Viscosity threshold

Solid hydrocarbon materials are not observable in process NMR instruments. In order to be observed the molecules under analysis must be entirely in the liquid state and must be above a nominal viscosity threshold. In heavy or waxy petroleum streams the samples must be heated to approximately 80 °C to lower viscosity and ensure melting and solubility of waxy components. 

Moskowitz and Arabic (1970) found that the taste intensity (sweetness, sourness, saltiness, and bitterness) was related to the apparent viscosity of carboxymethylcel-lulose solutions by a power function with a negative slope. Pangbom et al. (1973) observed that the influence of different hydrocolloids on the perception of some basic taste intensities (saltiness, bitterness, sourness) appeared to be more dependent on the nature of the hydrocolloid and the taste of the substance than on the viscosity level. In contrast, sweetness imparted by sucrose was found to be highly dependent on viscosity, that is, the hydrocolloid concentration above a certain viscosity threshold, it was shown that the sweetness intensity of sucrose was significantly depressed. Saltiness was the taste attribute less affected, sourness, imparted by citric acid, was significantly reduced by all hydrocolloids tested, and for the other taste substances, the presence of a hydrocolloid generally enhanced the taste intensity of saccharin and depressed that of sucrose and caffeine (bitterness). 

It is these solid carboxylic nitrile elastomers which began to show utility in the modification of epoxy resins. Processing needs for solid elastomer Inclusion, particularly in liquid epoxy resins, have not always been advantageous. Associated problems include gel, viscosity threshold limitations and achieving desired rubber levels in excess of 5-6 phr. Sometimes processing must be carried out in selected solvents, not always a desirable or tolerable step. 

Inspection of Fig. 3.9 suggests that for polyisobutylene at 25°C, Ti is about lO hr. Use Eq. (3.101) to estimate the viscosity of this polymer, remembering that M = 1.56 X 10. As a check on the value obtained, use the Debye viscosity equation, as modified here, to evaluate M., the threshold for entanglements, if it is known that f = 4.47 X 10 kg sec at this temperature. Both the Debye theory and the Rouse theory assume the absence of entanglements. As a semi-empirical correction, multiply f by (M/M. ) to account for entanglements. Since the Debye equation predicts a first-power dependence of r) on M, inclusion of this factor brings the total dependence of 77 on M to the 3.4 power as observed. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Each oil-dispersant combination shows a unique threshold or onset of dispersion [589]. A statistic analysis showed that the principal factors involved are the oil composition, dispersant formulation, sea surface turbulence, and dispersant quantity [588]. The composition of the oil is very important. The effectiveness of the dispersant formulation correlates strongly with the amount of the saturate components in the oil. The other components of the oil (i.e., asphaltenes, resins, or polar substances and aromatic fractions) show a negative correlation with the dispersant effectiveness. The viscosity of the oil is determined by the composition of the oil. Therefore viscosity and composition are responsible for the effectiveness of a dispersant. The dispersant composition is significant and interacts with the oil composition. Sea turbulence strongly affects dispersant effectiveness. The effectiveness rises with increasing turbulence to a maximal value. The effectiveness for commercial dispersants is a Gaussian distribution around a certain salinity value. 

Choice of liquid Vapor pressure Surface tension Viscosity Chemical reactivity Intensity of collapse Transient cavitation threshold Transient cavitation threshold Primary or secondary sonochemistry... 

Yarin and Weiss[357] also determined the number and size of secondary droplets, as well as the total ejected mass during splashing. Their experimental observations by means of a computer-aided charge-coupled-device camera and video printer showed that the dependence of the critical impact velocity, at which splashing initiates, on the physical properties (density, viscosity, and surface tension) and the frequency of the droplet train is universal, and the threshold velocity may be estimated by ... 

