Interfacial instability measurements

Some investigators have reported very high transfer rates which are attributed to "interfacial instability" or dimer formation of the acetic acid in the aqueous phase. However, we suspect that it is the data evaluation rather than system properties which causes difficulties. Even very slight inaccuracies in the measurement of concentration can cause the operating line to cross the equilibrium line, particularly when only the end concentrations are measured. Furthermore, considerable difficulties were met when handling unpleasantly smelling waste waters contaminated by butanol or MIBK. 

The agreement between measurement and (8.27) is not satisfactory for depth ratios of 0.8 and 0.833 in Fig. 8.29. In addition, for silicone oils 50, 100, and 350 the salt water attached to the top wall as the instability grew for the same depth ratios. That is, when the amplitude of the KHI is larger than Hi, the interfacial instability is disturbed by the top wall. 

The wavelength X increases with an increase in the kinematic viscosity of silicone oil. The analytical equation for A (8.30) agrees with the measured values of A for vi = 2 mm /s, but overestimates A as vi exceeds lOmm /s. The amplitude of the interfacial instability is also measured. 

There are two objectives triggering the interest in film instabilities. Since film instabilities must be caused by a force acting at one of the film surfaces, the structure formation process mirrors these forces. The observation of film instabilities can therefore be used as a sensitive measurement device to detect interfacial forces. The knowledge of these forces enables us, on the other hand, to control the morphology that is formed by the film break-up. 

The electric field experiment shown here can be considered as a test case for the quantitative nature of capillary instability experiments. It shows the precision, with which the capillary wave pattern reflects the underlying destabilizing force. In the case of electric fields, this force is well understood. Therefore, the good fit in Fig. 1.10b demonstrates the use of film instability experiments as a quantitative tool to measure interfacial forces. The application of this technique to forces that are much less well understood is described in the following section. 

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

A variant, with reminiscences to sec. 1.5, is based upon the capillary instability of jets, a topic that has drawn recent interest because of the increasing application of ink jet printers. Such printers are based on the deflection of a liquid jet in an electric field. The idea goes back to Sweet ), and has given rise to much printing technology. For the present purpose, oscillations in the jet are not produced by an elliptic orifice, but applied externally, say piezo-electrically. Dynamic surface or interfacial tensions can be obtained from, for Instance, the (quadrupole) oscillations of drops that have just broken away from the jet, or from the oscillations in the jet just before breaking. Measurements can be carried out down to lO" s" l... 

The Limits of Validity of the Experimental Data. Interfacial capacitance is generally measured through perturbation methods (pulse or a.c. methods). An earlier review has been strongly critical of the use of these experimental techniques because of the tendency of artifacts (such as stray capacitances and the high resistance of the organic phase) to affect the data and because of the mechanical/instrumental instability such experiments often induce [49, 58]. 

Depending on the nature of the sample, different techniques of observations and sample preparations are used in electron microscopy. For particles produced in solution " or from inert gas aggregation,clusters are generally collected on a microscope grid covered with an amorphous carbon film or with micro-crystals. Particles can, however, diffuse on the substrate and structural instabilities sometimes occur during the measurements. For particles supported on a soluble substrate (NaCl, KCl, MgO, etc.), a carbon layer is evaporated onto the particles and the carbon layer containing the particles is then stripped from the substrate by interfacial dissolution. The particles are observed directly in top view. The... 

Unfortunately, owing to experimental difficulties, interfacial measurements in molten salts often need to be made at solid metal substrates, and these give rather less reliable results.For example, the agreement between the reported values of the minimum capacitance for both solid and liquid metals tends to be quite poor. This may be ascribed to melt impurities, especially water and, at the higher temperatures, spurious parallel components arising from materials instabilities. However, progress has been achieved in a number of experimentally difficult situations, notably in the alumina-cryolite melt system which is so important in aluminum production. 

Dispersed products of carbonate rock weathering appear at interfacial contact of these two associations. They can be attributed to a group of problem soils, because of their considerable inhomogeneity, irregular variability of physical-mechanical properties, internal instability, susceptibility to suffosion and solubility. All of these properties suggest the difficulties in implementation of field tests and in sampling, as well as in measurement of physical-mechanical properties in laboratory conditions. 

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