Исследование реакции поверхности жидкости на импульсное воздействие наклонной газовой струи при малых числах Рэйнольдса

A mathematical description of the motion of a cavity on the liquid surface under an oblique action of a gas jet is obtained using the well-known expressions for the movement of a gas bubble in a liquid. The boundary of the viscous drag force domination over the form drag force is determined. The impingement of the gas jet on the liquid surface is considered as a dynamic object of the automatic control theory. It is found that the dynamic properties of the two-phase system "gas jet - liquid" are described by the integrator equations. Using a specially designed setup, the transient response of the "gas jet - liquid" system were experimentally obtained for the aerodynamic action at angles of 20º and 50º to the surfaces of liquids with the viscosities of 0.71 and 26.1 Pa•s (Reynolds number Re < 2). The research results are necessary for the analysis of the non-contact aerodynamic method of liquid viscosity measurements.

Non-monotonic Mpemba effect in binary molecular suspensions

The Mpemba effect is a phenomenon in which an initially hotter sample cools sooner. In this paper, we show the emergence of a non-monotonic Mpemba-like effect in a molecular binary mixture immersed in a viscous gas. Namely, a crossover in the temperature evolution when at least one of the samples presents non-monotonic relaxation. The influence of the bath on the dynamics of the particles is modeled via a viscous drag force plus a stochastic Langevin-like term. Each component of the mixture interchanges energy with the bath depending on the mechanical properties of its particles. This discrimination causes the coupling between the time evolution of temperature with that of the partial temperatures of each component. The non-monotonic Mpemba effect—and its inverse and mixed counterparts—stems from this coupling. In order to obtain analytical results, the velocity distribution functions of each component are approximated by considering multitemperature Maxwellian distributions. The theoretical results derived from the Enskog kinetic theory show an excellent agreement with direct simulation Monte Carlo (DMSC) data.

Dark Matter: Could It Be Vacuum Viscosity?

We test a hypothesis that stars located away from the center of the galaxy, moving under the effect of an emergent viscous drag force perpendicular to their velocities, might exhibit the behavior observed in the rotation curves of the spiral galaxies. We construct a simple model for such an assumption, then by using simple fitting technique, we are able to produce the rotation curves for a sample of 18 spiral galaxies. Results show good agreement with the observed rotation curves. The applicability of our hypothesis suggests that an emergent drag force perpendicular to the velocity of the stars might be the cause of the apparent dark matter effect.

Magnetic manipulation on the unlabeled nonmagnetic particles

This paper reports two basic microfluidic strategies for the magnetic manipulation of unlabeled nonmagnetic particles/cells. One is the deflection induced by a single magnet, and the other is the confusing effect produced by two magnets of opposite polarity. They can be combined into more completed particle manipulations like continuous flow separation, counting and detection, which are essential steps in biomedical applications. We experimentally studied the dynamics of 10.4 and 20 [Formula: see text]m nonmagnetic polystyrene particles within a flow rate range of 30, 50, 70 and 90 [Formula: see text]L/min in a straight channel. We defined the cross-section length that the particles occupy as the “particle bandwidth” to characterize the extent of deflection and focusing. To predict the trajectories of the particles, we established a simple theoretical model by considering the magnetic force and viscous drag force. Compared with the experimental results, the maximum deviation of the simulation is 9.28%. The influences of magnetic nanoparticle concentration, magnetic field parameters, size of microparticles and flow rate are systematically investigated. We also demonstrated that the effective deflection and focusing could be realized at low Fe3O4 nanoparticle concentrations, which means that this method can reduce the damage on cells in the practical applications.

Numerical Study on Ventilated Cavitation Influenced by Injection of Drag-Reducing Solution

The influence of injection of drag-reducing solution on ventilated partial cavitation and supercavitation for an axisymmetric underwater vehicle is analyzed by numerical simulation. Turbulence, cavitation and multiphase models are SST k-ω, Schnerr-Sauer and Mixture models, respectively. The Cross viscosity equation is adopted to represent the fluid property of aqueous solution of drag-reducing additives. First of all, for non-cavitating conditions, the pressure distribution is obtained to determine the positions of injecting drag-reducing solution and ventilation. Then natural cavitation at different cavitation numbers is investigated for acquiring inception cavitation number. Finally, numerical simulations are conducted on the ventilated cavitating flows with and without the injection of drag-reducing solution at the cavitation number slightly smaller than the inception cavitation number (partial cavitation) and much smaller than the inception cavitation number (supercavitation). It is shown that for partial cavitation, the shape of cavity with the injection of drag-reducing solution is larger and the resistance of underwater vehicle decreases in comparison with the case without the injection of drag-reducing solution. However, for supercavitation, just viscous drag force obviously decreases, while cavity shape does not change.

Bacterial handling under the influence of non-uniform electric fields: dielectrophoretic and electrohydrodynamic effects

This paper describes the modeling and experimental verification of a castellated microelectrode array intended tohandle biocells, based on common dielectrophoresis. The proposed microsystem was developed employing platinumelectrodes deposited by lift-off, silicon micromachining, and photoresin patterning techniques. Having fabricated the microdevice it was tested employing Escherichia coli as bioparticle model. Positive dielectrophoresis could be verified with the selected cells for frequencies above 100 kHz, and electrohydrodynamic effects were observed as the dominant phenomena when working at lower frequencies. As a result, negative dielectrophoresis could not be observed because its occurrence overlaps with electrohydrodynamic effects; i.e. the viscous drag force acting on the particles is greater than the dielectrophoretic force at frequencies where negative dielectrophoresis should occur. The experiments illustrate the convenience of this kind of microdevices to micro handling biological objects, opening the possibility for using these microarrays with other bioparticles. Additionally, liquid motion as a result of electrohydrodynamic effects must be taken into account when designing bioparticle micromanipulators, and could be used as mechanism to clean the electrode surfaces, that is one of the most important problems related to this kind of devices.

Particle Flow and Contamination in Slider Air Bearings for Hard Disk Drives

For a particle entrained in an air bearing, various forces, such as the viscous drag force, Saffmann and Magnus lift forces and gravity force, will act on it. Such particles may pass through the air bearing or impact the slider or disk and then adhere to the surface or bounce off. In this paper, particle flow in an air bearing is simulated. The contamination of particles on a slider’s surface is analyzed using the assumption of adhesion upon impact. The effect of particle size and density on particle paths in the air bearing is studied. The numerical results show that particles are likely to contaminate slider surfaces in the transition regions on the rails. The density of the particles and the pitch angle of the slider are also found to strongly affect the flying path of the particles, and therefore, the accumulation of the particles on slider surfaces.