Liquid surface, ripples

Capillary waves occur spontaneously at liquid surfaces or liquid liquid interfaces due to thermal fluctuations of the bulk phases. These waves have been known as surface tension waves, ripples, or ripplons for the last century, and Lamb described their properties in his book Hydrodynamics in 1932 [10]. Before that, William Thomson (Lord Kelvin) mentioned these waves in some of his many writings. 

Taylod205 also conducted mathematical analysis of the generation of ripples by wind blowing over a viscous fluid. Using a relationship between the growth of the amplitude of disturbance waves and the surface stress, Taylor derived a criterion for the instability of waves. In Taylor s instability theory, the disintegration of a liquid sheet/film is visualized as a process in which droplets are detached from the liquid surface with a wave of optimum amplitude. The diameter of the most frequent droplets is then formulated as a function of air velocity over the liquid surface, liquid density, surface tension and viscosity, as well as air density. 

In the ripple method a series of ripples is caused to travel over the surface of the liquid, the ripples being formed by means of an an electrically driven tuning fork dipping into the liquid. If viewed by means of intermittent illumination conveniently arranged by periodic interception of the light by interposition of a screen attached to one limb of the fork, apparently stationary waves may be observed and the mean wave length readily determined. 

In general, the observed mass-transfer rates are greater than those predicted by theory and may be related to the development of surface rippling, a phenomenon which increases in intensity with increasing liquid path. 

While the fluid dynamics of the actual film-flow process across the disc is daunt-ingly complex, a very approximate interim how model may be based upon Nusselt s treatment of the how of a condensate him. This assumes that the how is stable (i.e., ripple free), that there is no circumferential slip at the disc/liquid surface, and that there is no shear at the gas/liquid interface. The treatment is based... 

Entrainment and Mechanical Disintegration Gas can be entrained into a liquid by a solid or a stream of liquid falling from the gas phase into the liquicl, by surface ripples or waves, and by the vertical swirl of a mass of agitated hquid about the axis of a rotating agita-... 

On a molecular scale liquid surfaces are not flat, but subject to Jluctuations. These irregularities have a stochastic nature, meaning that no external force is needed to create them, that they cannot be used to perform work and are devoid of order. Their properties can only be described by statistical means as explained in sec. 1.3.7. Surface fluctuations are also known as thermal ripples, or thermal waves, in distinction to mechanically created waves that will be discussed in detail in sec. 3.6. Except near the critical point, the amplitudes of these fluctuations are small, in the order of 1 nm, but they can, in principle, be measured by the scattering of optical light. X-ray and neutron beams. From the scattered intensity the root mean square amplitude can be derived and this quantity can, in turn, be related to the surface tension because this tension opposes the fluctuations ). 

Figure 5.B-9 shows effective interfhcial areas that were deduced by Shulman et al.N from experiments on tha sublime ion of naphthalene Rase hi g rings. Many other studies have been inede in na attempt to relate effective area to specific packing surface, as a function or liquid and vapor flow rates. Generally, it is thought that the effective area reachas the specific area near the onset of flooding, but Pig. 3.8-9 shows that this mey not be the case. It also should be noted that the total area available for main transfer includes film surface ripples, entrained liquid within the bed, and vapor bubbling through pockets of liquid held up in the bed. Another important point relative specific surface of differeat packings might indicate relative mass transfer efficiencies, but this Is often sot the case.

The Schmidt numbers were varied from 0.60 to 2.5, and over this narrow range the difference between the exponents of 0,44 in Eq. (21.54) and 0.33 in Eq. (21,51) has only a small effect on the coefficient. The difference in exponents may have fundamental significance, since transfer to a liquid surface, which can have waves or ripples, should differ somewhat from transfer to a smooth rigid surface. 

Intensity fluctuation spectroscopy was used in our laboratory to study the dynamic behavior of surface ripples on thin liquid films. Both squeezing and bending modes were examined. To our knowledge one other group of researchers has obtained dynamic light-scattering data from thin soap films but as far as we know, nothing has been published in the official literature. Also some experiments were reported on lipid bilayers in water. ... 

If the gas flow rate is increased in the device in Fig. 16.4, ripples appear in the falling liquid film. As the gas flow rate is increased further, the film breaks and is pushed up and entrained into the gas stream. At low gas rates, if there are no liquid ripples, the interfacial area through which mass transfer occurs is simply the (measurable) liquid surface and the wetted-wall column can be used for mass transfer or hydrodynamic studies. 

The results in Eqs. ri5-40cl and fl5-43bl assume that the flow is laminar and the gas-liquid surface is flat. At Reynolds numbers less than 250, the flow is laminar f Sherwood et al 19751. However, even if the fluid flow remains laminar, ripples can appear on the surface. Ripples cause local mixing, which increases the liquid-side mass-transfer coefficient markedly. On the other hand, the area for mass transfer increases only slightly. If even small quantities of surfactant (e.g., soap or proteins) are added, the ripples are eliminated and the previously derived correlations for k, ijq remain valid. 

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