The interaction of water with solid surfaces: Fundamental aspects

This is a review of worldwide land degradation problems. Four themes are emphasized: delineating and estimating the magnitude of soil erosion, quantifying erosion and sedimentation impacts on land productivity, establishing quantitative values for erosion-causing parameters, and implementing global and regional soil and water conservation programs. Papers deal with both developing and developed countries and illustrate how erosion control techniques used in developed countries can or cannot be applied in developing countries.

https://gendocs.ru/docs/29/28948/conv_1/file1.pdf

Soil Erosion and Conservation

Jets, ie collimated streams of matter, occur from the microscale up to the large-scale structure of the universe Our focus will be mostly on surface tension effects, which result from the cohesive properties of liquids Paradoxically, cohesive forces promote the breakup of jets, widely encountered in nature, technology and basic science, for example in nuclear fission, DNA sampling, medical diagnostics, sprays, agricultural irrigation and jet engine technology Liquid jets thus serve as a paradigm for free-surface motion, hydrodynamic instability and singularity formation leading to drop breakup In addition to their practical usefulness, jets are an ideal probe for liquid properties, such as surface tension, viscosity or non-Newtonian rheology They also arise from the last but one topology change of liquid masses bursting into sprays Jet dynamics are sensitive to the turbulent or thermal excitation of the fluid, as well as to the surrounding gas or fluid medium The aim of this review is to provide a unified description of the fundamental and the technological aspects of these subjects

Physics of liquid jets

The conservation and distribution of water substance in atmospheric circulations is considered within a frame of continuity principles, model air flows, and models of microphysical processes. The simplest considerations of precipitation involve its vertical distribution in an updraft column, where condensate appears immediately as precipitation with uniform terminal fallspeed. The study also treats steady two-dimensional air circulations in which time-dependent distributions of water vapor, cloud and precipitation respond to model microphysical processes.

On the distribution and continuity of water substance in atmospheric circulations

The amyloid state of proteins in human diseases

Over most of the earth clouds and precipitation are the dominant elements of the weather, and their study includes, directly or indirectly, a large part of the science of meteorology. In the last fifteen years there has been a surge of activity in this science under the stimulus of development in civil and military aviation. The growth of cloud physics during this period has been fostered not only by this general invigoration, but also by recognition of the practicability of exerting some influence upon the behaviour of clouds and their capacity for producing rain, hail, lightning and other meteorological phenomena.

The physics of clouds

Ice crystals, growing on a fibre in a water-vapour diffusion cloud chamber under conditions in which the temperature and supersaturation of the environment can be varied indepen­dently, undergo the following transitions of habit in the temperature range 0 to —30°C: plates→needless→hollow prismatic columns→plates→dendritic crystals→plates→prismatic columns. These changes of habit are controlled mainly by the temperature of the environ­ment, large variations of supersaturation influencing only secondary features such as the development of dendritic forms. When crystals growing at a particular temperature are suddenly transferred into a different environment, further growth follows the habit charac­teristic of the new conditions; this allows the production of crystals of composite habit. In contradiction to a recent report by Nakaya, the crystal habit does not appear to be influenced by the presence or absence of atmospheric aerosols, but small quantities of organic vapours, e. g. camphor and iso butyl alcohol, modify the habit to an extent depending upon their concentration.

The Influence of Temperature and Supersaturation on the Habit of Ice Crystals Grown from the Vapour

