Medium Layer Thickness Research Articles

Nonlinearity in the growth of bacterial colonies: conditions and causes

The universally recognized kinetic model of colony growth, introduced by Pirt, predicts a linear increase of colony size. The linearity follows from the assumption that the colony expands through the growth of only such cells that are located immediately behind the moving colony front, in the so-called peripheral zone of constant width and density. In this work, Pirt's model was tested on two bacteria--Alcaligenes sp. and Pseudomonas fluorescens--having markedly distinct cultural properties and grown on agarized medium with pyruvate. The colony size dynamics was followed for different densities of the inoculum, ranging from a single cell to a microdroplet of bacterial suspension (10(5)-10(6) cells), and for different depths of the agar layer, determining the amount of available substrate. A linear growth mode was observed only with P. fluorescens and only in the case of growth from a microdroplet. When originating from a single cell, colonies of both organisms displayed nonlinear growth with a distinct peak of Kr (the rate of colony radius increase) occurring after 2-3 days of growth. The growth of P. fluorescens colonies showed virtually no dependence on the depth of the agarized medium, whereas the rate of colony size increase of Alcaligenes sp. turned out to be directly related to the medium layer thickness. The departure from linearity is consistently explained by a new kinetic chart stipulating a possible contribution to the colony growth not only of peripheral cells but also (much more distinct in Alcaligenes) of cells at the colony center. The colony growth dynamics is determined not only by the concentration of the limiting substrate but also by the amount of autoinhibitor, the synthesis of which is governed by age of cells. The distinctions of growth from a single cell and microdroplet could also originate as a result of dissociation into the R- and S-forms and competition between the corresponding subpopulations for oxygen and the common substrate.

Seismic attenuation in a heterogeneous porous rock

I study the attenuation of an elastic wave propagating in a macroscopic heterogeneous poroelastic medium due to the scattering (conversion) of the passing wave's energy into the highly attenuative Biot's slow wave. This is done by studying two particular geometrical configurations: (1) a thinly-layered porous medium and (2) porous saturated medium with ellipsoidal inclusions. The frequency dependence of the so-called mode-conversion attenuation has the form of a relaxation peak, with the maximum of the dimensionless attenuation (inverse quality factor) at a frequency at which the wavelength of the Biot's slow wave is approximately equal to the characteristic length of the medium (layer thickness or size of the inclusion). The width and the precise shape of this relaxation peak depend on the particular geometrical configuration. Physically, the mode-conversion attenuation is associated with wave-induced flow of the pore fluid across the interfaces between the host medium and the inclusions. The results of our study demonstrate how the local flow (or squirt) attenuation can be effectively modeled within the context of Biot's theory of poroelasticity.

Statistical modelling of the backscattered field in a one-dimensional non-stationary stochastic problem

Within the framework of an exact wave approach in the spatial-time domain, the one-dimensional stochastic problem of sound pulse scattering by a layered random medium is considered. On the basis of a unification of methods which has been developed by the authors, previously applied to the investigation of non-stationary deterministic wave problems and stochastic stationary wave problems, an analytical-numerical simulation of the behaviour of the backscattered field stochastic characteristics was carried out. Several forms of incident pulses and signals are analysed. We assume that random fluctuations of a medium are described by virtue of the Gaussian Markov process with an exponential correlation function. The most important parameters appearing in the problem are discussed; namely, the time scales of diffusion, pulse durations, the medium layer thickness or the largest observation time scale in comparison with the time scale of one correlation length for the case of a half-space. An exact pattern of the pulse backscattering processes is obtained. It is illustrated by the behaviour of the backscattered field statistical moments for all observation times which are of interest. It is shown that during the time interval when the main part of the pulse energy leaves the medium, the backscattered field is a substantially non-stationary process, having a non-zero mean value and an average intensity that decays according to a power law. There are various power indices for the different duration incident pulses, however, they are not the same as those of previous papers, which were obtained on the basis of an approximate and asymptotic analysis. We have also verified that the Gaussian law is valid for the probability density function of the backscattered field in the case of any incident pulse duration.

Mössbauer spectroscopy and magnetic properties of copper-304 stainless steel multilayer films

The magnetic behavior of multilayer films of Cu-304 stainless steel (304SS) with equal layer thickness fabricated by magnetron sputtering at ambient temperature is reported. For small layer thickness (t=0.5–1.0 nm), the films show epitaxial fcc growth of 304SS on fcc copper, and multilayers are nonmagnetic due to nonmagnetic nature of fcc 304SS. For larger layer thickness (t=1.0–50.0 nm), 304SS grows epitaxially as fcc near the copper interface accompanied by a bcc 304SS growth away from the interface. As a result, two regimes are observed in 1.0–50.0 nm thickness range. Medium layer thickness (t=1.0–1.35 nm) multilayers show weak ferromagnetism characterized by small magnetization due to comparable thickness of ferromagnetic bcc 304SS and nonmagnetic fcc 304SS. A strong ferromagnetic behavior is observed for thick samples (t=1.35–50.0 nm) due to 304SS layer being primarily ferromagnetic bcc phase with very small percentage of fcc near the interface. Mössbauer effect spectroscopy and magnetometry were utilized to observe these magnetic phases. We present the dependence of magnetization on external field at room temperature for the multilayers. In Mössbauer measurements a single absorption peak for nonmagnetic films is observed. A superposition of a six-peak spectrum and a single-peak spectrum representing the existence of ferromagnetic and nonmagnetic phases in each layer were observed for all other films.

Growth dynamics, lipid composition, and (β-carotene content in cells of Dunaliella viridis Teod. under cultivation in different types of photobioreactors

Growth dynamics of Dunaliella viridis Teod., its lipid composition, and β-carotene content were studied under cultivation in horizontal and vertical photobioreactors. The design of photobioreactor and thickness of culture medium layer are shown to affect the intensity of culture growth, lipid composition of cells, and carotene content. Inverse dependence is revealed between the culture growth and β-carotene content. The β-carotene content in D. viridis varies during its cultivation, character of variations may be both linear and rhythmical, which is determined by the photobioreactor design.