Viscous fluids, molecular

Mechanical and chemical methods for qualitative and quantitative measurement of polymer structure, properties, and their respective processes during interrelation with their environment on a microscopic scale exist. Bosch et al. [83] briefly discuss these techniques and point out that most conventional techniques are destructive because they require sampling, may lack accuracy, and are generally not suited for in situ testing. However, the process of polymerization, that is, the creation of a rigid structure from the initial viscous fluid, is associated with changes in the microenvironment on a molecular scale and can be observed with free-volume probes [83, 84]. 

When a tube or pipe is long enough and the fluid is not very viscous, then the dispersion or tanks-in-series model can be used to represent the flow in these vessels. For a viscous fluid, one has laminar flow with its characteristic parabolic velocity profile. Also, because of the high viscosity there is but slight radial diffusion between faster and slower fluid elements. In the extreme we have the pure convection model. This assumes that each element of fluid slides past its neighbor with no interaction by molecular diffusion. Thus the spread in residence times is caused only by velocity variations. This flow is shown in Fig. 15.1. This chapter deals with this model. 

Dependant on the molecular weights, PEGs range from viscous fluids to wax-like solids, with the tendency towards becoming solid increasing as the molecular weight increases (6). All of the PEGs are water soluble (6). 

The application of the hydrostatic pressure to the solvent of gel causes the permeation flow of the solvent. In this process, the gel behaves as a molecular sieve and imposes a frictional resistance on the flowing water. The permeation flow of water through a gel is a process analogous to the capillary flow of a viscous fluid. The frictional resistance of a single capillary is well described by the Hagen-Poiseuille s law by which the relationship between the dimension of the capillary, the applied pressure, and the flow rate is given as follows... 

This rule of thumb goes back to Maxwell (1867), who said that a viscous fluid with viscosity rjo can be thought of as a relaxing solid with modulus G that relaxes in a time period r hence, r]a Gx. Another handy rule is that the characteristic modulus of a liquid is roughly equal XovksT, where v is the number of structural units per unit volume. For a suspension of spheres, v is the number of spheres per unit volume, while for a small-molecule liquid, V is the number of molecules per unit volume thus, v = pNa/M, where p is the fluid density, M is the molecule s molecular weight, and is Avogadro s number. Hence, for a small-molecule liquid with density p = Ig/cm, Af = 100 g/mol, and T = 300 K, we estimate G 2.4 x 10 Pa = 24 MPa. 

Simethicone occurs as a translucent, gray-colored, viscous fluid. It has a molecular weight of 14 000-21000. 

For example, low viscosity may be attributed to molecular cracking (shear) or dilution by a lower viscosity fluid such as fuel. High viscosity may be attributed to oil oxidation or contamination by a more viscous fluid. 

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