343 research outputs found
Dynamical density functional theory with hydrodynamic interactions and colloids in unstable traps
A density functional theory for colloidal dynamics is presented which
includes hydrodynamic interactions between the colloidal particles. The theory
is applied to the dynamics of colloidal particles in an optical trap which
switches periodically in time from a stable to unstable confining potential. In
the absence of hydrodynamic interactions, the resulting density breathing mode,
exhibits huge oscillations in the trap center which are almost completely
damped by hydrodynamic interactions. The predicted dynamical density fields are
in good agreement with Brownian dynamics computer simulations
A Case Study of Sedimentation of Charged Colloids: The Primitive Model and the Effective One-Component Approach
Sedimentation-diffusion equilibrium density profiles of suspensions of
charge-stabilized colloids are calculated theoretically and by Monte Carlo
simulation, both for a one-component model of colloidal particles interacting
through pairwise screened-Coulomb repulsions and for a three-component model of
colloids, cations, and anions with unscreened-Coulomb interactions. We focus on
a state point for which experimental measurements are available [C.P. Royall et
al., J. Phys.: Cond. Matt. {\bf 17}, 2315 (2005)]. Despite the apparently
different picture that emerges from the one- and three-component model
(repelling colloids pushing each other to high altitude in the former, versus a
self-generated electric field that pushes the colloids up in the latter), we
find similar colloidal density profiles for both models from theory as well as
simulation, thereby suggesting that these pictures represent different view
points of the same phenomenon. The sedimentation profiles obtained from an
effective one-component model by MC simulations and theory, together with MC
simulations of the multi-component primitive model are consistent among
themselves, but differ quantitatively from the results of a theoretical
multi-component description at the Poisson-Boltzmann level. We find that for
small and moderate colloid charge the Poisson-Boltzmann theory gives profiles
in excellent agreement with the effective one-component theory if a smaller
effective charge is used. We attribute this discrepancy to the poor treatment
of correlations in the Poisson-Boltzmann theory.Comment: 9 pages, 7 figure
Depletion forces in non-equilibrium
The concept of effective depletion forces between two fixed big colloidal
particles in a bath of small particles is generalized to a non-equilibrium
situation where the bath of small Brownian particles is flowing around the big
particles with a prescribed velocity. In striking contrast to the equilibrium
case, the non-equilibrium forces violate Newton's third law, are
non-conservative and strongly anisotropic, featuring both strong attractive and
repulsive domains.Comment: 4 pages, 3 figure
Становище сільського господарства Чернігівської губернії на початку XX ст.
Chemically responsive hydrogels with embedded magnetic nanoparticles are of interest for biosensors that magnetically detect chemical changes. A crucial point is the irreversible linkage of nanoparticles to the hydrogel network, preventing loss of nanoparticles upon repeated swelling and shrinking of the gel. Here, acrylic acid monomers are adsorbed onto ferrite nanoparticles, which subsequently participate in polymerization during synthesis of poly(acrylic acid)-based hydrogels (PAA). To demonstrate the fixation of the nanoparticles to the polymer, our original approach is to measure low-field AC magnetic susceptibility spectra in the 0.1 Hz to 1 MHz range. In the hydrogel, the magnetization dynamics of small iron oxide nanoparticles are comparable to those of the particles dispersed in a liquid, due to fast Néel relaxation inside the particles; this renders the ferrogel useful for chemical sensing at frequencies of several kHz. However, ferrogels holding thermally blocked iron oxide or cobalt ferrite nanoparticles show significant decrease of the magnetic susceptibility resulting from a frozen magnetic structure. This confirms that the nanoparticles are unable to rotate thermally inside the hydrogel, in agreement with their irreversible fixation to the polymer network
Geometric origin of mechanical properties of granular materials
Some remarkable generic properties, related to isostaticity and potential
energy minimization, of equilibrium configurations of assemblies of rigid,
frictionless grains are studied. Isostaticity -the uniqueness of the forces,
once the list of contacts is known- is established in a quite general context,
and the important distinction between isostatic problems under given external
loads and isostatic (rigid) structures is presented. Complete rigidity is only
guaranteed, on stability grounds, in the case of spherical cohesionless grains.
Otherwise, the network of contacts might deform elastically in response to load
increments, even though grains are rigid. This sets an uuper bound on the
contact coordination number. The approximation of small displacements (ASD)
allows to draw analogies with other model systems studied in statistical
mechanics, such as minimum paths on a lattice. It also entails the uniqueness
of the equilibrium state (the list of contacts itself is geometrically
determined) for cohesionless grains, and thus the absence of plastic
dissipation. Plasticity and hysteresis are due to the lack of such uniqueness
and may stem, apart from intergranular friction, from small, but finite,
rearrangements, in which the system jumps between two distinct potential energy
minima, or from bounded tensile contact forces. The response to load increments
is discussed. On the basis of past numerical studies, we argue that, if the ASD
is valid, the macroscopic displacement field is the solution to an elliptic
boundary value problem (akin to the Stokes problem).Comment: RevTex, 40 pages, 26 figures. Close to published paper. Misprints and
minor errors correcte
Biological Synthesis of Size-Controlled Cadmium Sulfide Nanoparticles Using ImmobilizedRhodobacter sphaeroides
Size-controlled cadmium sulfide nanoparticles were successfully synthesized by immobilizedRhodobacter sphaeroidesin the study. The dynamic process that Cd2+was transported from solution into cell by livingR. sphaeroideswas characterized by transmission electron microscopy (TEM). Culture time, as an important physiological parameter forR. sphaeroidesgrowth, could significantly control the size of cadmium sulfide nanoparticles. TEM demonstrated that the average sizes of spherical cadmium sulfide nanoparticles were 2.3 ± 0.15, 6.8 ± 0.22, and 36.8 ± 0.25 nm at culture times of 36, 42, and 48 h, respectively. Also, the UV–vis and photoluminescence spectral analysis of cadmium sulfide nanoparticles were performed
Chemically and thermally stable silica nanowires with a β-sheet peptide core for bionanotechnology
Background: A series of amyloidogenic peptides based on the sequence KFFEAAAKKFFE template the silica precursor, tetraethyl orthosilicate to form silica-nanowires containing a cross-β peptide core.
Results: Investigation of the stability of these fibres reveals that the silica layers protect the silica-nanowires allowing them to maintain their shape and physical and chemical properties after incubation with organic solvents such as 2-propanol, ethanol, and acetonitrile, as well as in a strong acidic solution at pH 1.5. Furthermore, these nanowires were thermally stable in an aqueous solution when heated up to 70 °C, and upon autoclaving. They also preserved their conformation following incubation up to 4 weeks under these harsh conditions, and showed exceptionally high physical stability up to 1000 °C after ageing for 12 months. We show that they maintain their β-sheet peptide core even after harsh treatment by confirming the β-sheet content using Fourier transform infrared spectra. The silica nanowires show significantly higher chemical and thermal stability compared to the unsiliconised fibrils.
Conclusions: The notable chemical and thermal stability of these silica nanowires points to their potential for use in microelectromechanics processes or fabrication for nanotechnological devices
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