John C. Scott

The role of salt in whitecap persistence

Abstract Experiments have been performed concerning the coalescence and rupture of air bubbles in distilled water solutions of sodium chloride, in conditions free from contamination by organic surface-active materials. It is shown that the observed increased persistence a whitecap foam in salt-water systems compared with fresh-water systems is due, at least in part, to the mild surface activity of the salt itself. The result of the surface activity is to retard the coalescence of very small bubbles produced by breaking waves, decreasing the velocity of rise of the entrapped air and increasing the time during which bubbles are observed to reach the surface and break. A second effect is that bubbles may persist unbroken for a greater time after reaching the surface.

Stream meanders on a smooth hydrophobic surface

This paper reports an experimental study of a meandering water stream upon an inclined smooth hydrophobic surface. It was found that the sinuosity (ratio of the total stream length to the length of the projection of the stream on the line of maximum slope of the surface) increases with both increasing discharge rate and surface slope. It was observed that the meandering pattern is not always stable: once the discharge rate exceeds the upper critical value, the meandering pattern becomes unstable, whereas, when the discharge rate is smaller than the lower critical value, the water stream becomes discontinuous, and normally forms droplets, sliding successively down the sloping surface. It was found that, with increasing surface slope, the upper critical value decreases exponentially, while the lower critical value decreases only gradually. It was found that, when a system of stable meanders is formed on the surface, the meander loops are smoothly curved, swinging gradually from left-handed to right-handed deflections from the line of fastest descent, and vice versa , with an almost constant amplitude and wavelength. The stable meandering pattern migrates gradually down the sloping surface. The observations showed that the central axis of the meandering stream does not coincide with the locus of the highest points of the stream, the highest points being displaced towards the outside of each bend: the cross-sectional profile of the stream is thus usually asymmetrical. It was found that the cross-sectional area of the stream varies cyclically, with one increase and one decrease associated with each bend of the stream. This cyclic variation is repeated many times along the length of the stream, with each point of maximum cross-sectional area located close to a bend. A secondary reversing spiral flow was observed in the stream, and it was found that the sense of rotation of the flow is reversed at each bend. A plausible mechanism of these stream meanders is proposed on the basis of the present results, involving the existence of hysteresis of the contact angle between water and Plexiglas, the presence of asymmetrical surface-tension forces on the stream, and the acceleration and deceleration of the stream as it swings from loop to loop.

Flow beneath a stagnant film on water: the Reynolds ridge

Surface-active material is present in most naturally occurring water samples, and it naturally diffuses steadily to free surfaces, where it both reduces the surface tension and gives the surface elastic properties which enable it to resist compression. When the water flows so that the surface layer is trapped and compressed against a fixed shallow-draught barrier the film material makes the surface incompressible, and flow beneath the barrier forms a viscous boundary layer under the film. The stresses associated with this boundary layer are found to distort the surface in the region of the leading edge of the film, giving rise to a phenomenon which is commonly observed in nature and which has been called the Reynolds ridge. This paper describes experimental work on the measurement of the ridge, and compares the results with a theoretical model due to Harper & Dixon. Good agreement is indicated.

The Effect of Organic Films on Water Surface Motions

Water motions near surfaces are predominantly determined by two surface chemical factors: by the surface tension itself, and also by the surface dilatational elasticity, which is (simply) how much the surface tension changes with variations in surface compression and extension. Taking the example of ripples on water, whilst their velocity is strongly influenced by the surface tension, their dampingis determined mainly by the elasticity. For waves of larger wavelength, gravity waves, the surface tension loses most of its influence on the velocity, but it is found that the elasticity becomes even more important in determining the wave damping.

The Preparation of Clean Water Surfaces for Fluid Mechanics

This paper describes the criteria used in the preparation and characterization of clean and controlled water surfaces for experimental fluid mechanics. Small levels of surface contamination are shown to be capable of changing the whole character of certain free surface flows. The basis on which a surface can be considered clean is discussed, and acceptable techniques for determining the presence or absence of surface-active contamination are outlined.

The propagation of capillary-gravity waves on a clean water surface

Measurements are reported on the wavelength of small-amplitude water waves in a parallel-sided channel, covering the frequency range 2 to 10 Hz. Clean-surface techniques were employed in the experiments, and the results show good agreement with the predictions of linearized hydrodynamic theory.

Waves in narrow channels: faster capillary waves

DURING experiments on the propagation of water waves in channels with parallel sides made of Perspex, it was found that the phase velocities of small-amplitude waves with wavelengths in the range 40–100 mm were consistently higher than predicted by the standard hydrodynamic theory1,2. The original observations of this anomaly were made in a channel 100 mm wide filled to a depth of 50 mm. For these none too small dimensions, the standard theory might be expected to provide a close approximation to the wave velocity, but velocities ∼0.5% too high were observed. Highly surface-clean water and clean apparatus was used in all the experiments, and with these precautions the measurements proved to be wholly reproducible, prompting a closer examination, which is described here, of the wave motion to detect the cause of the anomaly. It was seen that the lines of contact between the water surface and the sides of the channel did not in fact rise and fall in step with the movement of the surface at the centre of the channel, instead remaining fixed in their original locations with the water at rest.

Techniques for Cleaning Liquid Surfaces

When we speak of “clean liquid surfaces” our meaning is often subtly different from considerations of “clean solid surfaces.” In most solid surface cases, what we require is a surface that is “pure”, i.e., composed solely of molecules of the desired substance.(1) Although there is a corresponding need for “pure” liquid surfaces in basic research, where delicate measurements of surface tension are related to intermolecular forces, what we require more often is a liquid surface whose fluid mechanics is simply specified or well defined.