Drop Detachment Research Articles

Dynamics of drop formation in granular suspensions: the role of volume fraction

The presence of grains strongly modifies the detachment of drops of a viscous liquid. We have shown previously that the detachment of drops of granular suspensions takes place via different regimes (Bonnoit et al. submitted, 2011). Here we study the influence of the volume fraction of particles on the formation and shape of the droplets by means of visual observations. We measure the minimal neck diameter as well as the height of the detachment as a function of time to quantify the evolution of the drop shape. We also address the question of the thinning dynamics of the neck in the different regimes. Linking the dynamics to the properties of the effective fluid or to rearrangements of individual grains in the thread gives insights in the origin of the different regimes.

Drop Detachment and Motion on Fuel Cell Electrode Materials

Liquid water is pushed through flow channels of fuel cells, where one surface is a porous carbon electrode made up of carbon fibers. Water drops grow on the fibrous carbon surface in the gas flow channel. The drops adhere to the superficial fiber surfaces but exhibit little penetration into the voids between the fibers. The fibrous surfaces are hydrophobic, but there is a substantial threshold force necessary to initiate water drop motion. Once the water drops begin to move, however, the adhesive force decreases and drops move with minimal friction, similar to motion on superhydrophobic materials. We report here studies of water wetting and water drop motion on typical porous carbon materials (carbon paper and carbon cloth) employed in fuel cells. The static coefficient of friction on these textured surfaces is comparable to that for smooth Teflon. But the dynamic coefficient of friction is several orders of magnitude smaller on the textured surfaces than on smooth Teflon. Carbon cloth displays a much smaller static contact angle hysteresis than carbon paper due to its two-scale roughness. The dynamic contact angle hysteresis for carbon paper is greatly reduced compared to the static contact angle hysteresis. Enhanced dynamic hydrophobicity is suggested to result from the extent to which a dynamic contact line can track topological heterogeneities of the liquid/solid interface.

溶滴移行現象の数値解析

Metal transfer modes in GMA welding process are different from welding variables: current, voltage, shielding gas, electrode wire and etc. Essentially they are influenced by physical properties of molten metal and electro-magnetic force. Then we numerically investigated the time-change of liquid transfer process using calculation model developed. The numerical result agreed very well with experimental result of water drop transfer from cylindrical nozzle. Numerical analysis shows that liquid transfer modes depend on the pressure balance at the tip of drop. When the dynamic pressure induced by liquid flow is lower than capillary pressure due to surface tension, globular transfer occurs. In contrast, when the dynamic pressure of liquid flow is higher, spray transfer occurs. In the case that the electrical current flows in the liquid, electro-magnetic force significantly influences on the transfer mode and the drop detachment. As for liquid flow-out from the nozzle of 1.2 mm in diameter, calculated results with use of physical properties of molten mild steel show that electro-magnetic force induced by higher current than 250 A changes the transfer mode from globular to spray. Whereas in real GMAW with pure argon gas shielding the transfer mode changes at around 230 A. The behavior of drop detachment is varied by both spatial distribution and time-change of the electro-magnetic force.

Numerical Model of Metal Transfer Using an Electrically Conductive Liquid

In order to understand the mechanism of the metal transfer process, the numerical model, which enables to calculate the time-change of liquid transfer process, was constructed. By using the model, we numerically investigated the influence of liquid properties and electro-magnetic force on the modes of liquid transfer. It is shown that liquid transfer modes depend on the pressure balance at the tip of drop. When the dynamic pressure induced by liquid flow is lower than capillary pressure due to surface tension at the tip of liquid drop, globular transfer occurs. In contrast, when the dynamic pressure of liquid flow is higher, spray transfer occurs. In the case that the electrical current flows in the liquid, electro-magnetic force significantly influences on the transfer mode and the drop detachment. Calculated results using physical properties of molten mild steel show that electro-magnetic pinch force induced by higher current than 250 A changes the transfer mode from globular to spray, for liquid flow-out from the nozzle of 1.2 mm in diameter. Whereas in real GMAW with pure argon gas shielding the transfer mode changes at around 230 A. The behaviour of drop detachment is varied by both spatial distribution and time-change of the electro-magnetic force.

