“…Classical viscometers, such as falling ball [ 6 ] (Hoeppler), cone-and-plate [ 7 ], rotating disc [ 8 ], and U-tube capillaries (e.g., Ostwald [ 9 ] or Ubbelohde [ 10 ]), despite some drawbacks (repetitive tests are required, large volume consumptions, and complex cleaning procedure [ 11 ]), are still used to measure the various fluids in the medical and biological fields. Moreover, optical technology also enables viscosity measurements with the use of optical tweezers [ 12 ], photoacoustic [ 13 ], and fluorescence [ 14 ] methods.…”