Magnet enhanced optical falling needle/sphere rheometer

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Abstract
A magnet enhanced falling needle/sphere rheometer equipped with a precision optical monitor and signal feedback system has been developed. The instrument which utilized the basic features of a magnetic sphere/needle rheometer could be controlled using a PC/AT(286) computer. The important advantages of this instrument over existing falling needle/sphere viscometers are as follows: (1) An extremely large viscosity range from the usual solvent viscosity (say, ∼1.0 cP) to that of polymer melts (say, 1010 cP) is available with an experimental precision of ≲1%. (2) The measurement time could be reduced by about 4 orders of magnitude (e.g., from hours to seconds) for a high viscosity fluid. (3) One single needle/sphere could cover a few orders of the viscosity range and two needles with appropriate densities could cover the whole viscosity range (1 cP≲η≲1010 cP). (4) Only a small amount (∼4 mL with the present setup) of sample is needed. (5) The sample chamber can be sealed hermetically and is suitable for toxic fluids. (6) The instrument permits measurements of the shear rate dependence of viscosity using a single needle/sphere. (7) For nonNewtonian fluids, the viscoelastic behavior could be measured. A set of viscosity standards was used to calibrate the device. Linear calibration curves of viscosity against the reciprocal of the falling needle velocity under both natural gravity field and fixed artificial magnetic fields were obtained. A flow curve was determined for a random copolymer solution using a single needle but varying the magnetic field, showing the applicability of the device for measuring the rheological properties of polymer solutions and melts.

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