Film pressure measurements

The float is effectively a two-dimensional manometer, and, like its open-ended counterpart, it measures the film pressure difference between the two sides of the float. This is another reason why it is imperative that no leakage occur past the float assembly Leakage would increase the pressure on the reference side of the float. For the same reason, the side of the float opposite the monolayer must be carefully checked for any possible source of contamination, not just misplaced surfactant. One way of doing this is to slide a barrier toward the float from that side to verify that no displacement of the float occurs. In all aspects of film pressure measurement, the torque must be measured with sufficient sensitivity to yield meaningful results. 

There are no measurements available for typical ocean situations, but it seems unlikely that slicks with film pressures significantly greater than 1 X 10" N m will be at all common on the surface of the open ocean. Firstly, the availability of organic material in these areas of relatively low primary production should be much reduced relative to coastal waters and, secondly, wind and wave conditions in the open sea are almost certainly too severe for the formation of visible slicks except under very calm and therefore unusual conditions. Goldacre (1949) made film pressure measurements on the surface of lake waters which are of special interest here as some of the findings, though obtained for inland waters, can be extrapolated to the oceanic situation. He found organic films (mainly protein and lipo-protein) on all of more than fifty natural bodies of water examined, with a few film pressure values in excess of 30 X 10 N m". Furthermore, at wind speeds as... 

A second aspect of the film properties of typical known surfactants, which is of relevance to the in situ film pressure measurements, concerns the usefulness of the film pressure as a measure of the amount of oi anic material present in a surface film. For solid and liquid-condensed films, the film pressure is almost independent of area/molecule in the region of interest, i.e., those pressures above 1 X 10" N m Fora liquid-expanded film. Fig. 1 suggests that the area/molecule might change by a factor of 2 or 3 if the film pressure was increased 30-fold from this lower limit to 30 X 10 N Finally for gaseous films, which may not in fact exist on the sea in this film pressure range, the film pressure should be approximately linear with the amount of organic material present per unit of surface. Only in this extreme case, therefore, is the film pressure data a reliable indicator of the amount of natural surfactant molecules at the water surface. For all of the other... 

In Figure 15, the curve (1) represents the strain measured by the thin-film strain sensor of Ni-Cr-Al. This strain causes the maximum error of 7MPa in the measured oil-film pressure according to the second term of equation [5]. The curve (2) with its maximum 155MPa represents the oil-film pressure measured directly by Ae pressure sensor of Cu-Mn-Ni according to the first term of equation [5]. The curve (3) with its maximum 162MPa is the corrected pressure and is the sum of the curves (1) and (2). 

Ellison and Zisman screened various compounds for surface activity in organic liquids by using a Langmuir film balance. The compound was spread on an organic liquid and the surface film pressure measured as a function of surface area [85]. The results indicated that the surface activity of fluorinated compounds in organic liquids can be predicted approximately from the Harkins equation for the spreading coefficient ... 

The film pressure is defined as the difference between the surface tension of the pure fluid and that of the film-covered surface. While any method of surface tension measurement can be used, most of the methods of capillarity are, for one reason or another, ill-suited for work with film-covered surfaces with the principal exceptions of the Wilhelmy slide method (Section II-6) and the pendant drop experiment (Section II-7). Both approaches work very well with fluid films and are capable of measuring low values of pressure with similar precision of 0.01 dyn/cm. In addition, the film balance, considerably updated since Langmuir s design (see Section III-7) is a popular approach to measurement of V. 

This method suffers from two disadvantages. Since it measures 7 or changes in 7 rather than t directly, temperature drifts or adventitious impurities can alter 7 and be mistakenly attributed to changes in film pressure. Second, while ensuring that zero contact angle is seldom a problem in the case of pure liquids, it may be with film-covered surfaces as film material may adsorb on the slide. This problem can be a serious one roughening the plate may help, and some of the literature on techniques is summarized by Gaines [69]. On the other hand, the equipment for the Wilhelmy slide method is simple and inexpensive and can be just as accurate as the film balance described below. 

Film pressure is often measured directly by means of a film balance. The principle of the method involves the direct measurement of the horizontal force on a float separating the film from clean solvent surface. The film balance has been considerably refined since the crude model used by Langmuir and in many... 

