Film pressure experimental measurements

The existence of asperity contacts in mixed lubrication causes great many local events and significant consequences. For example, the parameters describing lubrication and contact conditions, such as film thickness, pressure, subsurface stress, and surface temperature, fluctuate violently and frequently over time and space domain. It is expected that these local events would have significant effects on the service life of machine elements, but experimental measurements are difficult because of the highly random and time-dependent nature of the signals. Only a few successes were reported so far in experimental studies of mixed lubrication, mostly limited to the artificially manufactured... 

The experiment was carried out in a reaction cell shown in Fig. 3.3 with inner walls covered by a zinc oxide film having thickness 10 pm [20]. The surface area of the measuring film on the quartz plate was about 1/445 of the total film area on the wall of the vessel. The results of direct experimental measurements obtained when the adsorbent temperature was -196 C and temperature of pyrolysis filament (emitter of H-atoms) 1000°C and 1100°C, are shown on Fig. 3.4. One can see a satisfactory linear dependence between parameters A r (the change in film conductivity) and APh2 (reduction of hydrogen pressure due to adsorption of H-atoms), i.e. relations... 

The disjoining pressure vs. thickness isotherms of thin liquid films (TFB) were measured between hexadecane droplets stabilized by 0.1 wt% of -casein. The profiles obey classical electrostatic behavior. Figure 2.20a shows the experimentally obtained rt(/i) isotherm (dots) and the best fit using electrostatic standard equations. The Debye length was calculated from the electrolyte concentration using Eq. (2.11). The only free parameter was the surface potential, which was found to be —30 mV. It agrees fairly well with the surface potential deduced from electrophoretic measurements for jS-casein-covered particles (—30 to —36 mV). 

Alternatively, experimental measurement of the variation of equilibrium continuous liquid holdup with position for a concentrated oil-in-water emulsion can be employed to infer the variation of disjoining pressure with film thickness. Since the continuous phase liquid holdup e is known as a function of position, xp, Op and r can be calculated using equations 7,21 and 24. Equation 24 will then yield the disjoining pressure II at the film thickness xp. ... 

EXPERIMENTAL MEASUREMENT OF FILM PRESSURE 7.3a LANGMUIR FILM BALANCE... 

Several workers have reported experimental values of the ratio us/u for film flow, e.g., Friedman and Miller (F5), Grimley (Gil), and Chew (C6), who timed the movement of dye drops at the free surface, Brauer (B14) and Jaymond (J3) who used plastic confetti as surface tracers, and Asbj0rrisen (A6), who used an interesting residence-time technique. Jackson et al. (J2) have deduced the effective film surface velocities from pressure drop measurements in an adjoining gas stream, neglecting the effects of the surface roughness due to the waves. 

Examination of the relevant theory indicates that the adjuvant effect of surface-active agents on herbicide action is maximized when the quantity FI = yL cos 0, or the film pressure at the liquid/solid interface, has a maximum value. Measurement of surface tension of 1.0% aqueous solutions and of contact angle on a number of substrates (Teflon, paraffin) and plant-leaf surfaces (soybean, com) as a function of hydrophile-lipophile balance show at least one maximum, and these values are in good agreement with earlier experimental data on herbicidal activity. 

Schowalter and coworkers [49-51 ] developed a hot film anemometer to measure wall slip [49]. This particular experimental technique attempted to correlate the pressure and hot film oscillation with the alternation of boundary condition in slit capillary flow of monodisperse polybutadiene (PB). 

The thermodynamic state of the thin liquid film is described by the isotherm of disjoining pressure FI(/i). The opportunity the achieve a direct experimental measurement of the disjoining pressure is one of the main achievements in the study of thin liquid films. 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

Fig. 7.15 shows foam films formed in the measuring cell of Scheludko-Exerowa (variant A), in a porous plate cell (variant C) (see Section 2.1.2) and a foam. If the radii of the foam films in the measuring cells and the dispersity of the foam (respectively, the radii of the films in the foam) are properly chosen, as well as the applied capillary pressure in the films (variant B) and in the foam liquid phase, the experimental conditions with single foam films and foam films in the foam can be very close. 

