VISCOMETRY 



The Einstein equation relates the relative viscosity of a sus- 

 pension of spheres to the volume concentration. Thus, a viscosity 

 measurement on a dilute suspension of spheres would constitute a 

 means of determining the volume concentration of the suspended 

 matter. There are circumstances vmder which this information can 

 be of great value. For example, the water content of a pure prepa- 

 ration of particles of influenza virus can be determined by viscometry. 

 Electron micrographs show that the particles of this virus are spheres. 

 Therefore the total volume occupied by the particles (plus enclosed 

 or bound water) can be determined by measuring the viscosity. The 

 amount of solid matter can be determined by chemical analysis. Thus 

 the total volume in suspension associated with unit weight of dry matter 

 can be calculated; and, if the density of the dry matter is known, the 

 percentage by weight or by volume of water in the virus particle in 

 suspension can be calculated. 



This problem can be viewed in a slightly different manner. 

 The specific viscosity of the suspension of virus particles would be equal 

 to 2.5 times the true volume fraction of the particles. If these particles 

 were composed of 80% by volume of water, the volume fraction of the 

 solid matter would be only one-fifth of the true volume fraction. Thus 

 the specific viscosity would be equal to 5 X 2.5 or 12.5 times the volume 

 fraction of solids. Therefore, for such a suspension, the intrinsic vis- 

 cosity would be equal to 12.5, when concentration is expressed as 

 volume fraction of solid matter. It is not common to find biological 

 materials composed of very much more than 80% by volume of water. 

 Hence, one should not expect intrinsic viscosities of spherical biological 

 materials to have values much greater than 12.5. Yet many biological 

 systems have much higher intrinsic viscosities. For example, tobacco 

 mosaic virus has a minimum intrinsic viscosity of about 39, and values 

 two or more times as great are often observed. Under some circum- 

 stances, solutions of tobacco mosaic virus nucleic acid have an intrinsic 

 viscosity of more than 60, and thymus nucleohistone has an intrinsic 

 viscosity of about 82 (16). The interpretation of such high values 

 must be sought in an extension of the theory of Einstein. Today, it 

 is generally believed that the high intrinsic viscosities are caused by 

 extreme departure of the colloidal particles from the spherical shape. 



This question was first considered by Jeffery. He extended the 

 treatment of Einstein to inchide rodlike and platelike ellipsoids of 



