�ber den Mechanismus der Oberfl�cheninstabilit�t viskosimetrischer Str�mungen

Abstract

Several publications describing the measurement of rheological properties of non-Newtonian liquids have reported a surface-instability phenomenon, which places an upper limit on the useable shear rates. The disturbances grow with time and slowly extend into the inner part of the flow field. A semi-empirical criterion for the onset of the instability is also given in the literature. It involves the first normal-stress coefficient of the liquid. This paper, on the other hand, shows the importance of the second normal-stress coefficient with its negative sign as a possible cause of the instability.