The free-fall of viscoplastic drops

The free-fall of liquid drops under the action of gravity is studied experimentally by high-speed imaging, with focus on drops of viscoplastic fluids (i.e., those materials which exhibit a fluid behaviour only when the applied stress exceeds a certain threshold value, called the yield stress), and compared to the behaviour observed in Newtonian drops. The yield stress of the fluid is shown to alter significantly the drop shape during free-fall. The results can be interpreted in terms of a dimensionless number comparing the yield stress magnitude and the capillary pressure.