Rheology, thick-film measurement

Rheological characterization of the paste is to measure the relationship between shear stress and shear rate varying harmonically with the time, indicating the level of interparticle force or flocculation in the paste. The ideal paste for thick-film films should have a proper degree of pseudoplastic as well as thixotropic behavior (Wu et al., 2010). The thixotropic effect is a result of aggregation of suspended particles. Aggregation in the system caused by the attraction forces such as Van der Waals and repulsion forces due to steric and electrostatic... 

Firstly, the stability of the foam formed by P-casein and whole casein appears very different, the former being more stable. In order to further investigate this issne, we evalnate several surface properties of these two proteins. The surface tension and surface rheology do not seem to be accurate enough to account for this large difference in foam stability, since they show very similar values. However, the thickness of the foam films stabilized by the two proteins respectively seems to determine the ultimate behavior of the foam. Hence, the thicker foam film measured for P-casein probably prevents coalesce of air bubbles resulting in more stable foam formed by this protein as compared to whole casein. 

Another remarkable feature of thin film rheology to be discussed here is the quantized" property of molecularly thin films. It has been reported [8,24] that measured normal forces between two mica surfaces across molecularly thin films exhibit oscillations between attraction and repulsion with an amplitude in exponential growth and a periodicity approximately equal to the dimension of the confined molecules. Thus, the normal force is quantized, depending on the thickness of the confined films. The quantized property in normal force results from an ordering structure of the confined liquid, known as the layering, that molecules are packed in thin films layer by layer, as revealed by computer simulations (see Fig. 12 in Section 3.4). The quantized property appears also in friction measurements. Friction forces between smooth mica surfaces separated by three layers of the liquid octamethylcyclotetrasiloxane (OMCTS), for example, were measured as a function of time [24]. Results show that friction increased to higher values in a quantized way when the number of layers falls from n = 3 to n = 2 and then to M = 1. 

Rheological measurements were carried out at a Dynamic Analyzer Rheometer RDA II from Rheometrics. Parallel plate geometry with a plate diameter of 25 mm was used to perform the tests where thin films of materials of 1 mm thickness were inserted. To ensure the viscoelastic... 

The surface forces apparatus (SFA) measures forces between atomically flat surfaces of mica. Mica is the only material that can be prepared with surfaces that are atomically flat across square-millimetre areas. The SFA confines liquid films of a few molecular layers thickness between two mica surfaces and then measures shear and normal forces between them (figure C2.9.3)b)). In essence, it measures the rheological properties of confined, ultra-thin fluid films. The SFA is limited to the use of mica or modified mica surfaces but can be used to study the properties of a wide range of fluids. It has provided experimental evidence for the formation oflayered structures in fluids confined between surfaces and evidence for shear-induced freezing of confined liquids at temperatures far higher than their bulk freezing temperatures [14. 15]. 

Several other examples may be found in the literature, in which a correlation between the interfacial viscosity of macromolecular stabilized films with droplet stability was found. However, there are also a number of cases where stable emulsions could be prepared without any significant interfacial viscosity or elasticity. It can be concluded, therefore, that one should be careful in using interfacial rheology as a predictive test for emulsion stability. Other factors, such as film drainage and thickness, may be more important. In spite of these limitations, interfacial rheology offers a powerful tool for understanding the properties of surfactant and macromolecular films at the liquid/liquid interface. In cases where a correlation between the interfacial viscosity and/or elasticity and emulsion stability is found, one could use these measurements to screen various other components that have a marked effect on these parameters. 

The blister tests have also been extended to the evaluation of rheological properties of thin films, in particular, the nature of glass transition in thin film systems. McKenna and coworkers applied the blister test to the evaluation of the time-dependent creep compliance of nanoscale membranes (O Connell and McKenna 2005 O Connell et al. 2008). O Figure 22.4 is an image of the nano blister test, where a thin polystyrene membrane of thickness 17 nm is inflated under a pressure of 1 psi (= 6.895 kPa). By measuring the deflection... 

A transient numerical analysis has been carried out of the EHL shut-down conditions examined experimentally by Glovnea and Spikes [1]. The main features of the experimental measurements are reproduced in the numerical results obtained. There are differences observed in the detailed comparison of film thickness profiles. These may be the result of the particular form of non-Newtonian rheological model adopted for the numerical analysis. If so, such comparisons may be a means of distinguishing between such models within an EHL contact. 

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