共查询到20条相似文献,搜索用时 46 毫秒
1.
Dombrowski C Cisneros L Chatkaew S Goldstein RE Kessler JO 《Physical review letters》2004,93(9):098103
Suspensions of aerobic bacteria often develop flows from the interplay of chemotaxis and buoyancy. We find in sessile drops that flows related to those in the Boycott effect of sedimentation carry bioconvective plumes down the slanted meniscus and concentrate cells at the drop edge, while in pendant drops such self-concentration occurs at the bottom. On scales much larger than a cell, concentrated regions in both geometries exhibit transient, reconstituting, high-speed jets straddled by vortex streets. A mechanism for large-scale coherence is proposed based on hydrodynamic interactions between swimming cells. 相似文献
2.
A stationary substrate can suspend only small pendant drops even with excellent wetting ability because of gravity. We report the suspension of large pendant water drops by a copper substrate that vibrates ultrasonically with a frequency of 22 kHz. The mass of the largest pendant drop suspended by the vibrating substrate reaches 1.1 g, which is 9 times that by the same stationary substrate. The pendant drop deforms drasticaJly and quickly at both the beginning and the end of the vibration procedure. As the vibration power increases, the contact area between the drop and substrate expands and the drop height shrinks accordingly. Theoretical analysis indicates that the Bernoulli pressure induced by ultrasonic vibration may contribute strongly to enhancing the suspensibility of pendant drops. 相似文献
3.
When simply put on a solid, a liquid drop usually adopts the shape of a spherical cap or a puddle depending on its volume
and on the wetting conditions. However, when the drop is subjected to a periodic field, a parametric excitation can induce
a transition of shape and can break the drop’s initial axial symmetry, provided that the pinning forces at the contact-line
are weak enough. Therefore, a standing wave appears at the drop interface and induces a periodic motion, with a frequency
that equals half the excitation frequency. In the first part, we review the different situations where star drops can be generated
from various types of periodic excitations. In the second part, we show that similar star drops can occur in a much less intuitive
fashion when the drop is put on an air cushion, where no periodic motion is imposed a priori. Preliminary experiments as well as theoretical clues for a hydrodynamic interpretation, suggest that the periodic vibration
is due to an inertial instability in the air layer below the drop. 相似文献
4.
We report an experimental study of liquid drops moving against gravity, when placed on a vertically vibrating inclined plate, which is partially wetted by the drop. The frequency of vibrations ranges from 30 to 200 Hz, and, above a threshold in vibration acceleration, drops experience an upward motion. We attribute this surprising motion to the deformations of the drop, as a consequence of an up or down symmetry breaking induced by the presence of the substrate. We relate the direction of motion to contact angle measurements. This phenomenon can be used to move a drop along an arbitrary path in a plane, without special surface treatments or localized forcing. 相似文献
5.
G. Olalere Ajayi I. E. Owolabi I. A. Adimula 《International Journal of Infrared and Millimeter Waves》1987,8(2):177-197
The rain induced depolarization in a tropical environment has been studied using a tropical raindrop size distribution developed by Ajayi and Olsen (A-O). The differential attenuation, differential phase shift and cross polarization discrimination, XPD, were computed over a frequency range of 1GHz to 300GHz for spheroidal drops and up to 33GHz for Pruppacher-Pitter drops. The variations of XPD with frequency, rainfall rate and copolar attenuation, CPA, were investigated. A mathematical relationship was established between the XPD and the CPA, canting angle and frequency of propagation from 5GHz to 300 GHz for spheroidal drops and up to 33 GHz for Pruppacher-Pitter drops. The results obtained using the A-O drop size distribution have been compared with those assuming the Laws and Parsons (L-P) distribution. The Pruppacher-Pitter drop shape has been found to give rise to higher XPD, especially at low CPA and high frequencies. 相似文献
6.
