Viscoelastic Free Surface Instabilities During Exponential Stretching

A viscous Newtonian fluid and two model polystyrene Boger fluids are used to investigate the evolution of free-surface instabilities during the debonding of two cylindrical parallel rigid surfaces. A filament stretching rheometer is used to separate the surfaces at an exponential rate and simultaneously measure the evolution in the axial force. The experimental configuration also allows for simultaneous imaging of the fluid sample from beneath the endplate during the test. The test method is similar to both the probe-tack test used in adhesive testing and filament stretching rheometry experiments. Several geometric aspect ratios (defined as the ratio of initial sample height to radius) are used to vary the degree of sample confinement. These geometries bridge the gap between conventional adhesive testing and extensional rheometry. Three types of instabilities are seen in the experiments. The first is similar to the classical Saffinan-Taylor instability, in which fingers of a less viscous fluid penetrate into a more viscous one during pressure driven flow. In the present three-dimensional analog, the instability occurs at a critical rate of plate separation for a given geometry and fluid. Larger pressure gradients result from higher fluid viscosity, a more confined geometry and greater rate of plate separation. A second instability is caused by the extensional stress growth in the elastic fluids and occurs at a critical Hencky strain for a given Deborah number. This instability is characterized by the growth of fingers radially outward from the base of a stable, cylindrical fluid column. As this instability progresses, the fingers can repeatedly bifurcate resulting in complex surface morphologies. Finally, cavitation in the test fluids occurs below a critical negative gage pressure within the fluid. Modal interactions of the three instabilities are also observed and lead to complex evolution of the elastic instability. Thesis Supervisor: Gareth H. McKinley Title: Professor of Mechanical Engineering