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from Brown Corpus
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There is another means which should show the direction and relative value of the stresses in viscoelastic fluids that is not mentioned as such in the literature, and that is the shape of the suspended drops of low viscosity fluids in shear fields.
These droplets are distorted by the normal forces just as a balloon would be pulled or pressed out of shape in one's hands.
These droplets appear to be ellipsoids, and it is mathematically convenient to assume that they are.
If they are not ellipsoids, the conclusions will be a reasonable approximation.
The direction of the tension of minimum pressure is, of course, given by the direction of the major axis of the ellipsoids.
Mason and Taylor both show that the major axis of the ellipsoids is at 45-degrees at low rates of shear and that it approaches the direction of shear with increased rates of shear.
( Some suspensions break up before they are near to the direction of shear, and some become asymptotic to it without breakup.
) This is, of course, a similar type of behavior to that indicated by birefringence studies.
The relative forces can be calculated from the various radii of curvature if we assume: ( A ) The surface tension is uniform on the surface of the drop.
( B ) That because of the low viscosity of the fluid, the internal pressure is the same in all directions.
( C ) The kinetic effects are negligible.
( D ) Since the shape of the drop conforms to the force field, it does not appreciably affect the distribution of forces in the fluid.

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