Pneumatic foam generation in the presence of a high-intensity ultrasound field

Designer foams find applications in a wide range of industries. Foam quality is mostly determined by its complex cellular structure which defines its texture, rheology and stability. In addition to formulation design, the formation process is crucial to the development of a foam with an optimum structure. There is, therefore, a need for techniques that can assist in the generation of controlled foam structures. The work described in this paper demonstrates the potential of using high-intensity ultrasound to control foam structure during production. Foam generated in the presence of ultrasound usually exhibits a narrower bubble size distribution, i.e. a more uniform texture. Such enhanced homogeneity in texture is desirable to reduce the presence of aesthetically unattractive large cavities, and to reduce the destabilising effects of foam coarsening. In addition, a smaller mean bubble size and a slower rate of foam collapse usually result when ultrasound is applied. The work shows the effects on foams stabilised with different surfactants.     

Single-atom radiation effects in the presence of a damped fock-state field

Summary In this paper an exact analytic solution for the Jaynes-Cummings model in the presence of a damped Fock-state field is derived. In addition simpler approximate solutions are given for large and small cavity damping. Moreover, numerical results for atomic population inversion, atomic radiation rate and atom-field correlation are presented for different cavity dampings and initial photon numbers. In the case of small cavity damping damped periodic revivals as well as the existence of a new quasistationary state are pointed out.

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Riassunto In questo lavoro si deriva una soluzione analitica esatta per il modello di Jaynes-Cummings in presenza di un campo smorzato dello stato di Fock. Si danno anche soluzioni approssimate piú semplici per smorzamento a piccole e grandi cavità. Inoltre, si presentano risultati numerici dell'inversione della popolazione atomica, della quantità di radiazione atomica e della correlazione atomo-campo per diversi smorzamenti di cavità e numeri fotonici iniziali. Nel caso di smorzamento a piccola cavità, si mettono in evidenza riprese periodiche smorzate così come l'esistenza di un nuovo stato quasi stazionario.

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Резюме В этой статье выводится точное аналитическое решение для модели Джейнса-Камингса в присутствии поля затухающих фоковских состояний. Кроме того, приводятся более простые приближенные решения для больших и малых затуханий. Для различных затуханий и начальных чисел фотонов приводятся численные результаты для инверсии атомной заселенноси, интенсивности излучения атома и атом-полевой корреляции. В случае малого затухания отмечаются затухающие периодические восстановления, а также существование нового квази-стационарного состояния.

     

Plasma satellites in the presence of a non-coherent and unpolarized radiation field

Satellite lines produced by longitudinal electrostatic plasma waves in the presence of a non-coherent and unpolarized radiation field are investigated by perturbation theory expanded to fourth order. The results show that the Baranger and Mozer plasma satellites are shifted even in the zero radiation field limit, and that their strength can be enhanced by the radiation field.     

Stimulated Smith-Purcell radiation

JETP Letters - The design of free-electron lasers (FELs) based on submillimeter-range undulators involves difficulties associated with the fabrication of undulators with a very small period. To do...     

Brans-Dicke scalar field interactions in presence of radiation distribution

By considering the Robertson-Walker, line element exact solutions are obtained for radiation-filled cosmological differential equations of Brans-Dicke theory with the assumption thatk/k = 1/R, wherek denotes the gravitational variable andR is the radius of curvature and the dot denotes the differentiation with respect to time. Under this assumption, we obtain exact solutions corresponding to the three values of curvature indexK (1,0, –1). We can obtain physically realistic solutions in all the three cases for all values of coupling constant > –2. The radius of curvature increases linearly with respect to the age of the universe, while the gravitational variablek varies directly as the square of the radius of the universe. The solution obtained contradicts Dirac's hypothesis in which the gravitational constant should decrease with time in the expanding universe.     

Stimulated radiation pressure acting on an atom nonadiabatically interacting with the field of counterpropagating frequency-modulated waves

The stimulated radiation pressure acting on an atom nonadiabatically interacting with the field of counterpropagating frequency-modulated waves is shown to reach high values typical for the rapid adiabatic passage of the instantaneous frequency of the field of the counterpropagating waves through resonance with an atomic transition. Under the appropriate choice of interaction parameters, the radiation pressure changes insignificantly in a wide range of atomic velocities. Original Russian Text ? V.I. Romanenko, L.P. Yatsenko, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 12, pp. 868–872.     

Stimulated radiation pressure acting on an atom nonadiabatically interacting with the field of counterpropagating frequency-modulated waves

The stimulated radiation pressure acting on an atom nonadiabatically interacting with the field of counterpropagating frequency-modulated waves is shown to reach high values typical for the rapid adiabatic passage of the instantaneous frequency of the field of the counterpropagating waves through resonance with an atomic transition. Under the appropriate choice of interaction parameters, the radiation pressure changes insignificantly in a wide range of atomic velocities.     

