There were a number of elements to Extensional Flow apparatus development.
In the beginning there was the Bristol Extensional Flow centro symmmetric apparatus that was inspired by the original thinking of the late Sir Charles Frank. This started with the Double Jet, was followed by the Two Roll Mill, the Four Roll Mill and then the Six Roll Mill. With the exception of the Four Roll Mill, which was the invention of G.I.Taylor in 1934, all the other apparatus were unique to the Bristol stables.
Then there was Oscillatory Flow Mixing (OFM) which had as an important ingredient within the time dependant unsteady flow, strong extensional flow components. These produced stretch and fold motions which enhanced mixing.
Then there was the Cambridge Multipass Rheometer(MPR) and the addition to the apparatus of a Cross Slot geometry. This apparatus enabled high viscosity polymer melts to be tested in a flow geometry similar to the G.I.Taylor four roll mill.
Finally there was the Cambridge Trimaster a filament, stretch, decay and break up device. This apparatus was motivated mainly to characterise low viscosity fluids in an extensional flow that was similar to ink jet printing.