Precise measurements of surfaces

Precise measurement of surface tension of melts is difficult. Frequent use is made of the following methods ... 

USCT IT. The US block forms beam data by the mirror-shadow method and ensures simultaneously precise measurement of coordinates of sensors. It consists of two multichannel blocks, namely tomographic (USTB) for multiangle collection of projection data and coordinate (USCB) on surfaces waves for coordinates measurement of US sensors. 

It turns out to be considerably easier to obtain fairly precise measurements of a change in the surface free energy of a solid than it is to get an absolute experimental value. The procedures and methods may now be clear-cut, and the calculation has a thermodynamic basis, but there remain some questions about the physical meaning of the change. This point is discussed further in the following material and in Section X-6. 

Compared witii other direct force measurement teclmiques, a unique aspect of the surface forces apparatus (SFA) is to allow quantitative measurement of surface forces and intermolecular potentials. This is made possible by essentially tliree measures (i) well defined contact geometry, (ii) high-resolution interferometric distance measurement and (iii) precise mechanics to control the separation between the surfaces. 

Vitrinite Reflectance. The amount of light reflected from a poHshed plane surface of a coal particle under specified illumination conditions increases with the aromaticity of the sample and the rank of the coal or maceral. Precise measurements of reflectance, usually expressed as a percentage, ate used as an indication of coal rank. 

Several refinements of our experiments could test these theories further. By measuring etch pit densities as well as pit dimensions on sequentially-etched crystals, nucleation rate data and pit growth data could be collected, yielding information about the rate-limiting steps and mechanisms of dissolution. In addition, since the critical concentration is extremely dependent on surface energy of the crystal-water interface (Equation 4), careful measurement of Ccrit yields a precise measurement of Y. Our data indicates an interfacial energy of 280 + 90 mjm- for Arkansas quartz at 300°C, which compares well with Parks value of 360 mJm for 25°C (10). Similar experiments on other minerals could provide essential surface energy data. 

In this modification, the ionic micelle has been considered as the charged phase, which has difficulties from the thermodynamic viewpoint. The precise measurement of the surface tension of aqueous sodium dodecyl sulfate solutions revealed the cotlnuous decrease of surface tension above the cmc and indicated that the charged phase separation model is not correct (27). ... 

However, over the past decade, advances in, and in particular the availability of sophisticated instrumentation, and in the understanding of the instrumental techniques and the hosts and guests to which they are applied, mean that this need no longer be the case. A recent example in which a gamut of carefully chosen techniques, including such basic but essential measurements as elemental analyses, has led to the same precise characterization of surface species as has been the mainstay of molecular compounds is the study of the synthesis, characterization and reactivity of tantalum hydrides on silica, and their involvement in the dissociation of dinitrogen [203]. 

Whether or not an ionic species is desorbed directly from the surface or produced by field dissociation above the emitter surface can easily be determined with a precision measurement of the ion kinetic energy distribution. 

It is impossible to complete a discussion of the measurement of surface tension without saying something about the need for extreme cleanliness in any determination of 7. Any precision chemical measurement requires attention to this consideration, but surfaces are exceptionally sensitive to impurities. It is often noted that touching the surface of 100 cm2 of water with a fingertip deposits enough contamination on the water to introduce a 10% error in the value of 7. Not only must all pieces of equipment be clean, but also the experiments must be performed within enclosures or in very clean environments to prevent outside contamination. In addition, both surface tension and contact angle should be measured under constant temperature conditions. 

Barrier leak must always be suspected, and the film balance was periodically checked for leakiness either by maintaining an insoluble film of stearyl alcohol at constant tt for several hours (barrier leaks deduced if tt decreases) or by spreading talc on the water surface where barrier leak is most likely to occur. Careful preparation of the rim of the trough ensured a leakproof system. The mean deviation for k was < 2% for the more rapidly desorbing sulfate films, and — 5% for the phosphonate and carboxyl films. Experimental conditions were limited to systems in which the rates of desorption are neither too rapid nor too slow for the most precise measurements of k8. 

Dew point measurements are widely used in scientilic and industrial applications when precise measurement of water vapor pressure is needed. Dew point, the temperature at which water euudensaie heginx to form on a surface, can be accurately measured front -75 C to +101) C across the entire range of humidiiy with a condensation (chilled mirrort hygrometer. 

Instruments of this type may also be used quite effectively to evaluate kinetics of time-dependent changes in foods, be they enzymatic or reactive changes of other types. The computerized data-acquisition capabilities of these instruments allow precise measurement of absorbance or fluorescence changes, often over very brief time periods ( milliseconds). This is particularly useful for analysis of fluorescence decay rates, and in measurement of enzymatic activity in situ. A number of enzyme substrates is available commercially which, although non-fluorescent initially, release fluorescent reaction products after hydrolysis by appropriate enzymes. This kinetic approach is a relatively underused capability of computerized microspectrophotometers, but one which has considerable capability for comparing activities in individual cells or cellular components. Fluorescein diacetate, for example, is a non-fluorescent compound which releases intensely fluorescent fluorescein on hydrolysis. This product is readily quantified in individual cells which have high levels of esterase [50]. Changes in surface or internal color of foods may also be evaluated over time by these methods. 

Fundamentally, interferometric measurements entail bouncing a beam of coherent light from the sample surface, and then interfering the reflected beam with a reference beam. The light intensity is then a function of the displacement of the sample surface. When an alternating electric field is applied to the sample, then a lock-in amplifier can be used in conjunction with the detector to yield a precise measurement of the field-induced deflection of the sample surface. 

Robben and co-workers have exploited these facts to measure mean and rms temperature fluctuations in a turbulent flat flame (2) and above a catalytic surface (8). By measuring the postflame temperature on a flat flame burner, as a function of reactant flow rate, a precise measurement of laminar flame speed was reported by Muller-Dethlefs and Weinberg (9). 

ESR is known to be a very sensitive tool and can therefore be used in studying structural features of nanosized semiconductor particles doped with paramagnetic metal ions. In many studies vanadium impurities inside the Ti02 matrix or on the particle s surface were used as dopants. Moreover, V4+ ions are very convenient ESR probes since the 51V nuclei have a large magnetic moment leading to informative hyperfine structures (S = 1/2 / = 7/2). At low vanadium concentration, the EPR spectrum has well resolved sharp lines (Fig. 8.10) allowing precise measurement of spin-Hamiltonian parameters. 

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