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We demonstrate that sectoral coating by a hydrophobic fluoropolymer is an effective method for controlling flow separation and the turbulent wake behind a cylinder in high Reynolds number flows (Re = 2.2 × 105). Time-resolved particle image velocimetry measurements show that the shape of the wake and trajectory of large-scale vortex structures are inclined due to delayed flow separation on one side of the cylinder. Near-wall, high-resolution visualization reveals that this effect is related to micro-bubbles traveling along the coated surface. The properties of the coatings and bubble presence did not deteriorate, even after many hours of continuous facility operation.

We examine the motions of an autonomous Hamiltonian system with two degrees of freedom in a neighborhood of an equilibrium point at a 1:1 resonance. It is assumed that the matrix of linearized equations of perturbed motion is reduced to diagonal form and the equilibrium is linearly stable. As an illustration, we consider the problem of the motion of a dynamically symmetric rigid body (satellite) relative to its center of mass in a central Newtonian gravitational field on a circular orbit in a neighborhood of cylindrical precession. The abovementioned resonance case takes place for parameter values corresponding to the spherical symmetry of the body, for which the angular velocity of proper rotation has the same value and direction as the angular velocity of orbital motion of the radius vector of the center of mass. For parameter values close to the resonance point, the problem of the existence, bifurcations and orbital stability of periodic rigid body motions arising from a corresponding relative equilibrium of the reduced system is solved and issues concerning the existence of conditionally periodic motions are discussed.

Degradation-resistant two-dimensional metal–fluoropolymer composites consisting of gold nanoparticles coated with a thin fluoropolymer film were deposited on a substrate by hot wire chemical vapour deposition (HWCVD) and ion sputtering. The morphology and optical properties of the obtained coatings were determined. The thickness of the thin fluoropolymer film was found to influence the position of the surface plasmon resonance peak. Numerical calculations of the optical properties of the deposited materials were performed using Mie theory and the finite-difference time-domain (FDTD) method. The calculation results are consistent with the experimental data. The study shows that the position of the resonance peak can be controlled by changing the surface concentration of particles and the thickness of the fluoropolymer coating. The protective coating was found to prevent the plasmonic properties of the nanoparticles from changing for several months.

The paper presents the results of an experimental study of the effect of hydrophobic fluoropolymer coating on the multiscale characteristics of heat transfer at water boiling. New experimental data on dynamics of vapor bubble growth and detachment, evolution of contact line, nucleation site density, heat transfer coefficient were obtained using high-speed imaging techniques, including infrared thermography and video recording from the bottom side of transparent ITO heater. It was shown, that the using of hydrophobic fluoropolymer coating leads to heat transfer enhancement, to decrease of the superheat temperature at the onset of boiling, to increase of the active nucleation site density and to significant change in the dynamics of growth and departure of vapor bubbles and the evolution of the triple contact line.

In this paper we investigated the deposition process of the fluoropolymer coatings by the method of Hot Wire Chemical Vapor Deposition. The dependence of precursor gas pressure on deposition rate, structure and properties of the formed film was investigated. The study has shown that the increasing of precursor gas pressure leads to change of the film structure from porous to continuous with globules and further to solid that have different wettability. Depending on the mode of deposition was obtained the fluoropolymer coating with different structure: the wetting angle changed from 60 to 170°. A mechanism of the formation of fluoropolymer coating was discussed.

In contrast to two-level atoms, the interstate coherence of both three-level bosons and fermions contributes to the collisional frequency shift of internal transitions in a cold homogeneous gas. In particular, the measured contact shift of the b−c (a−d) hyperfine transition in 3D atomic hydrogen in the presence of incoherently populated state a (b) yields the difference at−as=30(5) pm between the triplet and singlet s-wave scattering lengths, which is a factor of 2 smaller than the previous result. The dynamic contact shift of the b−c resonance due to the Rabi oscillations between the coherently populated states a and b under the simultaneous continuous excitation of the a−b transition can explain the low-temperature electron-nuclear double resonance spectra of 2D hydrogen. The role of field inhomogeneity, spin diffusion, relaxation and recombination is discussed.

