483 research outputs found
Modeling Growth of Cellulomonas cellulans NRRL B 4567 under Substrate Inhibition During Cellulase Production
Cellulase production study was performed in shake flask and bioreactor system using Cellulomonas cellulans NRRL B 4567 for initial substrate concentration from ÎłS0 = 2 to 12 g Lâ1. The growth, substrate uptake profile and enzyme activity at different initial substrate concentrations were measured. The results inferred the presence of substrate inhibition kinetics. Various substrate inhibition models were tested and parameters were estimated, using non-linear regression analysis. Han-Levenspiel model was found
to be the best fitted model for both shake flask and reactor study. The highest volumetric enzyme activity was observed at initial substrate concentration of ÎłS0 = 12 g Lâ1 and 4 g Lâ1 in shake flask and bioreactor respectively
Global consequences of a local Casimir force: Adhered cantilever
Although stiction is a cumbersome problem for microsystems, it stimulates
investigations of surface adhesion. In fact, the shape of an adhered cantilever
carries information of the adhesion energy that locks one end to the substrate.
We demonstrate here that the system is also sensitive to the dispersion forces
that are operative very close to the point of contact, but their contribution
to the shape is maximum at about one third of the unadhered length. When the
force exceeds a critical value the cantilever does not lose stability but it
settles at smaller unadhered length, whose relation to adhesion energy is only
slightly affected by the force. Our calculations suggest to use adhered
cantilevers to measure the dispersion forces at short separations, where other
methods suffer from jump-to-contact instability. Simultaneous measurement of
the force and adhesion energy allows the separation of the dispersion
contribution to the surface adhesion.Comment: 5 pages, 3 figure
Substrate Inhibition Growth Kinetics for Cutinase Producing Pseudomonas cepacia Using Tomato-peel Extracted Cutin
Using tomato-peel extracted cutin, an economically viable substrate, cutinase production by Pseudomonas cepacia was studied at different initial substrate concentrations (2â20 g Lâ1). The highest volumetric enzyme activity was observed at 10 g Lâ1 of cutin, which was inhibited at further higher concentrations. Various 3-, 4- and 5- parametric Monod-variant models were chosen to analyze the inhibition kinetics. The model parameters as well as goodness of fit were estimated using non-linear regression analysis. The
4- parameter Webb model was the best-fit model (R2 = 0.933), followed by the 3-parameter Andrews model (R2 = 0.92). Parameter sensitivity analysis revealed that the maximum specific growth rate was the most sensitive parameter for both the models, and the
Webb constant was the least sensitive. Finally, based on a strong evidence ratio 190.65 from Akaikeâs information content criteria analysis as well as extra sum of square F test (P > 0.05), it was found that 3-parameter Andrews model gave the best fit
Body-assisted van der Waals interaction between two atoms
Using fourth-order perturbation theory, a general formula for the van der
Waals potential of two neutral, unpolarized, ground-state atoms in the presence
of an arbitrary arrangement of dispersing and absorbing magnetodielectric
bodies is derived. The theory is applied to two atoms in bulk material and in
front of a planar multilayer system, with special emphasis on the cases of a
perfectly reflecting plate and a semi-infinite half space. It is demonstrated
that the enhancement and reduction of the two-atom interaction due to the
presence of a perfectly reflecting plate can be understood, at least in the
nonretarded limit, by using the method of image charges. For the semi-infinite
half space, both analytical and numerical results are presented.Comment: 17 pages, 9 figure
Precision measurement of the Casimir-Lifshitz force in a fluid
The Casimir force, which results from the confinement of the quantum
mechanical zero-point fluctuations of the electromagnetic fields, has received
significant attention in recent years for its effect on micro- and nano-scale
mechanical systems. With few exceptions, experimental observations have been
limited to conductive bodies interacting separated by vacuum or air. However,
interesting phenomena including repulsive forces are expected to exist in
certain circumstances between metals and dielectrics when the intervening
medium is not vacuum. In order to better understand the effect of the Casimir
force in such situations and to test the robustness of the generalized
Casimir-Lifshitz theory, we have performed the first precision measurements of
the Casimir force between two metals immersed in a fluid. For this situation,
the measured force is attractive and is approximately 80% smaller than the
force predicted by Casimir for ideal metals in vacuum. We present experimental
results and find them to be consistent with Lifshitz's theory.Comment: 6 pages, 3 figures. (version before final publication
Comparison of the hydrodynamic and Dirac models of the dispersion interaction between graphene and H, He, or Na atoms
The van der Waals and Casimir-Polder interaction of different atoms with
graphene is investigated using the Dirac model which assumes that the energy of
quasiparticles is linear with respect to the momentum. The obtained results for
the van der Waals coefficients of hydrogen atoms and molecules and atoms of
metastable He and Na as a function of separation are compared with
respective results found using the hydrodynamic model of graphene. It is shown
that, regardless of the value of the gap parameter, the Dirac model leads to
much smaller values of the van der Waals coefficients than the hydrodynamic
model. The experiment on quantum reflection of metastable He and Na
atoms on graphene is proposed which is capable to discriminate between the two
models of the electronic structure of graphene. In this respect the parameters
of the phenomenological potential for both these atoms interacting with
graphene described by different models are determined.Comment: 15 pages, 4 figure
Lifshitz-type formulas for graphene and single-wall carbon nanotubes: van der Waals and Casimir interations
Lifshitz-type formulas are obtained for the van der Waals and Casimir
interaction between graphene and a material plate, graphene and an atom or a
molecule, and between a single-wall carbon nanotube and a plate. The reflection
properties of electromagnetic oscillations on graphene are governed by the
specific boundary conditions imposed on the infinitely thin positively charged
plasma sheet, carrying a continuous fluid with some mass and charge density.
The obtained formulas are applied to graphene interacting with Au and Si
plates, to hydrogen atoms and molecules interacting with graphene, and to
single-wall carbon nanotubes interacting with Au and Si plates. The
generalizations to more complicated carbon nanostructures are discussed.Comment: 11 pages, 5 figures, 2 tables; to appear in Phys. Rev. B; misprints
in Eqs.(33) and (34) are correcte
Ultrathin Metallic Coatings Can Induce Quantum Levitation between Nanosurfaces
There is an attractive Casimir-Lifshitz force between two silica surfaces in
a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin
(5-50{\AA}) metallic nanocoating to one of the surfaces results in repulsive
Casimir-Lifshitz forces above a critical separation. The onset of such quantum
levitation comes at decreasing separations as the film thickness decreases.
Remarkably the effect of retardation can turn attraction into repulsion. From
that we explain how an ultrathin metallic coating may prevent
nanoelectromechanical systems from crashing together.Comment: 4 pages, 5 figure
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
Universal behavior of dispersion forces between two dielectric plates in the low-temperature limit
The universal analytic expressions in the limit of low temperatures (short
separations) are obtained for the free energy, entropy and pressure between the
two parallel plates made of any dielectric. The analytical proof of the Nernst
heat theorem in the case of dispersion forces acting between dielectrics is
provided. This permitted us to formulate the stringent thermodynamical
requirement that must be satisfied in all models used in the Casimir physics.Comment: 6 pages, iopart.cls is used, to appear in J. Phys. A (special issue:
Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
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