Gravity filtration as a quantitative viscometric technique

Abstract

Gravity filtration tests of human blood have been carried out using a clear cylindrical tube standing vertically on a single filter element consisting of a glass capillary array with cylindrical pores 50 μm in diameter and 2 mm in length. Flow rates are controlled by means of a Parafilm template cut out to permit flow only through the desired number of pores. This experimental setup produces minimum wall shear stresses of the order of 1 dyne/cm 2 and offers a simple procedure which yields experimental data of column height vs time with extremely small scatter. Because of the very uniform cylindrical pores in the glass array, these data lend themselves to the analysis of Metzner and Reed for very general non-Newtonian fluids. Mathematical expressions are, therefore, derived for apparent viscosity as a function of strain rate in terms of the experimental measurements of column height vs time. The experimental data are closely matched by the Casson equation ( r 2 = 0.998) with a yield stress of about 0.02 dyne/cm 2 for normal human blood at 35% hematocrit. Agreement with rotary viscometer data for the same sample is good when the correction of Barbee and Cokelet is used to determine the hematocrit in each pore of the capillary array. It appears that gravitational viscometry using glass capillary arrays offers a simple, inexpensive test with results for blood which are comparable to those of rotary viscometers under more realistic flow conditions.