Liquid climb

Almost every biological solution of low viscosity [but also viscous biopolymers like xanthane and dilute solutions of long-chain polymers, e.g., carbox-ymethyl-cellulose (CMC), polyacrylamide (PAA), polyacrylnitrile (PAN), etc.] displays not only viscous but also viscoelastic flow behavior. These liquids are capable of storing a part of the deformation energy elastically and reversibly. They evade mechanical stress by contracting like rubber bands. This behavior causes a secondary flow that often runs contrary to the flow produced by mass forces (e.g., the liquid climbs the shaft of a stirrer, the so-called Weissenberg effect ). 

Since a2 oc Rch, a has units of length. Note that the height to which a liquid climbs in a capillary (assuming 6 < 90°) increases as Rc decreases. As would be expected, h is larger for large y and small Ap. By measuring h for a capillary of known radius, Equation (5) permits an approximate value of 7 to be determined if 6 is known. 

Capillary action, the rise of liquids up narrow tubes, occurs when there are favorable attractions between the molecules of the liquid and the tube s inner surface. These are forces of adhesion, forces that bind a substance to a surface, as distinct from the forces of cohesion, the forces that bind the molecules of a substance together to form a bulk material. By finding a relation between the height to which a liquid climbs in a tube of known diameter, we can use the height to determine the surface tension. 

A concave meniscus occurs in a liquid in which the adhesive forces are greater than the cohesive forces. The liquid climbs to a height where the weight of liquid column balances the attraction to the walls. 

If the liquid wets the glass, 6 is either zero or close to zero and not easily observable. Contact angles are more easily observable if the liquid does not wet the glass, 9 > 90°, so that instead of the liquid climbing inside the tube, the liquid is depressed to a lower level relative to the free liquid surface. This is the case, for example, if the glass tube is immersed in mercury or if the glass tube is coated with paraffin wax internally and is submerged in water. 

FIGURE 3.31 Experiments demonstrating the normal force (Weissenberg) effect. Liquid climb on rotation (a) in channels drilled into a cone and (b) in a coaxial cylinder. 

When a small-diameter glass tube, or capillary, is placed in water, water rises in the tube. The rise of liquids up very narrow tubes is called capillary action. The adhesive forces between the liquid and the walls of the tube tend to increase the surface area of the liquid. The surface tension of the liquid tends to reduce the area, thereby pulling the liquid up the tube. The liquid climbs until the force of gravity on the liquid balances the adhesive and cohesive forces. Capillary action helps water and dissolved nutrients move upward through plants. 

FIGURE 6.12. Some special examples of capillary flow (a) a liquid climbing a partially immersed rod (b) wicking—the spontaneous movement of a liquid from a nonwetting to a wetting situation. 

Another serious limitation on the magnitude of the torque is avoidance of the Weissenberg effect in which, owing to normal stresses (Section C below), the liquid climbs up the inner cylinder during rotation. This alters the geometry and falsifies the form factor b calculated from equation 7. Observation of the sample is therefore highly desirable to make sure that the torque is sufficiently small to render this effect negligible. 

There are several ways of demonstrating, experimentally, that polymeric fluids exhibit elastic characteristics. One very well known experimental observation is the behavior of liquid climb-up on a rotating rod in a polymer solution. Figure 1.1 demonstrates a dramatic difference in the behavior of liquid climb-up on a rotating rod between (a) 4 wt % aqueous solution of polyacrylamide and (b) glycerin. It is seen in Figure 1.1 that the polyacrylamide solution climbs the rod rotating within it, whereas no climb-up... 

Figure 1.1 Difference in liquid climb-up behavior on a rotating rod between (a) 4 wt % aqueous solution of polyacrylamide (viscoelastic fluid) and (b) glycerin (Newtonian fluid).

The twelve experiments of Figure 1 help to develop intuition about how polymeric liquids behave (a) The surface of a Newtonian liquid near a rotating rod shows a depression near the rod, whereas the polymeric liquid climbs the rod. This rod-climbing is called the Weissenberg effect . ... 

Mercury Porosimetry and Capillary Flow Porometry - Pore Size Determination In a mercury porosimetry measurement, pressure is used to force mercury into filling the pores and voids of the material. The method is based on the capillary rise phenomenon which exists when a non-wetting liquid climbs up a narrow capillary. As the pressure is increased, mercury infiltrates the pores to occupy a subset of the total pore space, the extent of which depends on the applied external pressure. The injected volume of mercury as a function of pressure is recorded. The pore size and distribution can be resolved using the Young and Laplace model [43]. The pore sizes that can be determined by mercury porosimetry range from a few nanometers to a few hundreds of microns. The method is invasive in that not all the mercury will be expelled from the pores and pores may collapse as a result of the high pressures. Due to this and environmental concerns about mercury pollution mercury porosimetry method is becoming less popular. 

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