Thin films flowing down inverted substrates: Three dimensional flow

We study contact line induced instabilities for a thin film of fluid under destabilizing gravitational force in three dimensional setting. In the previous work (Phys. Fluids, {\bf 22}, 052105 (2010)), we considered two dimensional flow, finding formation of surface waves whose properties within the implemented long wave model depend on a single parameter, $D=(3Ca)^{1/3}\cot\alpha$, where $Ca$ is the capillary number and $\alpha$ is the inclination angle. In the present work we consider fully 3D setting and discuss the influence of the additional dimension on stability properties of the flow. In particular, we concentrate on the coupling between the surface instability and the transverse (fingering) instabilities of the film front. We furthermore consider these instabilities in the setting where fluid viscosity varies in the transverse direction. It is found that the flow pattern strongly depends on the inclination angle and the viscosity gradient

Three-dimensional localized coherent structures of surface turbulence. III Experiment and model validation

The paper continues a series of publications devoted to the 3D nonlinear localized coherent structures on the surface of vertically falling liquid films. The work is primarily focussed on experimental investigations. We study: (i) instabilities and transitions leading to 3D coherent structures; (ii) characteristics of these structures. Some nonstationary effects are also studied numerically. Our experimental results, as well as the results of other investigators, are in a good agreement with our theoretical and numerical predictions.Comment: 42 pages, 15 figure

Mean first-passage times for an ac-driven magnetic moment of a nanoparticle

The two-dimensional backward Fokker-Planck equation is used to calculate the mean first-passage times (MFPTs) of the magnetic moment of a nanoparticle driven by a rotating magnetic field. It is shown that a magnetic field that is rapidly rotating in the plane {\it perpendicular} to the easy axis of the nanoparticle governs the MFPTs just in the same way as a static magnetic field that is applied {\it along} the easy axis. Within this framework, the features of the magnetic relaxation and net magnetization of systems composed of ferromagnetic nanoparticles arising from the action of the rotating field are revealed.Comment: 7 pages, 1 figur

Stochastic stabilization of cosmological photons

The stability of photon trajectories in models of the Universe that have constant spatial curvature is determined by the sign of the curvature: they are exponentially unstable if the curvature is negative and stable if it is positive or zero. We demonstrate that random fluctuations in the curvature provide an additional stabilizing mechanism. This mechanism is analogous to the one responsible for stabilizing the stochastic Kapitsa pendulum. When the mean curvature is negative it is capable of stabilizing the photon trajectories; when the mean curvature is zero or positive it determines the characteristic frequency with which neighbouring trajectories oscillate about each other. In constant negative curvature models of the Universe that have compact topology, exponential instability implies chaos (e.g. mixing) in the photon dynamics. We discuss some consequences of stochastic stabilization in this context.Comment: 4 pages, 3 postscript figures in color which are also appropriate for black and white printers; v2 emphasizes relevance to flat as well as negatively curved cosmologies; to appear in J. Phys.

Observation of Resonant Diffusive Radiation in Random Multilayered Systems

Diffusive Radiation is a new type of radiation predicted to occur in randomly inhomogeneous media due to the multiple scattering of pseudophotons. This theoretical effect is now observed experimentally. The radiation is generated by the passage of electrons of energy 200KeV-2.2MeV through a random stack of films in the visible light region. The radiation intensity increases resonantly provided the Cherenkov condition is satisfied for the average dielectric constant of the medium. The observed angular dependence and electron resonance energy are in agreement with the theoretical predictions. These observations open a road to application of diffusive radiation in particle detection, astrophysics, soft X-ray generation and etc.. `Comment: 4pages, 4figure

