The effect of surface tension in capillary viscometry of biological materials

Article type: Research Article

Authors: McMillan, Donald E.

Affiliations: Sansum Medical Research Foundation, 2219 Bath Street, P.O. Drawer LL, Santa Barbara, California 93102

Abstract: Surface tension has a measurable effect on the rate of flow in capillary viscometers of aqueous solutions of biological materials. Changing the surface tension of saline from 70 to 33 dyn/cm by adding sodium dodecyl sulfate reduces its flow time in Cannon-Ubbelohde semi-micro viscometers by 0.6 per cent. High surface tension not only increases flow time, but it decreases measurement reproducibility by producing variable drainage patterns. Addition of detergent to saline improves reproducibility of measurement so that calibration can be carried out more accurately and far more rapidly. Frequent calibration appears necessary in regular use when removal of biologic materials from the viscometer requires strong acid and/or alkali. Analysis of surface tension effect in the Ubbelohde viscometer demonstrates three components: (1) a reduction in effective fluid column height due to the bulb meniscus, (2) an upward force due to concavity of the meniscus, and (3) a downward force produced by concavity of the fluid surface in the spherical area below the capillary. Estimations of the magnitude of these three components were in reasonable agreement with observed effects and demonstrated the importance of surface effects in the fiducial line and upper viscometer bulb areas. A method for adjusting solvent flow time to the surface tension of other solutions is described; such an adjustment is more appropriate for Ubbelohde than for Ostwald viscometers. Careful surface tension correction appears particularly important when developing derived viscometric values such as reduced or intrinsic viscosity.

DOI: 10.3233/BIR-1975-12103

Journal: Biorheology, vol. 12, no. 1, pp. 11-19, 1975