Visual Observations

A droplet size of 2.4 mm diameter was employed for most of the mass transfer experiments and attempts were made to maintain the droplet size at this value for the photographic studies. Unfortunately the technique used to form individual droplets was not as reproducible as that employed to produce a controlled stream of droplets for the mass transfer experi­ ments, and hence some size variation will be apparent in the subsequent photographs.

The initial photographs were taken with a Zenith B camera with an f2 Helios lens and Sunpak BC7 electronic flash unit. Plate 3 shows a droplet of amalgam, of approximately 2 mm diameter and containing init­ ially about 0.5 wt io La, during free fall through 1.1 IT Hydrochloric acid, glycerol-water solution. This photograph illustrates the general, visual appearance of all the reacting droplets observed. The stream of hydro­ gen bubbles evolved as a result of the reaction between sodium and the acid

is visible, but no information can be gained concerning the distrib­ ution of gas 011 the surface of the droplet.

immediately the droplet entered the aqueous phase. For the system ill­ ustrated 011 Plate 3» there appears to be a period, equivalent to a distanc of fall of about 4 cm, during which reaction is either slow or does not occur. The length of this "incubation period" was found to be very sensitive to the composition of the aqueous phase.- Further observations on this feature will be described in the next section.

The trail of bubbles produced by a reacting droplet tended to be somewhat irregular. This behaviour is illustrated more clearly on Plate 4 The regions of high bubble density tended to rise rapidly, contracting into spherical formations as they did so, before ultimately breaking up into clusters of small individual bubbles, which rose more slowly to the top of the column.

Two high speed cine films were taken of droplets falling through the same 1.1 N HC1 solution. The irregularities in the stream of bubbles produced by the reacting droplet appeared to be due to periodic variations in the size of the wake formed behind the droplet. Gas appeared to be evolved at a steady rate from the droplet surface, allowing the volume of bubbles in the wake to increase progressively until some critical size was reached, when a group of bubbles could break away. This con­ sequently left a much diminished volume of gas in the wake region, which would proceed to steadily grow again.

The droplets, during their fall through the aqueous phase, took the shape of an oblate spheroid, with their major axis perpendicular to the direction of fall. The larger droplets appeared to be oscillating slightly and rotating about an axis parallel to the direction of fall. In some cases the wake appeared to be non-symmetrical about the vertical axis of the drop and rotation about this axis caused the bubble stream leaving the wake to assume a helical pattern.

The high speed cine films obtained were of only moderate quality, exhibiting both excessive contrast and limited definition. This is an inherent problem with the technique, caused by the necessity to use high intensity lighting with high A.S.A. films. This line of research was consequently not pursued any further. In order to obtain more in­ formation concerning the surface topography of reacting droplets, it was necessary to develop the photographic techniques described in the pre­ vious section, involving the use of a micro-flash unit with a Nikon F camera and bellows unit.

The appearance of reacting amalgam droplets, containing initially about 0.55 wt <f> Na, in 0.5N, 1.1 N, 1.5 N and 2.1 N hydrochloric acid solutions are illustrated on Plates 5> 6, 7 and 8 respectively. It will be noted that on each of the photographs there appear three roughly circular discs lyingalong the horizontal axis of the droplets. The central dark area is a reflection of the camera lens, while the lighter areas on the left and right hand sides of the droplet are respectively, the reflections of the micro flash unit and a white card used as a light reflector.

In 0.5 N solution (Plate 5) "the hydrogen gas bubbles are confined to a relatively small area at the back of the droplet, leaving the front surface clear. The slightly irregular shape of this droplet suggests that it was oscillating. This is a little surprising in view of the fact that its diameter is well below that corresponding to the maxirnnrn term­ inal velocity for the system, (Figure 16), which is generally associated with the onset of oscillation. By increasing the acid concentration to 1.1 N a larger proportion of the droplet surface became covered by bubbles (Plate 6). The front of the drop still appears to be free from bubbles, although some can be seen adhering to the droplet surface at the point at which flow separation occurs. The wake has a slightly

stepped appearance suggesting that the droplet may have been rotating.