Stability of sessile and pendant liquid drops

The solution space of axisymmetric liquid drops attached to a horizontal plane is investigated, and the stability of hydrostatic shapes is assessed by a novel numerical linear stability analysis involving discrete perturbations. For a given contact angle and Bond number, multiple interfacial shapes exist with compact, lightbulb, hourglass, and more convoluted pearly shapes. It is found that more than one solution branch can be stable, and that negative curvature at the contact line of a pendant drop is not a prerequisite for instability. Numerical simulations based on the boundary-integral method for Stokes flow illustrate the process of unstable drop detachment. Unstable drops transform into elongated threads with a spherical head whose volume is determined by a Bond number expressing the significance of surface tension. A complementary investigation of the shape and stability of two-dimensional drops attached to a horizontal or inclined plane reveals that hydrostatic shapes are least stable in the inclined configuration and most stable in the pendant or sessile configuration.

Wire-in-a-Nozzle as a New Droplet-on-Demand Electrogenerator

Many engineering applications assume the generation of single spherical droplets on demand. Piezo and thermo droplet-on-demand (DOD) instruments are not able to produce droplets with a broad range of size distributions from the same nozzle. We show that this challenge can be resolved using the principles of electrostatic generation and capillarity. A thin conductive wire threaded through a needle can be used as the droplet electrogenerator (or DOD generator). By applying a weak-bias electric field, one can deliver a drop from the needle edge to the free wire tip. When the drop moves toward the wire tip, it leaves behind a thin coating film. The capillary pressure of this cylindrical film helps isolate the droplet from the liquid source: the film spontaneously thins, breaking the connection between the droplet and the liquid source. The detachment of a pendant drop from the wire is then achieved by applying a short voltage pulse. Using the wire-in-a-needle nozzle, we were able to produce single droplets ranging from 50 to 500 μm, and we were able to deposit them with controlled velocity in a prescribed position. Therefore, the proposed method offers new opportunities in fields dealing with the DOD applications.

Interfacial tension determination of liquid systems in which one of the phases is non-Newtonian using a rheo-optical method

The interfacial tension between two low-density-difference liquids, one of them being non-Newtonian, was determined using a rheo-optical method with a computer-controlled parallel bands apparatus. This determination is featured measuring the steady-state deformation suffered by a Newtonian drop sheared by a non-Newtonian liquid matrix as a function of the applied shear rate. The application of Taylor's theory to the linear region of deformations, taking into account a mean viscosity of the matrix, enables us to determine the interfacial tension between both liquids as a fitting parameter. Such calculated σ values were checked for several oils with different viscosities, immersed in the same continuous phase, finding excellent agreement with the conventional drop detachment measurement.

Effect of Nanometer Far-Infrared Materials on the Cleanability of Ceramic Glazes

Nanometer far-infrared materials (NFIM) were doped into ceramic glazes to prepare easy-to-clean ceramic glazes，and the effect of NFIM on the cleanability properties was investigated. Static contact angle measurements were used to describe the surface hydrophilic property and calculate the surface free energy. The effect of far-infrared radiance measured by Fourier Transform Infrared Spectrometer on the O/W interface tension was tested by a Dynamic Contact Angle Meter and Tensiometer. The cleanability was estimated by the ultraviolet spectrophotometry technique using oleic acid as soiling agent. It was shown that the better cleanability of the ceramic glazes, the higher surface free energy and its polar contribution. The experiments were carried out on detachment of oleic acid drop from the easy-to-clean ceramic glazes. The three-phase contact line shrinked spontaneously, and eventually the oil drop detached from the surface. The spontaneous detachment of a drop corresponded to two different driving factors: (1) the decrease of oil-water interfacial tension caused by the far-infrared radiation of the glazes, (2) the spontaneous advancement of aqueous meniscus owing to the penetration of water between the oil and super hydrophilic glaze surfaces.

