Alexander I. Fedorchenko

On some common features of drop impact on liquid surfaces

The impact of a drop on liquid surfaces is studied experimentally and theoretically in the region of the fully developed splashing. In order to reveal the influence of viscosity and target liquid depth on the resulting flow patterns, the experiments were carried out with water and 70% glycerol–water solution, and for different target liquid depths. Based on the experimental observations, a dynamic model of the central jet formation at the cavity collapse is developed. This model predicts an emergence of a liquid flow up into the central jet and simultaneously a small flow velocity downward and allows us to evaluate the velocities of these two flows. A theoretical model for the cavity submergence is presented. This model gives the constant velocity of the cavity submergence which is half the initial drop impact velocity. Analytical solution for the gravity–capillary cavity collapse has been derived and provides a good fit to the experimental results. Theoretical analysis and experiments have shown that the...

Thickness dependence of nanofilm elastic modulus

Young’s modulus is a fundamental physical parameter that determines not only the mechanic but also the electronic properties of a solid thin film. In here, we show that the elastic modulus is not a constant as that of conventional treatment but varies with film thickness. Scaling behavior is found based on the theoretical analysis of the free energy of surface-to-volume ratio of the film and results of the elastic modulus measurement. It has been shown that there exists some film thickness hb when the surface energy of the film comes into play. The hb is inverse proportional to the bulk Young’s modulus and depends strongly on the in-plain strain e0 as e0−2. For Si nanofilms, the variation of dimensionless elastic modulus Ψ=E/Ebulk with the dimensionless film thickness η=h/hb can be represented in the following form: Ψ=η0.226. The present investigation illustrates the importance of the effect of dimensionality on the basic parameter of a thin film as well as providing important implications for electronic ...

Effect of capillary and viscous forces on spreading of a liquid drop impinging on a solid surface

The theoretical models for the deformation of a liquid drop impinging on a solid flat surface at the initial and late stages are proposed. It was found that at the initial stage of the drop impact, the thickness of the emerging film decreases rapidly along its radius r, as r−6, that is similar to the splash jet induced by the blunt-body impact on the liquid surface. The thickness of the film levels off with time due to the viscous force, and the late stage of the drop spreading is controlled by the action of viscous and capillary forces. The influence of the capillary forces is localized in the vicinity of the triple line, and it causes the formation of the thick border (blob) on the edge of the spreading drop. An analytical solution of the model in viscous limit reveals that the minimum film thickness scales as Re−2∕5 and the drop maximum radius in its maximum extension as Re1∕5. The analytical solution for the dynamics of the blob mass growth is also obtained. The kinetic energy of the drop at its maxim...

Strain-induced wrinkling on SiGe free standing film

This study reports both experimental and theoretical investigation on the strain-induced wrinkling on Si1−xGex∕Si free standing film. Clear periodical pattern is observed and attributed to the strain relaxation of the SiGe film. With increasing lateral length of the film, both wavelength and amplitude increase. Nonlinear Von Karman plate theory is employed to model the structure. From the analysis, it shows that the formation of the wrinkling pattern is a trade-off between bending energy and stretching energy. Based on the modeling, it is found that wavelength decreased with increasing Ge content, while vice versa for the amplitude.

Strain on wrinkled bilayer thin film

The authors report strain measurement on bilayer wrinkling silicon-germanium (SiGe) thin film. In combination of atomic force microscopy and ultraviolet micro-Raman measurement, the relationship between the spatial profile and strain distribution is established. Theoretical analysis on the mechanism of strain relaxation shows that, in contrast to self-rolling nanotube, both bending and shearing force play an important role in determining the morphology. The behavior demonstrated in the SiGe system should also exist in a wide range of material systems under various strain situations. This study advances the understanding of nanostructure engineered by strain and has significant implications on the performance of nanodevices.

The formation and dynamics of a blob on free and wall sheets induced by a drop impact on surfaces

A physical model for describing an inviscid motion of free and wall liquid sheets induced by drop impact is presented. This model takes into account the formation of thick borders at the edge of a spreading drop, and the influence of advancing and receding contact angles on the dynamics of blob formation and motion. It has been shown that the blob motion on a free liquid sheet is described by a universal relationship (independent of the Weber number) in terms of dimensionless blob coordinates at both advancing and receding stages. For the case of blob formation and motion on a wall sheet at high Weber numbers, only the advancing stage is described by a universal relationship. The receding stage depends on the ratio of advancing and receding contact angles, but not on the Weber number. At the instant when the drop is at its maximum extension, the blob speed becomes equal to the liquid sheet velocity and the total kinetic energy of the drop is greater than zero. It is shown that the ratio of the instantaneo...

WRINKLING OF A DEBONDED INITIALLY COMPRESSED Si1-XGeX FILM

A compressively strained pseudomorphic Si1-xGex film being debonded from Si substrate by selective etching forms wrinkles with a uniform space periodicity. The present study provides experimental evidences and a theoretical model for the wrinkling process. To allow large deflection, non-linear Von Karman plate theory is employed. The amplitude and wavelength of wrinkles are determined by minimizing the total free energy of a debonded wrinkled film. The wrinkling analysis has shown that the amplitude and wavelength of wrinkled film are an outcome of a subtle compromise between bending energy, and normal and shearing components of the stretching energy. The wave number nondimentionalized over the depth of etch is a function of the membrane strain of a bonded film, Poissons ratio, and the nondimensional film thickness.

On the effective temperature concept in the problem of laminar vortex shedding behind a heated circular cylinder

Prediction of the nonisothermal vortex shedding based on the effective temperature Teff=T∞+c(TW−T∞) is always linked with the specific material properties of the working fluid, while a desirable universal prediction is unknown. Here we show that the thermal effect in diluted gases, where cylinder heating stabilizes wake flow (thus delaying the onset of vortex shedding), can be quantified in the extraordinary compact form of the proportionality between the critical Reynolds number ReC,∞ and the dimensionless film temperature Tf*. The theoretically derived c value for any dilute gas is related to an exponent n of the kinematic viscosity-temperature power law by a simple formula c=(2n)−1. The universal character of the proportionality has been proven; the linear increase of the critical Reynolds number is independent of the specific material properties for all diluted gases.

Viscosity measurement using fiber bend loss sensor

A fluid viscosity sensor using bend loss theory is presented. The sensing principle makes use of the damping characteristic of a vibrating optical fiber probe with fix-free end configuration. By measuring the frequency response of the fiber probe, the viscosity can be determined from the displacement of the fiber. The physical and mathematical model of the vibrating fiber submerged in a viscous fluid has been put forward. The impact of the viscosity and density on the resonance frequency shift has been elucidated. Based on the solution of the model methods of the viscosity extraction from maximum vibrational amplitude and bandwidth have been proposed.

Wrinkled SiGe Nanofilms as a Source of Terahertz Radiation

We demonstrate that the wrinkled Si_1-xGe_x/Si_1-yGe_y films can serve as radiation sources, which emit electromagnetic waves in a very wide range of the frequencies including the terahertz band from 0.3 to 3 THz. The emission mechanism is based on the change of acceleration of carriers, when they travel along the sinusoidal trajectory in wrinkled Si_1-xGe_x/Si_1-yGe_y films, a manner similar to synchrotron radiation with undulators, or like a free-electron laser.