The low-temperature dc mobility of the two-dimensional electron system localized above the surface of superfluid helium is determined by the slowest stage of the longitudinal momentum transfer to the bulk liquid, namely, by the interaction of surface and volume excitations of liquid helium, which rapidly decreases with temperature. Thus, the temperature dependence of the low-frequency mobility is ��_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V s), where n_e is the surface electron density. The relation T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in kV/cm) and temperature (in K) and the value ��< 10^8 T^5 K^{-5}s^{-1} of the driving-field frequency have been obtained, at which the above effect can be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and ��/2��< 30 Hz., 4 pages, 1 figure

We propose the explanation of a small yet finite frequency shift of a hyperfine transition in two-dimensional atomic hydrogen bound to the surface of superfluid helium at T < 0.1 K. The nonzero shift is caused, first, by the interaction between adsorbed atoms, which effectively reduces their binding energy to the helium surface by a value proportional to their density and, consequently, reduces the deviation of the hyperfine constant from the free-atom value. Second, the transition between the hyperfine states b and c is shifted owing to a difference in the interaction of the b and c atoms with the residual atoms in the state a that appear due to one and two-body nuclear relaxation. This shift is also linear in the density of the two-dimensional gas. The net effect of the two contributions qualitatively agrees with the experimentally observed value of the shift.

Seeking for manifestations of superfluidity in 2D spin-aligned atomic hydrogen (H↓) adsorbed on the surface of liquid helium at T∼0.1 K we consider possible acoustic modes in this 2D Bose gas depending on frequency and on characteristic times of energy and momentum transfer. At high frequencies, the analogues of ordinary and second sound are realized in 2D H↓. At low frequencies, the 2D analogue of the fourth sound is realized: the normal component of hydrogen and ripplons are immobile and only the superfluid component of hydrogen participates the oscillations. We also estimate the rate of momentum relaxation between superfluid hydrogen and ripplons, τHR−1∝T13/3. The most promising for observing the superfluidity in 2DH↓ is the fourth sound, which could be excited, e.g., by ESR.

Experimental results on the achievement of high degrees of quantum degeneration in two-dimensional atomic hydrogen (2D H↓) by the magnetic-compression method are analyzed by taking into account current data on the binding energy Ea = 1.14(1) K of hydrogen atoms with the 4He surface and the constant Kab of the two-particle exchange recombination of adsorbed H atoms. The behavior of pair and three-particle correlation functions, as well as the transverse delocalization of the wavefunction of adsorbed atoms due to their interaction with each other, is taken into account self-consistently. A new mechanism of cooling of the compression region by means of the flow of H atoms on the helium surface with the subsequent evaporation and emission from a magnetic trap is proposed. This mechanism prevails at high densities, whereas the heat transfer at low densities occurs owing mainly to the interaction of ripplons with the phonons of the helium film. Existing data corroborate the achievement of the phase density σλ2 ≳ 10, which is certainly higher than the density necessary for both the arrangement of local coherence in 2D H↓ and its transition to the superfluid state. The results agree with the representations on quasi-condensation; however, direct evidence of this phenomenon is not revealed. The probability of three-particle dipole recombination that is corrected for the quantum correlation and delocalization is equal to 7(2) × 10−26 cm4 s−1 (T = 0.15, …, 0.21 K, B = 6.6 T, and σλ2 = 1, …, 9). The results are compared with other theoretical and experimental data.

Within the framework of quantum hydrodynamics of a superfluid helium surface, the momentum relaxation rate caused by the annihilation of two ripplons with phonon creation, inelastic phonon scattering with ripplon annihilation, and in the case of helium films one-particle ripplon scattering from the surface-level inhomogeneities introduced by the substrate roughness (new relaxation mechanism) was obtained for a ripplon gas at T≲0.25 K. The contribution from the inelastic phonon scattering is negligible at these temperatures. For a film at T≤0.15 K, one-particle scattering dominates, leading to a temperature dependence of the form K∝T5/3 for the convective thermal conductance.At higher temperatures, phonon creation with annihilation of two ripplons is the dominant mechanism, giving K∝T−3. These results are in quantitative agreement with the available experimental data.

We report on new experiments at Turku where two-dimensional atomic hydrogen gas has been compressed magnetically to high degrees of quantum degeneracy on the surface of liquid 4 He at about 200 mK. The results are in good agreement with our earlier observations: The probability of 3-body dipolar recombination is found to decrease gradually by an order of magnitude but to show no levelling yet, when the 2D phase-space density increases from 3 to 9. This effect is attributed mainly to a decrease in the local 3-particle correlator caused by the appearance and growth of a more or less local 2D condensate. The roles of interactions and out-of-plane delocalization of adsorbed H atoms are also discussed.