A detailed study has been made of the conditions under which uncharged water drops of radius 60 to 200 $\mu m$ coalesce or rebound at a clean water/air interface. The variable parameters in the system are the drop radius, r, its impact velocity, V$\_i$, and the angle of impact, $\theta\_i$; and the dependent parameters are the time of contact, $\tau$, between a rebounding drop and the water surface, the velocity, V$\_b$, and the angle $\theta\_b$ with which it leaves the surface. All these have been measured. Relations are established between the drop radius and the critical values of V$\_i$ and $\theta\_i$ at which coalescence occurs between uncharged drops and plane or convex water surfaces. Drops impacting at nearly normal incidence remain in contact with the surface for about 1 ms, lose about 95% of their kinetic energy during impact, and rebound with an effective coefficient of restitution of about 0.2. Drops carrying a net charge and drops polarized in an applied electric field coalesce more readily than uncharged drops of the same size and impact velocity. The magnitudes of the critical charges and critical fields required to cause coalescence are determined as functions of V$\_i$, $\theta\_i$ and drop radius. Typically, drops of radius 150 $\mu m$ impacting at 100 cm/s coalesce if the charge exceeds about 10$^{-4}$ e.s.u. or if the field exceeds about 100 V/cm. If the motion of a drop rebounding from a plane water surface is treated as simple harmonic and undamped, one may derive expressions for the depth of the crater, x, and the restoring force, F, at any stage, and also for the time of contact. These yield values that are in reasonable accord with experiment. However, the collision is clearly inelastic, and a second solution is obtained when F is assumed to be proportional, not only to the displacement, x, but to x/t. This leads to a slightly different expression for the time of contact and to a calculated energy loss of 84% compared with the measured value of 95%. If the drop is to coalesce with the water surface, it must first expel and rupture the intervening air film. Treating the undersurface of the drop as a flattened circular disk, an expression is determined for the minimum thickness, $\delta$, achieved by the film during the period of contact, in terms of V$\_i$, $\theta\_i$ and the drop radius r. This predicts values of $\delta \sim$0.1 $\mu m$ below which fusion may well take place under the influence of van der Waals forces. Several features of the observed relations between V$\_i$, $\theta\_i$ and r are accounted for by this simplified theory, but the behaviour of drops impacting at nearly glancing incidence, and of relatively large, energetic drops impacting nearly normally is not. In the latter case, the observed distortion of the drop is thought to play an important role in permitting more rapid thinning of the air film and, in the case of charged and polarized drops, by producing intense local electric fields that may cause the final rupture.

The coalescence and bouncing of water drops at an air/water interface

1. The rationale, design and managements of Surface Waters Acidification Programme/ Sir John Mason Part I. Hydrochemical Studies in Catchments Part II. Catchment Process Studies Part III. Catchment Manipulation Experiments Part IV. Chemical Processes Part V. Palaeolimnological Studies Part VI. Effects of Acidification on Fish and Other Aquatic Life Part VII. Modelling Studies Part VIII. Discussion of Conclusions General discussion Index.

The Surface waters acidification programme

In extension and confirmation of the work of Bigg, observations on the freezing of large numbers of drops of distilled water, ranging from 10 $\mu $ to 1 cm in diameter and suspended at the interface of two immiscible liquids in order to avoid the nucleating effects of solid surfaces, have established proportionality between the logarithm of the droplet volume and its freezing temperature for a constant rate of cooling. Such a relationship appears to characterize the heterogeneous nucleation of water droplets containing small foreign nuclei, and it is interpreted in terms of the droplets being infected with atmospheric aerosols whose ice-nucleating ability is known to increase roughly logarithmically with decreasing temperature. A technique has been developed for purifying water so that drops of up to 1 mm diameter may be supercooled to temperatures at which freezing occurs spontaneously without the aid of foreign nuclei. The empirical relationship between droplet volume and nucleation temperature, which differs from that representing heterogeneous nucleation, shows good agreement with theory. Photographs taken of freezing droplets upwards of 50 $\mu $ in diameter reveal that, in some cases, solidification was accompanied by rupture of the ice surface and the liberation of a number of small ice splinters. This suggests a secondary mechanism of ice crystal formation in natural clouds.

Basil John Mason

Papers

The physics of clouds

The Influence of Temperature and Supersaturation on the Habit of Ice Crystals Grown from the Vapour

The coalescence and bouncing of water drops at an air/water interface

The Surface waters acidification programme

The heterogeneous and homogeneous nucleation of supercooled water