C.L. Shen 《Physics letters. A》2010,374(23):2301-4048
Liquid drops can be suspended in air with acoustic levitation method. When the sound pressure is periodically modulated, the levitated drop is usually forced into an axisymmetric oscillation. However, a transition from axisymmetric oscillation into sectorial oscillation occurs when the modulation frequency approaches some specific values. The frequency of the sectorial oscillation is almost exactly half of the modulation frequency. It is demonstrated that this transition is induced by the parametric resonance of levitated drop. The natural frequency of sectorial oscillation is found to decrease with the increase of drop distortion extent. 相似文献
7.
We investigate experimentally and theoretically the coalescence dynamics of two spreading droplets on a highly wettable substrate. Upon contact, surface tension drives a rapid motion perpendicular to the line of centers that joins the drops and lowers the total surface area. We find that the width of the growing meniscus bridge between the two droplets exhibits power-law behavior, growing at early times as t1/2. Moreover, the growth rate is highly sensitive to both the radii and heights of the droplets at contact, scaling as ho3/2/Ro. This size dependence differs significantly from the behavior of freely suspended droplets, in which the coalescence growth rate depends only weakly on the droplet size. We demonstrate that the scaling behavior is consistent with a model in which the growth of the meniscus bridge is governed by the viscously hindered flux from the droplets. 相似文献
8.
When a current is applied to a type-I superconducting strip containing a narrow channel across its width, magnetic flux spots nucleate at the edge and are then driven along the channel by the current. These flux "drops" are reminiscent of water drops dripping from a faucet, a model system for studying low-dimensional chaos. We use a novel high-bandwidth Hall probe to detect in real time the motion of individual flux spots moving along the channel. Analyzing the time series consisting of the intervals between successive flux drops, we find distinct regions of chaotic behavior characterized by positive Lyapunov exponents, indicating that there is a close analogy between the dynamics of the superconducting and water drop systems. 相似文献
9.
10.
We show that small amounts of 3He atoms, added to a 4He drop deposited on a flat cesium surface at zero temperature, populate bound states localized at the contact line. These edge states show up for drops large enough to develop well defined surface and bulk regions together with a contact line, and they are structurally different from the well-known Andreev states that appear at the free surface and at the liquid-solid interface of films. We illustrate the one-body density of 3He in a drop with 1000 4He atoms, and show that for a sufficiently large number of impurities the density profiles spread beyond the edge, coating both the curved drop surface and its flat base and eventually isolating it from the substrate. 相似文献
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13.
A model of evaporation of a multiply charged liquid drop is developed. The model self-consistently takes into account the main factors influencing the charged drop evaporation, including effects of the drop surface curvature and charge on the saturated vapor pressure, repeated fragmentation of drops during evaporation, and the capability of drops having a unit charge and a certain stabilization radius not to evaporate even in an unsaturated vapor medium. Analytical dependences are derived that can be used to calculate an integral lifetime of a charged drop with allowance for its fragmentation into smaller drops. Our estimates demonstrate that the evaporation time of charged drops is much smaller than that of uncharged drops. 相似文献
14.
We study the spreading of pure water drops or water drops
with surfactine (surfactant produced by bacteria Bacillus Subtilis) on gels
(Agar/Water gel). We find that, surprisingly, the drops do not
spread indefinitely, but remain in a state of partial wetting.
Eventually the liquid diffuses into the gel on a time scale short
with respect to evaporation times. The drops containing surfactant
show a complex dynamics: at first the spreading velocity decreases,
until the front stops and starts receding at about constant
velocity. Concurrently, a second front detaches from the rim of the
drop if the agar concentration is sufficiently low, and continues to
move outwards. 相似文献
15.
C. Altucci S. Amoruso R. Bruzzese M. Nisoli I. Procino G. Sansone V. Tosa R. Velotta C. Vozzi J. Xia X. Wang 《The European physical journal. Special topics》2009,166(1):11-14
We report an experimental study of liquid drops moving against gravity, when placed on a vertically vibrating inclined plate,
which is partially wet by the drop. Frequency of vibrations ranges from 30 to 200 Hz, and above a threshold in vibration acceleration,
drops experience an upward motion. We attribute this surprising motion to the deformations of the drop, as a consequence of
an up/down symmetry-breaking induced by the presence of the substrate. We relate the direction of motion to contact angle
measurements. 相似文献
16.