Migone s group [46] also conducted more detailed studies of the different phases present on the Xe films. They compared their first- and second-layer data (between 112 and 150K) to computer-simulated isotherms for this system. The experimentally measured values for the temperature dependence of the midpoint pressure of the two substeps in the first layer, and that for the midpoint pressure of the second-layer step agreed very well with the values for these same quantities obtained in the simulations. The lower pressure substep in the first layer was identified as a one-dimensional phase formed by Xe adsorbed in the grooves. 

More pertinent, however, is the consideration introduced above relating to the vapor saturation conditions applicable to experimental measurements of contact angles. Such a measurement involves a liquid drop resting on a solid surface and forming a three-phase line of contact. But in the immediate vicinity of the contact line, the adsorbed film on the solid surface must be exposed to vapor which is saturated [2]. Hence, the insensitivity of the contact angles in the experiments reported by Fox and Zisman [33] to the saturation conditions of the bulk vapor is to be expected. The results therefore, are not conclusive evidence for negligible values of the film pressures. 

Verification of the estimates presented will be aided by further experimental studies, particularly direct measurements of film pressures. Also of distinct interest will be theoretical calculations of the solid-vacuum tension for a paraffin crystal. 

Experiments have clearly shown the formation of a dry spot at the central area of the layer (Gaver and Grotberg 1990 Ahmad and Hansen 1972 Fraaije and Cazabat 1989). This is also one of our findings and it is a part of the results reported here. An attempt to explain the dry spot formation due to the action of surface forces appeared in ref. (Jensen and Grotberg 1992), where the disjoining pressure is taken a function of the film thickness as for non-wettable surfaces. However this is not our case for we have complete wetting. We provide the appropriate explanation also accounting for the expected limited precision of experimental measurements, and for evaporation. 

Disjoining Pressure, Fig. 8 Calculated and experimentally measured isotherms of disjoining pressure, 11(A), of films of water on a quartz surface at KCl concentration of C = 10 mol/1, pH = 7, and dimensionless potential of the quartz surface equal to - 6 [I], (a) Within the region of large thicknesses, dimensionless potential of the film-air interface equals — 2.2 (curve 1), — 1 (curve 2),... 

The portions of the disjoining pressure curves that have negative slope and thus correspond to stable films can be measured experimentally. One technique involves formation of a film in a hole in a porous plate. At equilibrium, disjoining pressiue is equal to the capillary pressure applied to the film, which can be measured. Film thickness is determined by interferometry. Further information may be foimd elsewhere (Bergeron and Radke, 1992 Exerowa and Kruglyakov, 1998). 

To evaluate liquid hydrogen properly as a coolant, more experimental information is required concerning the heat-transport mechanism through the thermal film for both subcritical and supercritical conditions. For this report, subcritical pressures 30-70 psi and a narrow range of bulk temperatures are included in the experimental conditions. The heat-transfer characteristics of hydrogen were measured in an electrically heated vertical tube. The tube was instrumented for surface temperature and pressure-drop measurements. The temperature difference between the wall and bulk fluid was varied from approximately 40° to 1000°R. The maximum heat flux was 0.8 Btu/in. -sec. 

FIGURE 4.36 Plot of disjoining pressure, II, vs. film thickness, h comparison of experimental data for a foam film from Ref. [461] (thin-film pressure balance) with the theoretical curve (the solid line) calculated by means of Equation 4.230. The film is formed from 200 mM aqueous solution of the nonionic surfactant Tween 20. The volume fraction of the micelles (cj) = 0.334) is determined from the film contact angle the micelle diameter id = 1.2 nm) is determined by dynamic light scattering. The points on the horizontal axis denote the respective values of h for the stratification steps measured by a thin-film pressure balance. 

In order to solve this problem, some workers have Investigated the oil film pressure distribution in journal bearings under dynamic load (2-5), and one of the authors has also presented two papers ii.,5) which had dealt with this problem. Nevertheless, the boundary condition for dynamic loading has remained a big question, because there is infinite combinations of experimental parameters and the measurement of an oil film pressure under dynamic condition is a difficult task. 

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