Magnetic fluid bridge between two cones and a cylinder in the magnetic field of a current-carrying wire

A magnetic fluid (MF) changes its surface shape in the magnetic field of a current-carrying wire while the current is changing. In the present paper, we propose to study a MF bridge which can open or close the channel formed by two cones and a cylinder by imposing the magnetic field of a wire. Numerical modelling of the bridge behavior for different values of MF volumes and currents in the wire is done for two cases: when the MF wets and does not wet surrounding solid boundaries. It is shown that the presence of limiting cones allows the MF to sustain the pressure drop which is much higher in case of non-wetting than in case of wetting. In case of wetting, the MF cannot sustain any pressure drop at low currents, but in case of non-wetting, the MF can do it even at zero current. It is found that in case of non-wetting, spasmodic and hysteresis phenomena are possible for some values of MF volumes and currents in the wire. The use of a MF in valves, dispensers and pumps is one of possible actuation methods.

Meniscus of a ferrofluid around a vertical cylindrical wire carrying electric current

We study the meniscus profiles of ferrofluids in the magnetic field of a vertical current-carrying wire. Measurements of the free ferrofluid surface profile are quantitatively compared with numerical calculations. The theoretical model leads to a second-order ordinary differential equation. All material parameters are determined in independent experiments, therefore no fitting parameters are involved in the calculations. The experimental results can be modeled by the equilibrium of magnetic, gravitational, and interface tension forces. The classical model that neglects interface tension yields significant deviations from the experimental profiles in the parameter range studied.

Evaluation of the force required to move a coalesced liquid droplet along a fiber

This work presents a theoretical model describing the force required to move a coalesced liquid droplet along an oleophilic filter fiber. Measurements have been made using the atomic force microscope (AFM) to examine these forces over a range of fiber and droplet diameters as well as oil properties. Good agreement between measured and modeled forces was found. The influence of droplet displacement perpendicular to the fiber on the force required to move the droplet has also been determined experimentally and theoretically. It was found that fiber surface inhomogeneities are likely to influence results. This work has also established empirical relationships that can be used to predict the force, based on a known droplet volume, for the liquid types used.

Spreading of liquids on highly curved surfaces

Because of surface tension, liquid films coating fibers or the insides of capillary tubes are usually unstable and break up into a periodic array of droplets. However, if these films are very thin (of thickness in the range of tens of angstroms), they can be stabilized by long-range van der Waals forces. A simple method for making such wetting films consists of slowly drawing the fiber out of a bath of liquid; the thickness of the film is then measured using a method based on gas chromatography. If these liquid films are thick, and are forced to flow, they may then not break up: the instability becomes "saturated."

Hydrodynamic Instability of Liquid Films on Moving Fibers

The stability of liquid films on moving fibers is studied with a focus on effects caused by film-solid adhesion interactions and hydrodynamic interactions between the film and the surrounding gas. We show that at high fiber velocities (large Reynolds numbers) the film-gas hydrodynamic interactions induce instability in films, which would, otherwise, be stabilized by the adhesion interactions at static conditions. Two types of unstable modes caused by hydrodynamic factors are found: the first corresponds to the Kelvin-Helmholtz waves induced by inertia effects; the second, induced by viscosity effects, is observed at smaller (yet still large) Reynolds numbers when the Kelvin-Helmholtz instability is suppressed. Linear stability analysis of the system of coupled hydrodynamic equations of film and gas flow is performed by the method of normal modes. We derive the general dispersion relation as an implicit equation with respect to the mode frequency. In the limit of vanishing fiber velocities, the obtained equation provides the dispersion equation for motionless fibers. The conditions of film stability and unstable modes are analyzed in terms of two dimensionless parameters, the adhesion factor, which quantifies the intensity of film-fiber interactions, and the Weber number, which quantifies the intensity of film-gas interactions. The Reynolds number determines the regions of validity of the inviscid and viscous regimes of film instability. The results are illustrated by estimates of the stability conditions for moving fibers in terms of the stability diagram, the critical film thickness, the fastest unstable mode, and the corresponding characteristic break-up time. Although the methods developed are applicable for any type of liquid-solid interaction, the estimates were made for the long-range van der Waals interactions. Practical applications include various technologies related to fabrication and chemical modifications of fibers and fiber products, e.g., spin finishing and lubrication. Copyright 1999 Academic Press.