Neutralizing the effect of the angle variations on the drop detachment in automatic GMAW system

Two forces play the main role on the drop detachment in gas metal arc welding process: the electromagnetic and gravitational forces. Since in an automatic pipeline system, the welding angle varies from 0° to 180° in each pass, the total force will vary, so the drop detaching will not remain invariable during the welding time. In the previous works Doodman Tipi (Int J Adv Manu Technol 50:137–147, 2010 [1]), Doodman Tipi (Int J Adv Manu Technol 50:149–161, 2010 [2]), the angle variation effects on the metal transfer was studied on both transfer modes (free flight and short circuit). In this paper, the effect of the input parameters on the drop detaching is studied. Also, a welding current pattern is presented in order to neutralize the effect of the angle variation around the pipe. Furthermore, suitable patterns for other parameters (arc voltage, travel speed, and electrode speed) are introduced to keep constant the molten electrode volume, heat input, voltage to current relation, and energy density. This would guarantee stabilization of the other welding specifications. Finally, several experimental and simulation examples illustrate to achieve the regular detachment using the presented method, and results are compared with the un-neutralized case.

Volume of a liquid drop detaching from a sphere

AbstractA theoretical and experimental study is conducted to investigate the detached volume from a pendant drop on the surface of a sphere. Observation of drop detachment by high‐speed video camera reveals that the movement of the upper part of the neck of the drop is quite slow compared to that of the detaching lower part. The surface profile of the upper part was calculated approximately as a static problem using the axisymmetric Laplace equation. Using the drop profile, the system energy, including the work done by the solid–liquid wetting behavior, was calculated. Based on the condition of minimum energy, the volume of the detached part V was calculated. The volume V increases with the sphere diameter and approaches the value for the pendant drop attached to a plate. In addition, V is strongly dependent on the wettability between the sphere and the liquid and decreases with the receding contact angle. The detached volume of the water drop was measured for spheres of porous brick of various diameters. The experimental and theoretical results were found to be in good agreement. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/htj.20305

The study on the drop detachment for automatic pipeline GMAW system: short-circuit mode

In the automatic pipeline welding systems, the welding properties are varied regarding to the robot angle variations around the pipe. In this paper, a modified dynamic model—presented for the free-flight mode [1]—is extended to include the short-circuit mode. Consequently, a complete dynamic model is introduced including some modifications in characteristics equations. Some events and phenomena, like frequency jumping and combined transfer mode—caused by angle variations—are illustrated using time-frequency representation through the simulation and experimental examples.

Formation of viscoplastic drops by capillary breakup

The process of growth and detachment of drops from a capillary nozzle is studied experimentally by high-speed imaging. Newtonian drops are compared to shear-thinning and viscoplastic drops. Both Newtonian and shear-thinning fluid drops grow on the end of the capillary until a maximum supportable tensile stress is reached in the drop neck, after which they become unstable and detach. The critical stress is not influenced by variations in viscosity or in the degree of shear thinning. Viscoplastic fluids show a different behavior: at low values of the yield stress, the critical stability behavior is similar to that of Newtonian and shear-thinning drops. Above a threshold value, characterized in terms of the drop size, surface tension and tensile yield-stress magnitude, yield-stress forces are larger than surface forces, and the maximum tensile stress achievable in the drop neck at the point of critical stability is governed by the von Mises criterion.

The study on the drop detachment for automatic pipeline GMAW system: free flight mode

In the automatic pipeline welding systems, the mechanical properties and weld bead geometry are changed following to angle variation around the pipe. It occurred since during a weld pass, there are all of possible positions. In this work, first an augmented state-space model of process is presented that includes the travel motion and angle of robot. Also, gravitational force and arc voltage equations are modified. This model covers the free flight transfer modes (spray and globular). Then, accuracy of model is shown by model validation and comparing to real experiments. Finally, the angle effect on detachment is studied for all positions using simulation results. Detachment frequency variations, disturbance effects, and irregularity in detachments are analyzed using time–frequency representation.

Short time dynamic interfacial tension as studied by the growing drop capillary pressure technique

For the characterisation of interfaces between two immiscible liquids, capillary pressure tensiometry is the most versatile method. To measure short time interfacial tensions it has to be applied under very dynamic conditions for which the following protocols have been developed: 1—continuously growing drop (CGD); 2—pre-aged growing drop (PGD); 3—stopped growing drop (SGD). The CGD is a procedure analogous to the well-known maximum bubble pressure tensiometry for liquid–gas interfaces, however, the processes of drop detachment and formation of the subsequent drop require a rather complex data analysis. For the PGD protocol based on a growing drop with an initially pre-established equilibrium adsorption layer, the complexities are less, but for higher surfactant concentrations it is not optimum. The SGD protocol provides a drop with a fresh surface and the dynamic interfacial tension can be monitored in the absence of hydrodynamic effects. All three protocols complement each other and provide a good set of data for dynamic interfacial tensions at short adsorption times.

