Wetting instabilities

This form of nanopartides organization is related to a method which is based on the wetting instabilities of monomolecular layers when transferred onto solid... 

One method of overcoming some of the instability and loss in strength of oxychloride cements when exposed to water has been to modify them by impregnation with sulphur (Beaudoin, Ramachandran Feldman, 1977). The resulting material appears to be a composite in which the respective components complement each other. The magnesium oxychloride part has relatively poor resistance to water as initially formed, whereas the sulphur is difficult to wet and is completely insoluble in water. 

Introduction of a second solvent, incompatible with the continuous phase of a normally stable dispersion, produces an unstable condition if the new solvent wets the surface of the solid better than does the original vehicle. The instability induced by the introduction of a oily vehicle to an aqueous dispersion of an organic pigment is exploited commercially in the so-called flushing of pigments from an aqueous liquor into a vehicle. 

As mentioned in the previous -section, the existence of a liquid film of uniform thickness around the base of a slug was pointed out by Nicklin, Wilkes, and Davidson (N4). In an extension of this work, Nicklin and Davidson (N3) have carried the analogy between this situation and a wetted-wall column to the instability point known to occur in wetted-wall columns at high gas-flow rates. 

At low liquid rates, the onset of instability occurs at a constant value of the total superficial velocity, and is predictable from holdup and flooding data for wetted wall columns. As liquid flow rates increase, Nicklin and Davidson predict that unstable flow begins at lower values of the gas flow rate. For high liquid flow rates, however, the slug length becomes important, and the unstable flow will begin at higher values of gas flow rate. Therefore, a definite liquid flow rate exists at which an unstable flow pattern appears with a minimum gas flow rate. 

A horizontal interface between two fluids such that the lower fluid is the less dense tends to deform by the process known as Rayleigh-Taylor instability (see Section UFA). Spikes of the denser fluid penetrate downwards, until the interface is broken up and one fluid is dispersed into the other. This is observed, for example, in formation of drops from a wet ceiling, and of bubbles in film boiling. For low-viscosity fluids, the equivalent diameter of the particle formed is of order Ja/gAp. 

Both methods have advantages and limitations. The latter approach allows dry development, for example with ammonia vapour, but there also exists the possibility of premature, indiscriminate coupling. The one-component system does not suffer from this instability, but its processing is a wet operation, which itself leads to instability of the final image. 

Mechanisms Symptoms Carbon corrosion (air-air start) Gas impurities (e.g., CO, H2S, Sp2, ) Contaminants (e.g. some transition metal cations, anions) Catalyst instability (pt sintering, dissolution, re-crystallization) GDL loss of wet-proof (flooding) Seal failure (gross leaking) Membrane failure (pinholing, and tear)... 

Nearly all metals are thermodynamically unstable in most environments and the result of this instability is corrosion, such as oxidation or some other reaction with the environment. In both "wet" and "dry" corrosion three general phenomena occur. First, material from the metal can dissolve in the environment. This takes forms such as evaporation and volatile compound formation at high temperatures and material dissolution in aqueous solutions. Material loss by such processes may weaken a structure or cause loss of a protective layer. Second, a reaction layer may form on the surface of the metal. Frequently, these layers reduce the rate of a reaction and thus protect the material (passivate a... 

While it is perfectly acceptable to tone prints that have just been fixed and washed the most consistent results are obtained from toning dry prints that have been re-wet for 5 minutes. This is because the emulsion of freshly processed prints is still in a state of flux. This instability may cause minor variations between prints, even within the same batch. If the prints are to be stored for extended periods prior to toning, use proper storage techniques, as you would for any fine print. Prints that have been stored in or around materials that off-gas may exhibit staining. 

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