Dipolar superfluidity in electron-hole bilayer systems

Bilayer electron-hole systems, where the electrons and holes are created via doping and confined to separate layers, undergo excitonic condensation when the distance between the layers is smaller than typical distance between particles within a layer. We argue that the excitonic condensate is a novel dipolar superfluid in which the phase of the condensate couples to the {\it gradient} of the vector potential. We predict the existence of dipolar supercurrent which can be tuned by an in-plane magnetic field and detected by independent contacts to the layers. Thus the dipolar superfluid offers an example of excitonic condensate in which the {\it composite} nature of its constituent excitons is manifest in the macroscopic superfluid state. We also discuss various properties of this superfluid including the role of vortices.Comment: 5 pages, 1 figure, minor changes and added few references; final published versio

Short-Wave Excitations in Non-Local Gross-Pitaevskii Model

It is shown, that a non-local form of the Gross-Pitaevskii equation allows to describe not only the long-wave excitations, but also the short-wave ones in the systems with Bose-condensate. At given parameter values, the excitation spectrum mimics the Landau spectrum of quasi-particle excitations in superfluid Helium with roton minimum. The excitation wavelength, at which the roton minimum exists, is close to the inter-particle interaction range. It is shown, that the existence domain of the spectrum with a roton minimum is reduced, if one accounts for an inter-particle attraction.Comment: 5 pages, 5 figures, UJP style; presented at Bogolyubov Kyiv Conference "Modern Problems of Theoretical and Mathematical Physics", September 15-18, 200

Volume element structure and roton-maxon-phonon excitations in superfluid helium beyond the Gross-Pitaevskii approximation

We propose a theory which deals with the structure and interactions of volume elements in liquid helium II. The approach consists of two nested models linked via parametric space. The short-wavelength part describes the interior structure of the fluid element using a non-perturbative approach based on the logarithmic wave equation; it suggests the Gaussian-like behaviour of the element's interior density and interparticle interaction potential. The long-wavelength part is the quantum many-body theory of such elements which deals with their dynamics and interactions. Our approach leads to a unified description of the phonon, maxon and roton excitations, and has noteworthy agreement with experiment: with one essential parameter to fit we reproduce at high accuracy not only the roton minimum but also the neighboring local maximum as well as the sound velocity and structure factor.Comment: 9 pages, 6 figure

Результаты геодезических измерений баланса массы некоторых ледников Заилийского Алатау

SummaryThe current state and mass balance of some glaciers of the Trans-Ili Alatau (Zailiyskiy Alatau, Tien Shan, Kazakhstan) is estimated in the paper. The remote sensing data (images of the Pléiades satellites) and detailed field geodetic measurements (differential global positioning system with the South G6 instrument)were used for the analysis. The field works were carried out in August-September 2021on the northern slope of the Zailiyskiy Alatau ridge. A digital relief model had been built for the analysis as well. Negative trends in the area of the studied glaciers have been revealed, which generally correspond to the average rate of degradation of the glaciation of the Trans-Ili Alatau from 1955 to the present. A comparison of glaciological (contact, i.e., obtained as a result of measurements made on the glacier) and geodetic (resulted from analysis of remote sensing data) methods for estimating the annual mass balance of the Central Tuyuksu glacier for the period 2016-2021 showed that the final values calculated by these two methods are very close in values. For example, the sum of annual balances calculated by the glaciological method for this period was -2.9 m w.e., or -0.58 a-1 m w.e. per a year, while the balance calculated by the geodetic method for the same period was - 0.63 a-1 m w.e. Thus, the results of comparison of the above methods of the glacier mass-balance investigation confirm that the geodetic one based on satellite measurements and the ground surveys using the South G6 instrument is in a good agreement with results of glaciological methods of measurements and calculations.Деградация оледенения вызывает изменения в горных экосистемах, оказывая непосредственное влияние на жизнедеятельность людей, вызывает изменения в гидрологическом режиме, биогеохимии рек и гляциальных озёр, влияя на качество и доступность водных ресурсов. Рассмотрены вопросы оценки баланса массы ледников северного склона Заилийского (Иле) Алатау посредством применения данных дистанционного зондирования Земли (ДЗЗ) в сочетании с полевыми исследованиями и измерениями