Lubricated Squeezing Flow: A New Biaxial Extensional Rheometer

Squeezing flow between two disks with lubricated surfaces was found to generate a homogeneous compression or equal biaxial extension in a high viscosity polydimethylsiloxane sample. The apparatus is extremely simple: two steel disks with a central rod to provide alignment and prevent sample slip, an LVDT to measure displacement, and a silicone oil bath. The mass and area of the upper disk provide for a constant force boundary condition. The biaxial viscosity was found to be approximately six times the shear viscosity over biaxial extension rates ε̇r from 0.003 to 1.0 s−1. Lubrication could be achieved up to Hencky strains of about 2.5. Some data were also taken on the same polyisobutylene sample used by Stephenson and Meissner in their biaxial stretching study. Agreement was very good.