Forward three- and four-wave mixing in a photorefractive crystal are studied theoretically for the case of an AC field applied to the crystal and illuminated by a standing or moving interference pattern. The gain coefficients are obtained in the undepleted pump approximation. A comparison is made of the forward wave gain for the different mechanisms of signal beam amplification.

We consider nonlinear spectroscopic effects - interaction-enhanced double resonance and spectrum instability - that appear in ultracold quantum gases owing to collisional frequency shift of atomic transitions and, consequently, due to the dependence of the frequencies on the population of various internal states of the particles. Special emphasis is put to two simplest cases, (a) the gas of two-level atoms and (b) double resonance in a gas of three-level bosons, in which the probe transition frequency remains constant., 7 pages, 4 figures; Proceedings of the XXXVI Conference on Low Temperature Physics NT-36 (in Russian), July 2-6, 2012, Ioffee Physicotechnical Institute, St. Petersburg, Russia; accepted for publication in JETP. This paper draws partly from arXiv:1101.2559

The mechanism of recording a static grating in a planar photorefractive waveguide by a moving interference pattern and an external ac electric field is discussed. The direction of the external field is perpendicular to the waveguide layer. The recording efficiency is studied theoretically.

A theoretical investigation is reported of the suppression of noise waves in a thin-layer mutual conjugator. The methods of matrix and wave optics are used to show that the displacement of thin layers from the exact image planes in the direction towards a lens results in mutual phase conjugation of Gaussian beams when the self-oscillation threshold is exceeded slightly.

A new type of double-resonance spectroscopy of a quantum gas based on interaction-induced frequency modulation of a probe transition has been considered. Interstate interaction of multilevel atoms causes a coherence-dependent collisional shift of the transition between the atomic states |1> and |2> due to a nonzero population of the state |3>. Thus, the frequency of the probe transition |1>-|2> experiences oscillations associated with the Rabi oscillations between the states |1> and |3> under continuous excitation of the drive resonance |1>-|3>. Such a dynamic frequency shift leads to a change in the electromagnetic absorption at the probe frequency and, consequently, greatly enhances the sensitivity of double-resonance spectroscopy as compared to traditional "hole burning", which is solely due to a decrease in the population of the initial state |1>. In particular, it has been shown that the resonance linewidth is determined by the magnitude of the contact shift and the amplitude of the drive field and does not depend on the static field gradient. The calculated line shape and width agree with the low-temperature electron-nuclear double-resonance spectra of two-dimensional atomic hydrogen., 6 pages, 3 figures

In this Comment, we reanalyze the experiments on the collision frequency shift of the b-c and a-d hyperfine transitions in three-dimensional atomic hydrogen in the presence of, respectively, a and b-state atoms. Accurate consideration of the symmetry of the spatial and spin part of the diatomic wavefunction yields the difference a_T-a_S=0.30(5) \AA between the triplet and singlet s-wave scattering lengths of hydrogen atoms. This corrects the factor-of two error of the commented work [Phys. Rev. Lett. 101, 263003 (2008)]., Comment: 1 page

Hologram recording and subharmonic generation by means of ac field and phase modulated laser beams are studied. Phase locked detection mechanism in the geometry when the direction of the external ac field is perpendicular to the direction of the grating vector is discussed also.

Fractions of spin-polarized atomic hydrogen gas samples have been compressed magnetically from a low-density buffer volume at 0.25 to 0.5 K into the pole gap of a ΔB≈6T miniature field intensifier. The density of the compressed fraction has been calculated from the observed time evolutions of the buffer volume density and nuclear polarization. It is shown that excessive recombination overheating limits the maximum density to about 2·1019 cm-3 and thwarts the attainment of Bose condensation.

Two- and four-wave interactions by means of a photorefractive grating recorded by a moving interference pattern are investigated theoretically. The primary stationary grating is recorded by two waves of different frequency in an externally applied ac field. Secondary shifted and nonshifted gratings are recorded by frequency-degenerate interference; their influence is analyzed qualitatively. The depletion of all the waves is taken into account.