‘Gobbling drops’: the jetting–dripping transition in flows of polymer solutions

This paper discusses the breakup of capillary jets of dilute polymer solutions and the dynamics associated with the transition from dripping to jetting. High-speed digital video imaging reveals a new scenario of transition and breakup via periodic growth and detachment of large terminal drops. The underlying mechanism is discussed and a basic theory for the mechanism of breakup is also presented. The dynamics of the terminal drop growth and trajectory prove to be governed primarily by mass and momentum balances involving capillary, gravity and inertial forces, whilst the drop detachment event is controlled by the kinetics of the thinning process in the viscoelastic ligaments that connect the drops. This thinning process of the ligaments that are subjected to a constant axial force is driven by surface tension and resisted by the viscoelasticity of the dissolved polymeric molecules. Analysis of this transition provides a new experimental method to probe the rheological properties of solutions when minute concentrations of macromolecules have been added.

Forces and charges on a slightly deformed droplet in the DC field of a plate condenser

A conducting drop in partial wetting on the lower electrode of a plate condenser and surrounded by a dielectric fluid is considered. When a DC field is applied the drop, acquiring electric charges, is subjected to an electrostatic force normal to the electrode. The force exerted on undeformable drops was previously calculated. In this paper, the distortion from a spherical shape is asymptotically calculated at low electric Bond numbers to generalize previous developments. A mechanism is proposed to explain the drop detachment, leading to an electrical field strength threshold. Some experiments were performed confirming the mechanism.

Force–displacement model for analysis of pulsed-GMAW

A force–displacement model (FDM) is introduced in this work to analyse pulsed gas metal arc welding (GMAW). The drop detaching criterion is established based on the displacement of the pendant drop instead of the previous force balance criterion. The displacement of the drop is calculated by integrating the acceleration of the drop, and the force exerted on the drop is predicted using the modified force balance model coupled with wire melting. When the peak current, base current and time of pulsed-GMAW are given, the peak time for the one-drop one-pulse condition is determined using the FDM. While the peak current has the most significant effect on drop detachment, the initial drop mass prior to the peak time also influences the drop transfer. The FDM is applied to the dc as well as pulsed-GMAW, and the calculated results show good agreement with the experimental data.

The impact and deformation of a viscoelastic drop at the air–liquid interface

The parameters that influence the deformation of viscoelastic drops impinging on a viscoelastic bath, and that lead to the detachment of these drops from the surface were investigated. A range of PEO solutions with different viscosities and molecular weights was used and the deformation of each drop during impact was observed using a high speed camera. The use of image analysis to measure the evolution of interfacial areas between the drop and the cavity formed during impact allowed the estimation of the potential and interfacial energies. This gave valuable information for the understanding of drop detachment. The drops need to retain a sufficiently high kinetic energy after impact in order to pass through the surface. It is therefore necessary to limit the deformation of the drop as well as the deformation of the bath (i.e. cavity depth) by increasing the drop viscosity. Reducing the kinetic energy of the drop at the moment of impact also limits deformation and promotes detachment of the drop.

Effects of cross-flow velocity, capillary pressure and oil viscosity on oil-in-water drop formation from a capillary

The formation of oil drops from a single capillary with a diameter of 200 μm into a cross-flowing continuous water phase has been studied experimentally with the particle image velocimetry (PIV) technique and numerically with the computational fluid dynamics (CFD) software Fluent. The drop formation time and the volume of the detached drop were used as validation parameters and the results from the two methods corresponded well, with a difference of less than 5% for the drop formation time and 10% for the drop volume. The cross-flow velocity has a major impact on drop size, which decreases as the cross-flow increases. An increase in cross-flow, oil viscosity and capillary pressure displace the position of necking and drop detachment away from the capillary opening, which will have a decreasing effect on the final size of the drop.

Les nouveaux procédés de décontamination des effluents liquides et des solides du nucléaire

Les nouveaux procédés de décontamination des effluents liquides et des solides du nucléaire