17.
On the basis of Born-Green-Yvon integral equations for the density distribution functions, an approximate integral equation is established for the profile of the surface of the drop. Numerical solutions and analytical solutions for limiting cases are obtained for this profile. Equations relating the angle at the leading edge and in its vicinity to parameters characterizing the interaction forces between the molecules of the liquid and between those of the liquid and solid are derived for large and for very small drops on a horizontal solid surface. One concludes that there is a rapid spatial variation of shape near the leading edge, that for large drops the measured macroscopic wetting angle is reached at a distance of about 20 to 40 Å from the leading edge, and that for very small drops the wetting angle is weakly size dependent. A condition for drop stability is established, which if not satisfied, the liquid will spread over the surface of the solid. 相似文献
18.
Haidar J. Lowke J.J. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1997,25(5):931-936
We have developed a unified arc electrode model that enables us to make predictions of the time development of molten drops from the welding wire in gas metal arc welding. The wire is taken as the positive electrode, and the effects of surface tension, magnetic pinch forces, and convection within the drop are taken into account to predict drop detachment for any given arc current. For pure argon, we have previously predicted the sharp transition that is observed experimentally at about 300 A between globular transfer at low current, when drop diameters are larger than the wire diameter, and spray transfer, for currents above 300 A, when drop diameters are smaller than the wire diameter. In this paper, we predict that addition of 25% of CO2 to the argon leads to an increase in the transition current to more than 325 A, also in agreement with published experimental results. For pure CO2, we find a significantly different drop behavior due to the more constricted arc. Both small and large drops are produced, with many very small drops being produced successively between each large drop 相似文献
19.
G. Barbero 《Physics letters. A》2008,372(12):2079-2085
We present a simple approach to introduce Debye's relaxation frequency. Our model is based on a kinetic equation at the electrodes, describing the time variation of the ionic charge pushed at the limiting surfaces by the effective electric field. In a first approximation the surface region where the ions are confined is assumed of negligible thickness. We show that this approximation allows to define a frequency at which the effective resistance of the sample begins to decrease. This frequency coincides with Debye's relaxation frequency. An equivalent electric circuit describing the frequency dependence of the effective impedance of the cell, valid in this approximation, is presented. In a second approximation we discuss the influence of a perfectly dielectric surface layer on the relaxation frequency, where the electrolytic cell is described by means of a Maxwell-Wagner system. The equivalent electrical circuit is discussed. 相似文献
20.
The resonant modes of sessile water drops on a hydrophobic substrate subjected to a small-amplitude lateral vibration are
investigated using computational fluid dynamic (CFD) modeling. As the substrate is vibrated laterally, its momentum diffuses
within the Stokes layer of the drop. Above the Stokes layer, the competition between the inertial and Laplace forces causes
the formation of capillary waves on the surface of the drop. In the first part of this paper, the resonant states of water
drops are illustrated by investigating the velocity profile and the hydrostatic force using a 3d simulation of the Navier-Stokes
equation. The simulation also allows an estimation of the contact angle variation on both sides of the drop. In the second
part of the paper, we investigate the effect of vibration on a water drop in contact with a vertical plate. Here, as the plate
vibrates parallel to gravity, the contact line oscillates. Each oscillation is, however, rectified by hysteresis, thus inducing
a ratcheting motion to the water droplet vertically downward. Maximum rectification occurs at the resonant states of the drop.
A comparison between the frequency-dependent motion of these drops and the variation of contact angles on their both sides
is made. The paper ends with a discussion on the movements of the drops on a horizontal hydrophobic surface subjected to an
asymmetric vibration. 相似文献