 research outputs found
Magnetization of rotating ferrofluids: the effect of polydispersity
The influence of polydispersity on the magnetization is analyzed in a
nonequilibrium situation where a cylindrical ferrofluid column is enforced to
rotate with constant frequency like a rigid body in a homogeneous magnetic
field that is applied perpendicular to the cylinder axis. Then, the
magnetization and the internal magnetic field are not longer parallel to each
other and their directions differ from that of the applied magnetic field.
Experimental results on the transverse magnetization component perpendicular to
the applied field are compared and analyzed as functions of rotation frequency
and field strength with different polydisperse Debye models that take into
account the polydispersity in different ways and to a varying degree.Comment: 11 pages, 7 figures, to be published in Journal of Physics
A history of First Lutheran Church, Sioux Falls, South Dakota
The first white inhabitants of the Valley of the Sioux River came from opposite directions, but in search of the same reward: good homesites in the new Eden of the Dakota Country. After reading of the beautiful falls of the river called Te-han-kas-an-data by the Indians, Dr. George M. Staples of Dubuque, Iowa, organized the Western Two Company in 1856. The possibilities of future land exploitation in the area attracted further support of this undertaking, and two frontiersmen were hired to gain possession of the area under the laws of the United States, which allowed up to three hundred twenty acreds of land to settlers. In November of 1856, Ezra Millard of Sioux City, and an accomplice, reached the Falls of the Big Sioux River, built a log dwelling above the Falls, and returned to Sioux City for the winter
Magnetization of polydisperse colloidal ferrofluids: Effect of magnetostriction
We exploit magnetostriction in polydisperse ferrofluids in order to generate
nonlinear responses, and apply a thermodynamical method to derive the desired
nonlinear magnetic susceptibility. For an ideal gas, this method has been
demonstrated to be in excellent agreement with a statistical method. In the
presence of a sinusoidal ac magnetic field, the magnetization of the
polydisperse ferrofluid contains higher-order harmonics, which can be extracted
analytically by using a perturbation approach. We find that the harmonics are
sensitive to the particle distribution and the degree of field-induced
anisotropy of the system. In addition, we find that the magnetization is higher
in the polydisperse system than in the monodisperse one, as also found by a
recent Monte Carlo simulation. Thus, it seems possible to detect the size
distribution in a polydisperse ferrofluid by measuring the harmonics of the
magnetization under the influence of magnetostriction.Comment: 23 pages, 4 figures. To be accepted for publication in Phys. Rev.
Charakterisierung der Hybridsensorkinase LuxN und des Antwortregulators LuxO des Quorum sensing-Systems in Vibrio harveyi
Die Signaltransduktionskaskade des komplexen Quorum sensing-Systems in Vibrio harveyi umfasst die drei Hybridsensorkinasen LuxN, LuxQ und CqsS, das Histidinphosphotransferprotein LuxU und den Antwortregulator LuxO. Bei niedriger Zelldichte funktionieren die Hybridsensorkinasen als Autokinasen. Die Phosphorylgruppe wird zunĂ€chst intramolekular ĂŒbertragen und anschlieĂend auf LuxU und LuxO weitergeleitet. Phosphoryliertes LuxO aktiviert die Expression von fĂŒnf regulatorischen RNAs, die im Zusammenspiel mit dem RNA-Chaperon Hfq die Translation der mRNA des Masterregulators LuxR inhibieren. Bei hoher Zelldichte wird die KinaseaktivitĂ€t der Hybridsensorkinasen durch die jeweiligen Autoinduktoren (LuxN: HAI-1, LuxQ: AI-2, CqsS: CAI-1) inhibiert, sodass es zum Abschalten der Phosphorylierungskaskade und zur Anreicherung von LuxR kommt.
Im Rahmen dieser Arbeit wurden die Proteine LuxN und LuxO biochemisch nĂ€her charakterisiert. Mit Hilfe diverser Methoden konnte die Topologie des Membranproteins LuxN, zusammen mit der Lage des N-Terminus, gelöst werden: das Protein besteht aus neun TransmembrandomĂ€nen mit einem periplasmatisch lokalisierten N-Terminus. Im Zuge der biochemischen Charakterisierung von LuxN wurden zwei an der Bindung des Autoinduktors HAI-1 beteiligte AminosĂ€uren identifiziert. Das Membranprotein LuxN wurde erfolgreich gereinigt und in Escherichia coli- und V. harveyi-basierten Proteoliposomen rekonstituiert. Ebenso wurde der in Form von Inclusion Bodies heterolog in E. coli ĂŒberproduzierte Antwortregulator LuxO gereinigt und renaturiert. Das renaturierte Protein konnte erstmalig mit dem niedermolekularen Phosphodonor [Îł-32P]-Acetylphosphat markiert werden und eine Bindung an die DNA in phosphorylierter und nicht-phosphorylierter Form im Bereich der hypothetischen Ï54- und LuxO-Bindestelle gezeigt werden
Reversible magnetomechanical collapse: virtual touching and detachment of rigid inclusions in a soft elastic matrix
Soft elastic composite materials containing particulate rigid inclusions in a
soft elastic matrix are candidates for developing soft actuators or tunable
damping devices. The possibility to reversibly drive the rigid inclusions
within such a composite together to a close-to-touching state by an external
stimulus would offer important benefits. Then, a significant tuning of the
mechanical properties could be achieved due to the resulting mechanical
hardening. For a long time, it has been argued whether a virtual touching of
the embedded magnetic particles with subsequent detachment can actually be
observed in real materials, and if so, whether the process is reversible. Here,
we present experimental results that demonstrate this phenomenon in reality.
Our system consists of two paramagnetic nickel particles embedded at finite
initial distance in a soft elastic polymeric gel matrix. Magnetization in an
external magnetic field tunes the magnetic attraction between the particles and
drives the process. We quantify the scenario by different theoretical tools,
i.e., explicit analytical calculations in the framework of linear elasticity
theory, a projection onto simplified dipole-spring models, as well as detailed
finite-element simulations. From these different approaches, we conclude that
in our case the cycle of virtual touching and detachment shows hysteretic
behavior due to the mutual magnetization between the paramagnetic particles.
Our results are important for the design and construction of reversibly tunable
mechanical damping devices. Moreover, our projection on dipole-spring models
allows the formal connection of our description to various related systems,
e.g., magnetosome filaments in magnetotactic bacteria.Comment: 14 pages, 7 figure
First-order reversal curve analysis of magnetoactive elastomers
The first magnetization loop and the first stressâstrain cycle of magnetoactive elastomers (MAEs) in a magnetic field differ considerably from the following loops and cycles, possibly due to the internal restructuring of the magnetic filler particles and the matrix polymer chains. In the present study, the irreversible magnetization processes during the first magnetization of MAEs with different filler compositions and tensile moduli of the matrix are studied by first-order reversal curve (FORC) measurements. For MAEs with mixed magnetic NdFeB/Fe fillers the FORC distributions and magnetization distributions of the first major loop reveal a complex irreversible magnetization behavior at interaction fields Hu 600 kA mâ1
Magnetic properties of colloidal suspensions of interacting magnetic particles
We review equilibrium thermodynamic properties of systems of magnetic
particles like ferrofluids in which dipolar interactions play an important
role. The review is focussed on two subjects: ({\em i}) the magnetization with
the initial magnetic susceptibility as a special case and ({\em ii}) the phase
transition behavior. Here the condensation ("gas/liquid") transition in the
subsystem of the suspended particles is treated as well as the
isotropic/ferromagnetic transition to a state with spontaneously generated
long--range magnetic order.Comment: Review. 62 pages, 4 figure
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