Drops, falling, oscillations

Polarography is the classical name for LSV with a DME. With DME as the working electrode, the surface area increases until the drop falls off. This process produces an oscillating current synchronized with the growth of the Hg-drop. A typical polarogram is shown in Fig. 18b. 10a. The plateau current (limiting diffusion current as discussed earlier) is given by the Ilkovic equation... 

Figure 5.6. Variation of drop base radius (a) and contact angle (b) with time for A1 on steel at T = 700°C in dry H2 using the dispensed drop technique (drop mass of about 50 mg). In the experiment of Ebrill, oscillations of the triple line are due to the kinetic energy of the drop falling from a height of 12-13 mm (see Section 2.1.1). Data from works reported in (Tarasova et al. 1980, Ebrill...

The current oscillations arise from the dislodging of the Hg drop the current drops to minimum but then increases monotonically until the next drop falls. An undamped x-y or strip-chart recorder is used to measure the maximum current (i.e., the current at the end of the drop life) on the plateau of the wave. If the wave is diffusion controlled, the maximum current is called the diffusion current, i, and is given, in microamperes, by ... 

Figure 7.1.4 Polarogram for 1 mM CrO in deaerated 0.1 M NaOH, recorded at a DME. The Ilkovic equation describes current flow in the plateau region, at potentials more negative than about -1.3 V. The lower curve is the residual current observed in the absence of CrOl. The recorder was fast enough to follow the current oscillations through most of each drop s life, but not at the moment of drop fall, as one can see by the fact that the trace does not reach the zero-current line before starting a fresh rise with the new drop.

Note the oscillations in current as the mercury drop grows and falls. As the current increases, the magnitude of the oscillations increases in direct proportion. The recorded current does not fall to zero at the instant the drop falls because of the slow response of the current-recording device. 

O. Liquid drops in immiscible liquid, free rise or fall, discontinuous phase coefficient, oscillating drops... 

Now commence the voltage sweep using a scan rate of 5 mV per second, or with a manual polarograph, increase the voltage in steps of 0.05 V. The recorder plot will take the form shown in Fig. 16.4 if a manual instrument is used, then since the current oscillates as mercury drops grow and then fall away, the plot will have a saw-tooth appearance, and for measurement purposes a smooth curve must be drawn through the midpoint of the peaks of the plot. 

The value of Rl within a falling drop of liquid is of interest in view of the applications of spray absorbers. A wind-tunnel (59) for the study of individual liquid drops, balanced in a stream of gas, has shown (60) that Rl for a drop depends on its shape, velocity, oscillations, and internal circulation. The drop will remain roughly spherical only if... 

Large drops (De =1 cm) of chlorobenzene will fall through water with a somewhat erratic oscillatory motion (L3). The drop pitches and rolls. The flight is not vertical but is erratically helical in nature. A series of oscillations, accompanied by waves moving over the interface, can cause the drop to drift several inches in a horizontal direction in a range of a foot or two of fall. Such drops can not oscillate violently as described above, due to the damping action of such movement by the sliding side-wise motion of the wobble. Motion pictures indicate that internal circulation is also considerably damped out by this type of oscillation. Rate of... 

Violent oscillations of the axially symmetric type can be induced in single drops formed at a nozzle. Drops of chlorobenzene (Dg = 0.985 cm) were so formed, and allowed to fall in water. At about five inches below the nozzle two types of rupture were observed. A small droplet was formed at the front and hurled ahead of the drop by the next oscillation. A second mode of formation caused a droplet to be formed by inertial pinch at the rear of the oscillating drop. This rear-formed droplet was always larger than the very small one formed in front. There were, on occasion, two successive pinch-formed droplets from the rear. In a few instances both front and rear formation occurred, as shown in Fig. 13 in selected... 

In general, oscillations may be oblate-prolate (H8, S5), oblate-spherical, or oblate-less oblate (E2, FI, H8, R3, R4, S5). Correlations of the amplitude of fluctuation have been given (R3, S5), but these are at best approximate since the amplitude varies erratically as noted above. For low M systems, secondary motion may become marked, leading to what has been described as random wobbling (E2, S4, Wl). There appears to have been little systematic work on oscillations of liquid drops in gases. Such oscillations have been observed (FI, M4) and undoubtedly influence drag as noted earlier in this chapter. Measurements (Y3) for 3-6 mm water drops in air show that the amplitude of oscillation increases with while the frequency is initially close to the Lamb value (Eq. 7-30) but decays with distance of fall. 

As for steady motion, shape changes and oscillations may complicate the accelerated motion of bubbles and drops. Here we consider only acceleration of drops and bubbles which have already been formed formation processes are considered in Chapter 12. As for solid spheres, initial motion of fluid spheres is controlled by added mass, and the initial acceleration under gravity is g y - l)/ y + ) (El, H15, W2). Quantitative measurements beyond the initial stages are scant, and limited to falling drops with intermediate Re, and rising... 

Lenard1 has observed the oscillations of falling drops, and further measurements have been made by his pupils. 

P. Liquid drops in immiscible liquid, free rise or fall, discontinuous phase coefficient, oscillating drops T L,d,0icdp Nsh- 032(pA (a3g ph010 [E] Used with a log mean mole fraction difference. Based on ends of extraction column. Nfedrop = 411 < < 3114 dp = diameter of sphere with volume of drop. Average absolute deviation from data, 10.5%. Low interfacial tension (3.5-5.8 dyn), [ic < 1.35 centipoise. [141] p. 406 [144] p. 435 [145]... 

Figure 33. Effect of increasing proton activity, a through e, on the polarogram of anthracene. For the sake of clarity the five polarograms have been shifted horizontally. Note also that the oscillations caused by the growth and fall of the mercury drops are not shown. (From Ref. 76).

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