摘要:

The deformation and break‐up of single drops which are freely suspended in shear fields is examined both theoretically and experimentally. For creeping flows, a comprehensive theory is summarized which combines small deformation analysis, a slender‐body asymptotic expansion, as well as exact numerical computations using an integral equation formulation of the solution to the relevant mathematical system. Theoretical predictions are shown to be in excellent agreement with experimental results.

ISSN:0094-243X    

DOI:10.1063/1.38974

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The circulation produced in a drop by an electric field is examined. Here the leaky dielectric, used in the prototypal solution of Sir Geoffrey Taylor, is replaced with an electrokinetic model of the charge transport. Singular perturbation methods are employed to obtain a description of the macroscopic response of the drop to the field. The remarkable result is that complete agreement is found between the predictions of the two models. Previously, it was thought that effects due to space‐charge accumulation at the drop surface, which had presumably been omitted from the leaky dielectric, explained extant discrepancies between theory and experiment. The electrokinetic model shows that, in an aggregate sense, the leaky dielectric accounts for the space charge, and, so, the differences with the data remain unexplained.

ISSN:0094-243X    

DOI:10.1063/1.38956

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The first measurements of the effects of charge on the coalescence of small precipitation drops initially at terminal velocity in widely separated pairs have been obtained in a laboratory experiment. Observations were taken for a small drop (190 &mgr;m radius) with a constant positive charge interacting with a larger drop (340 &mgr;m radius) with both positive and negative charge. Data were also obtained for both drops carrying minimal charge and for a case with the small drop having a larger positive charge. Coalescence, bounce, temporary coalescence and temporary coalescence with satellites were observed. The results indicate that ‖ QR−Qr ‖ may be adequate to characterize the outcomes of drop collisions. New features of charged drop interactions were also observed: a unique impact angle dividing the coalescence from the non‐coalescence region that is independent of charge; a two‐order of magnitude range of relative charge between drop bounce and complete charge‐induced coalescence; a wide range of charge where a satellite drop occurs with temporary coalescence; and at higher charge levels, the beginning of charge‐induced permanent coalescence at the most grazing collisions.

ISSN:0094-243X    

DOI:10.1063/1.38982

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The hydrodynamic force resisting the relative motion of two drops moving along their line‐of‐centers is determined for Stokes flow conditions. The drops are assumed to be in near‐contact and to have sufficiently high interfacial tension that they remain spherical. The squeeze flow in the narrow gap between the drops is analyzed using lubrication theory, and the flow within the drops near the axis of symmetry is analyzed using a boundary integral technique. The two flows are coupled through the nonzero tangential stress and velocity at the interface.Depending on the ratio of drop viscosity to that of the continuous phase, and also on the ratio of the distance between the drops to their reduced radius, three possible flow situations arise, corresponding to nearly rigid drops, drops with partially mobile interfaces, and drops with fully mobile interfaces. The results for the resistance functions are in good agreement with an earlier series solution using bispherical coordinates. The new results for near‐contact motion have important implications for droplet collisions and coalescence.The theory is also extended to consider the normal motion of a drop toward a solid boundary. As expected, the resistance to this motion is less than that experienced by a solid particle moving toward a solid boundary. In particular, the force on a drop as it becomes very close to the boundary approaches one‐fourth of that on a solid sphere with the same size and relative velocity.

ISSN:0094-243X    

DOI:10.1063/1.38966

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In previous studies, the authors introduced an analytical model to predict the final state for both single sibling and multi‐sibling breakups. In order to verify the validity of this model, charged droplets of water were formed at the tip of capillary tube raised to high potential and subjected to external electric fields. The nozzle characteristics were tested to identify the different ejection modes. Under some conditions of electric field at the nozzle tip, breakup of the ejected droplets resulted. These droplets were collected on water sensitive paper and then examined to estimate the droplets size. The results showed good agreement with the analytical predictions and also supported the validity of the concept of treelike secondary breakups introduced in the model.

ISSN:0094-243X    

DOI:10.1063/1.38973

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity and from a free liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range of 5 to 300 MHz, with corresponding droplet diameters from 300 to 5 microns. The physics of droplet formation is essentially unchanged over this frequency range. For acoustic focusing elements having similar geometries, droplet diameter has been found to scale inversely with acoustic frequency. We summarize the results of a simple model which is used to obtain analytical expressions for the key parameters of droplet formation and their scaling with acoustic frequency. We also describe a more detailed theory which includes the linear propagation of the focused acoustic wave and the subsequent non‐linear hydrodynamics of droplet formation. This latter phase is modeled numerically as an incompressible, irrotational process using a boundary integral vortex method. For simulations at 5 MHz, this numerical model is very successful in predicting the key features of droplet formation.