While the canal viscometer provides absolute viscosities and the effect of the substrate drag can be analyzed theoretically, the shear rate is not constant and the measurement cannot be made at a single film pressure as a gradient is required. Another basic method, more advantageous in these respects, is one that goes back to Plateau... 

The most common situation studied is that of a film reacting with some species in solution in the substrate, such as in the case of the hydrolysis of ester monolayers and of the oxidation of an unsaturated long-chain acid by aqueous permanganate. As a result of the reaction, the film species may be altered to the extent that its area per molecule is different or may be fragmented so that the products are soluble. One may thus follow the change in area at constant film pressure or the change in film pressure at constant area (much as with homogeneous gas reactions) in either case concomitant measurements may be made of the surface potential. 

Values for x , the film pressure of the adsorbed film of the vapor (of the liquid whose contact angle is measured), are scarce. Vapor phase adsorption data, required by Eq. X-13, cannot be obtained in this case by the usual volumetric method (see Chapter... 

Experiments of propane pyrolysis were carried out using a thin tubular CVD reactor as shown in Fig. 1 [4]. The inner diameter and heating length of the tube were 4.8 mm and 30 cm, respectively. Temperature was around 1000°C. Propane pressure was 0.1-6.7 kPa. Total pressure was 6.7 kPa. Helium was used as carrier gas. The product gas was analyzed by gas chromatography and the carbon deposition rate was calculated from the film thickness measured by electron microscopy. The effects of the residence time and the temperature... 

For osmotic drug delivery systems, Eq. (2) is of critical importance. This equation demonstrates that the quantity of water that can pass a semipermeable film is directly proportional to the pressure differential across the film as measured by the difference between the hydrostatic and osmotic pressures. Osmotic delivery systems are generally composed of a solid core formulation coated with a semipermeable film. Included in the core formulation is a quantity of material capable of generating an osmotic pressure differential across the film. When placed in an aqueous environment, water is transported across the film. This transported water in turn builds up a hydrostatic pressure within the device which leads to expulsion of the core material through a suitably placed exit port. 

Rates of photodegradation of copolymer films were measured in air using a filtered medium pressure Hg arc as the source of radiation and o-nitrobenzaldehyde as the actinometer. Table 1 gives the dosage levels incident on these films. 

The surface pressure-area (tc-A) isotherm measurements and LB film transfer were performed with the use of a KSV 5000 minitrough (KSV Instrument Co., Finland) operated at a continuous speed for two barriers of 10 cm2/min at 20°C. The buffer used in the present work was composed of 10 mM MES, 2 mM ascorbic acid sodium salt, and a given concentration of salt or polymers (pH =7.0). The accuracy of the surface pressure measurement was 0.01 mN/m. Monolayers of the PS I were transferred at 10 mN/m on hydrophobic substrate surface by horizontal lifting method. 

In addition to absolute pressure measurements, pressure sensors can be used to determine flow rates when combined with a well-defined pressure drop over a microfluidic channel. Integration of optical waveguide structures provides opportunities for monitoring of segmented gas-liquid or liquid-liquid flows in multichannel microreactors for multiphase reactions, including channels inside the device not accessible by conventional microscopy imaging (Fig. 2c) (de Mas et al. 2005). Temperature sensors are readily incorporated in the form of thin film resistors or simply by attaching thin thermocouples (Losey et al. 2001). 

Optical absorption, of hydrogenated and hydrogen- free films, 17 206 Optical amplifiers, 11 145-146 Optical applications U.S. patents in, 12 614t of vitreous silica, 22 440-441 Optical cavities, 14 849 Optical cells, for high pressure measurements, 13 417-419 Optical coatings, cerium application,... 

Figure 10. Development of equilibrium spreading pressure of film spread from pure crystals. Film pressure is measured on a Langmuir balance with barriers stationary. From Thompson (101).

This view of the process of spreading and the conditions of equilibrium possesses certain advantages because many solids such as the fatty esters and higher fatty acids spread upon water and their spreading coefficients can only be determined by a direct measure of the film pressure which exists when equilibrium is attained. 

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