Field-emission in the presence of a magnetic field

Transverse effective-mass effects are studied in a pseudo-one-dimensional Kronig-Penney model. Three-dimensional effects are discussed qualitatively. It is shown that the current must be analyzed in terms of the normal, and not the total energy, of electrons in the vacuum and that band effets appear via a structural dependent modulation which does not reduce to the “simplified effective-mass” approximation.     

Stimulated light scattering by a fluid surface in the presence of total fresnel reflection

Stimulated scattering of light by a free fluid surface in the case of total Fresnel reflection was considered. For scattering directions which are close to the direction of specular reflection of the ray, simple analytic expressions are derived for the threshold intensity and the spectrum of the scattered radiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 15, No. 2, pp. 233–241, February, 1972.     

Chaotization and decay instability of a diffraction-catastrophe field in the presence of focusing of high-power laser radiation in a nonlinear medium

Focusing of a high-power laser beam, whose initial wave front is deformed by spherical aberration and astigmatism, in a medium with refractive-index nonlinearity is investigated by the computer experiment method. It was found that the extended formations which arise near the focus are no longer diffraction catastrophes, since they are structurally unstable. It is shown that weak disturbances of the shape of the initial wave front produce extremely strong distortions of the optical field in the focusing region. As the wave propagates in the z direction, spatial chaos in the distribution of the optical field develops in the region of focusing (from 137 to 142 mm). Optical turbulence arises in the focusing region as a result of self-diffraction of light by self-induced nonuniformities of the refractive index of the medium. After the region with the smallest cross section of the formation near the focus, the three-dimensional optical field has the form of chaotically dispersing “splashes” and extended “filamentary ejecta.” Zh. Tekh. Fiz. 68, 74–83 (September 1998)     

Broadband radiation transport in an optically dense gas in the presence of an RF field

The variational Monte Carlo method is applied to investigate the ground state and some excited states of the lithium atom and its ions up to Z = 10 in the presence of an external magnetic field regime with γ = 0–100 arb. units. The effect of increasing field strength on the ground state energy is studied and precise values for the crossover field strengths were obtained. Our calculations are based on using accurate forms of trial wave functions, which were put forward in calculating energies in the absence of magnetic field. Furthermore, the value of Y at which ground-state energy of the lithium atom approaches to zero was calculated. The obtained results are in good agreement with the most recent values and also with the exact values.     

Modeling of high-intensity focused ultrasound-induced lesions in the presence of cavitation bubbles

The classical "Bio Heat Transfer Equation (BHTE)" model is adapted to take into account the effects of oscillating microbubbles that occur naturally in the tissue during high-intensity focused ultrasound (HIFU) treatment. First, the Gilmore-Akulichev model is used to quantify the acoustic pressure scattered by microbubbles submitted to HIFU. Because this scattered pressure is not monochromatic, the concept of harmonic attenuation is introduced and a global attenuation coefficient is estimated for bubble-filled tissues. The first results show that this global attenuation coefficient varies significantly with respect to several parameters such as the frequency and the density of microbubbles in the medium, but also with respect to the incident acoustic pressure which thus becomes a transcendental function. Under these conditions, a layer-by-layer modeling, in the direction of propagation, is proposed to calculate the ultrasonic beam. Finally, the BHTE is solved and the HIFU-induced lesions are estimated by the calculation of the thermal dose. Using this model, it can be observed first that, when the firing power increases, the lesion develops clearly in the direction of the transducer, with a shape agreeing with in vivo experimentation. Next, it is observed that the lesion can be significantly modified in size and position, if an interface (skin or inner wall) is simulated as a zone with multiple cavitation nuclei. With a firing power increase, it is also shown how a secondary lesion can appear at the interface and how, beyond a certain threshold, this lesion develops at the main lesion expense. Finally, a better in-depth homogeneity of lesions is observed when the acoustic frequency of HIFU is increased.     

Stimulated collision induced processes in sodium vapor in the presence of helium

Stimulated resonant emission and stimulated Raman effects in sodium vapor in the presence of helium are investigated experimentally. The intensity dependence of these effects on the buffer gas and sodium vapor pressures, and on the intensity and detuning of the exciting radiation are obtained in experiment. Our experimental results agree with the most recent theoretical calculations.This work was carried out under the Polish Central Program of Fundamental Research CPBP 01.06     

Ionization and stabilization of atoms in a high-intensity,low-frequency laser field

The dynamics of a model silver atom in the strong radiation field of a Ti:sapphire laser is studied in the Keldysh parameter regions γ ⩾ 1 and γ ⩽ 1. It is found that in the entire range of Keldysh parameter variations, along with ionization, the efficient excitation of Rydberg states of the atom with principal quantum numbers n = 6−14 is observed. A Rydberg wavepacket appearing in this case proved stable with respect to ionization; i.e., the atomic system in strong low-frequency electromagnetic fields becomes stable with respect to ionization. The physical reasons behind the stabilization are discussed.