Since it is necessary for the negative pressure in the rarefaction cycle to overcome the natural cohesive forces acting in the liquid, any increase in these forces will increase the threshold of cavitation. One method of increasing these forces is to increase the viscosity of the liquid. Tab. 2.1 shows the influence of viscosity on the pressure amplitude (Pft) at which cavitation begins in several liquids at 25 °C, at a hydrostatic pressure of 1 atm. 

The final factor to be considered here, and known to affect the cavitation threshold, is the temperature. In general, the threshold limit has been found to increase with decrease in temperature. This may in part be due to increases in either the surface tension (a) or viscosity (rj) of the liquid as the temperature decreases, or it may be due to the decreases in the liquid vapour pressure (P ). To best understand how these parameters (a, q, Py) affect the cavitation threshold, let us consider an isolated bubble, of radius Rq, in water at a hydrostatic pressure (Pjj) of 1 atm. 

Let us now consider the effect of solvent viscosity on the cavitation threshold. According to Tab. 2.1, an increase in the solvent viscosity required the application of a... 

Whilst vapour pressure may be the major solvent factor involved in the degradation process, there could also be a contribution from solvent viscosity or even, yet less likely, from surface tension. It has already been argued (see Section 2.6.2) that although an increase in viscosity raises the cavitation threshold, (i. e. makes cavitation more difficult), provided cavitation occurs, the pressure effects resulting from bubble collapse... 

Zero shear viscosities have been determined in solution over a wide range of concentrations with a cone-plate Rheometrics Stress Rheometer. For linear macromolecules, the viscosity is proportional to c below the so called "entanglement concentration", c above c, is proportional to c. However, the viscosity will rise steeply at some concentration below c in the case where particular interconnections are formed at the concentration at which the molecules come into contact with one another. Ideally this will be the overlap threshold c. Below c, the molecules may associate partially but cannot form a network continuous over the entire sample space. Above c, plastic flow will require separation and... 

An interesting class of polymer matrices widely employed for CSE consists of temperature-dependent, viscosity-adjustable polymer solutions that are filled into the capillary at one temperature at low viscosities and are used in separation at another temperature at entanglement threshold concentrations and higher viscosities. Such viscosity-adjustable polymers are termed thermoresponsive... 

Loveless DM, Jeon SL, Craig SL. Chemoresponsive viscosity switching of a metaUo-supra-molecular polymer network near the percolation threshold. J Mater Chem 2007 17 56-61. 

Fig. 11. The relationship between shear stress (t) and shear rate ( y) for a polymer disperse system showing creeping flow, with very high viscosity, at stresses smaller than the threshold yield stress [1]...

It is important to note that there is a critical threshold molecular weight below which there is little if any entanglement of polymer chains. Melt viscosity is a measure of the tendency (speed) of melted materials to flow. 

In contrast, the viscosity increases exponentially as the molecular weight increases above the threshold molecular weight. Since more energy is required to process these high-molecular-weight polymers, an optimum or commercial range is often selected for commercial general purpose polymers. 

Fluids that show viscosity variations with shear rates are called non-Newtonian fluids. Depending on how the shear stress varies with the shear rate, they are categorized into pseudoplastic, dilatant, and Bingham plastic fluids (Figure 2.2). The viscosity of pseudoplastic fluids decreases with increasing shear rate, whereas dilatant fluids show an increase in viscosity with shear rate. Bingham plastic fluids do not flow until a threshold stress called the yield stress is applied, after which the shear stress increases linearly with the shear rate. In general, the shear stress r can be represented by Equation 2.6 ... 

The rheological properties of all HMHEC polymers are profoundly affected by the hydrophobe molar substitution (MS) and the hydrophobe chain length. For any given hydrophobic moiety, there is a threshold hydrophobe MS below which there are no significant associative interactions. The most common phenomenological evidence for associative behavior is a dramatic increase in the solution viscosity of HMHEC polymers as a function of hydrophobe MS. The solution viscosity of HMHEC polymers continues to increase as a function of hydrophobe MS until the maximum limit of solubility is reached, as which point the HMHEC polymer becomes insoluble in water.33... 

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