ISSN:0094-243X    

DOI:10.1063/1.38949

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In containerless science for material processing, the acoustic fields is used to levitate and to control the position of a heated or cooled sample. The interaction between the temperature and the acoustic fields leads to complicated fluid flow phenomena, resulting in the perturbation of the sample position and the heat transfer process. The physical mechanisms in this thermal‐acoustic field were investigated using the technique of holographic interferometry and thermometry. Of particular interest was the heat transfer rate from the sample associated with the sound intensities, normal frequencies of the acoustic standing wave field, and gravitational effects. For metallic spheres with high thermal conductivity, the surface temperature was found to be uniform. The thermal flow phenomenon, which is associated with the circulating flow inside the resonant chamber, was recorded. The heat transfer coefficient at the sample surface was correlated with the acoustic and the gravitational parameters, based on the classical theory of convective heat transfer. These correlations can be used to predict the heat transfer from a spherical object in a zero‐gravity environment.

ISSN:0094-243X    

DOI:10.1063/1.38954

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

When a liquid drop is levitated on earth in a one‐axial acoustic levitator, it deforms under the action of radiation and Bernoulli pressure. With increasing product of Bond numberBo=&rgr;s/&sgr;s&agr;0 2g0(&sgr;s−surface tension, &rgr;s—density,a0—radius,g0=9.81 m/s2) and levitation safety factor &fgr;s, its shape varies from spheroid to ‘‘doughnut,’’ until, at a critical value ofBo ⋅ &fgr;sthe drop will either disintegrate or (at large viscosity) self‐inflate to a shell or a multibubble foam structure. Measurements of the drop aspect ratio are presented and compared with the theoretical prediction for an optimized levitator. Drop disintegration and self‐inflation is demonstrated with video recordings.Practical applications such as acoustical measurement of the Bond number and atomization of drops or liquid jets, fed continuously into the tuned standing wave, are presented. The ‘‘contactless standing wave atomization’’ is a new, valuable technique for containerless supercooling of melts, providing a wide spectrum of droplet size and respective rates with small quantities of any desirable material of technical or scientific interest.

ISSN:0094-243X    

DOI:10.1063/1.38955

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An acoustic method for suspending a drop in another liquid and deforming the drop has been combined with an optical detection scheme to measure the free quadrupole oscillation frequency and damping constant of the drop. The data for the pure water system support the theoretical predictions. For hexane drops in SDS (surfactant) aqueous solutions, it is found that the most important interfacial property is the Gibbs elasticity. Furthermore, the measured damping constants for SDS concentrations lower than 20% of the critical micelle concentration (CMC) are well described by employing an ideal equation of state for surfactants at the interface. The acoustic method, which is nonperturbative and requires very little amount of sample, may supplement other methods to measure the dynamic properties at a liquid‐liquid interface.

ISSN:0094-243X    

DOI:10.1063/1.38957

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Shapes and stabilities of freely rotating charged drops are investigated experimentally. Liquid drops approximately 3 mm in diameters were suspended in air by an electrostatic levitator and rotated by a torque which was acoustically generated in the levitation chamber. As the drop angular velocity was increased from the static state, families of axisymmetric, triaxial, dumbbell shapes, and eventual fissioning have been observed. With the assumption of ‘‘effective surface tension’’ by which the surface charge simply modified the surface tension of neutral liquid, the results from drops carrying low level surface charges agree quantitatively well with the Brown and Scriven’s prediction. The normalized angular velocity at the bifurcation point agrees with the predicted value of 0.56 to within 3%. However, as the sample charges approached the Rayleigh limit, the results indicated a marked deviation from those of low charges. Drops of high charges showed asymmetric dumbbell shapes and the drop break‐up directly from the axisymmetric shapes. Also discussed in this paper are methods of measuring drop charges and surface tension of highly viscous liquids in microgravity laboratories.

ISSN:0094-243X    

DOI:10.1063/1.38958

出版商:AIP    

年代:1990

数据来源: AIP