At water-like viscosities, the bulk liquid of the sample rotates in phase with the shaker, its centre of mass pointing in the direction of centrifugal acceleration (Fig. Simultaneously, a thin liquid film adheres to the inner flask wall, if the liquid is aqueous and the wall is a hydrophilic material such as glass 39, 40, 41. The translational orbital movement of the sample vessel on a circular path causes the bulk liquid of the sample to rotate due to its inertia 19. Although other geometries (such as cylinders) are suitable, we chose Erlenmeyer flasks as our standard sample vessels because they are easy to handle and well characterized 35, 36, 37, 38, 39. The measurement principle is based on detecting the angular position of a liquid sample in an orbitally shaken vessel relative to the direction of centrifugal acceleration. Here we describe a new method for the quantitative determination of viscosity (η) which addresses the limitations of current viscometers. Liquid distribution as a function of viscosity They also have limitations such as inaccurate measurement of the true sample radius 18, 29, hydrodynamic instability 30, 31, low torque limit 32, 33 and wall slip behaviour 34. These advanced rotation and oscillation rheometers are more expensive and more sensitive than the simpler pressure-driven and drag flow viscometers because they contain components such as air bearings. Cone-plate systems are versatile but they are unsuitable for fluids containing dispersed particles larger than the gap size 17. The gap height in the centre depends on the angle and the radius, and is in the range 50–200 µm 17. Typically, cones with a radius of 10–30 mm are used with a 0.3–6° gap angle 28. ![]() Instead of a rotating plate, cone-plate systems use a truncated cone as a rotating element, resulting in a conical gap and uniform shearing of the sample over the conic radius 19. The plate-plate system consists of fixed and rotating plates with an adjustable gap and can be used with particulate and high-viscosity systems 17. Typically, concentric cylinders are used for low-viscosity or particulate systems and are difficult to clean 19, 27. Concentric cylinders consist of a measuring cup and a bob with a narrow gap, and can be defined as Couette type (outer part rotates) or Searly type (inner part rotates) 18. Absolute measuring systems include rotation and oscillation rheometers that use concentric cylinders, plate-plate or cone-plate systems to determine the flow behaviour of Newtonian and non-Newtonian fluids. All the pressure-driven and drag flow viscometers described above are relative measuring systems. The calculation of parameters such as shear stress, shear rate and viscosity is not recommended in relative measuring systems, but raw data such as torque or speed can be used instead 19. The former have a standard “two plates” geometry and defined shear conditions in a relatively narrow shearing gap, whereas the latter do not meet the conditions of the two-plates model 17. ![]() ![]() Viscometers can also be classed as absolute or relative measuring systems. Mixer-type rheometers with various rotating stirrers are particularly suitable for the analysis of building materials and foodstuffs containing large dispersed particles, e.g. Different geometries are available (cylinders, disks, cones, pins and T-bars) and such devices are often used for quality assurance 22, 23, 24. Spindles are immersed in the sample and rotated at a specified rate to measure the resulting torque 19. Drag flow viscometers include spindles and mixer-type rheometers. Falling sphere viscometers additionally require transparent samples, unless variants based on induction or electromagnetic fields are available 19, 20, 21. In all three cases, flow is induced by gravitational force and measurement is dependent on weight and density, and can only be determined for Newtonian fluids with low viscosity 3. Typical pressure-driven viscometers include flow cups, falling sphere viscometers and glass capillary viscometers. The most common are pressure-driven and drag flow viscometers 16. ![]() Several devices have been developed to measure the viscous, viscoelastic and elastic properties of liquids and solids 17, 18, 19. The rheological behaviour of viscous liquids is relevant in the food 1, 2, 3, 4, pharmaceutical 5, 6, 7, 8, 9 and cosmetic 10, 11, 12 industries, as well as technical chemistry 13, 14, 15 and production engineering 16.
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