Abstract
This chapter describes a low shear falling ball viscometer that can be used to measure consistency changes in cellular extracts and the gelation of actin filament networks. It is particularly useful for routine assays of actin filament interactions during the purification of molecules that either promote or inhibit the network formation. Although this device cannot yet be used for rigorous quantitative analysis, the assay does have a number of practical advantages over other routine assays. The falling ball apparatus consists of a capillary tube containing the sample and a steel ball that will fall through the sample, provided that the yield strength of the material is low enough. The force balance in the system and Stokes' equation yield an expression relating the observed velocity of the ball to the absolute viscosity of Newtonian samples and the apparent viscosity of non-Newtonian samples. The ball centrifugation assay is a simple approach to measuring yield strength. Its major limitations are some uncertainty about the value of the geometrical constant and the crude, stepwise application of force by centrifugation. A better method would be to observe the ball continuously as the centrifugal force is gradually increased.
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