Vortex viscosity, magnetic fluids

D. Antisymmetric Stresses, Internal Spin Fields, and Vortex Viscosity in Magnetic Fluids... 

These suspension viscosity concepts are of growing technological importance in rationalizing and quantifying the behavior and properties of so-called magnetic fluids (Rosensweig, 1982, 1985, 1987). In a novel proposal, Brenner (1984) outlined a potentially useful scheme to use the apparently rigid-body rotation of a dipolar suspension to measure its vortex viscosity... 

Since, in certain applications of glyoxal-treated xanthan guin, it is desirable to delay the development of viscosity (hydration delay), a simple test was developed to monitor this behavior (Figure 8). In the hydration delay test discussed, the gum is added to the vortex (created by a magnetic stirrer) of the hydration fluid (usually synthetic tap water) and the time that is required for the vortex to dlsappeeu Isecause of viscosity development is called the hydration delay time. 

In the original method (39), 2 g of polymer sample is mixed with 50 mL of the desired test fluid, which is stirred by means of a magnetic stirring bar in a small beaker. As the fluid absorption proceeds, the viscosity of the suspension increases until the stirring vortex disappears at time ty The volume fraction of swollen polymer in the suspension when the vortex disappears was estimated to be equal to 0.52 (42). For the most accurate and comparable results across samples with different swelling capacities, the mass of polymer added must be adjusted based on the value of the specific absorbency. As the specific absorbency increases, less polymer is used in the test so that each sample reaches the same relative swelling extent at time ty (43). 

A solution of 0.9% sodium chloride in water was used as the swelling fluid. The experimental set up consisted of a 1000 mL beaker into which was suspended the 1 spindle of a Brookfield LVT Viscometer. Seven hundred fifty milliliters of 0.9% NaCI solution was put into the beaker and stirred at a constant speed with a magnetic stirrer. The viscometer spindle was offset in the beaker so that the reading was not affected by the stirring vortex. The viscometer was started and a zero time reading obtained. Then 7.50 g of polymer powder was added to the stirring solution, and the timer was started. The viscosity was determined at various times until no further increase was apparent. 

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