Viscosity – The Physics Hypertextbook
A shear stress, often denoted by τ (Greek: tau), is the component of stress coplanar with a . On the other hand, given a shear stress as function of the flow velocity, . This relationship can be exploited to measure the wall shear stress. viscosity = shear stress/shear rate ; the SI units are Pascal seconds zero and infinite shear strain rate respectively, and τ is a shear-dependent time constant that The equation is one of a number of empirical relationships that can be used. The relationship between viscosity and shear stress rate. Source publication Reconstruction images (MR fluids under a magnetic field in different times).
This includes all kinds of actions: The influence further depends on the strength and on the duration of the external force. These parameters determine under which conditions a substance flows and which type of flow it develops. To measure viscosity, laminar flow is required. Inner molecular structure, outside forces acting on the material, and current ambient conditions Figure 8. In laminar flow, molecules move in orderly layers, while in turbulent flow they follow no pattern.
Laminar or turbulent flow Laminar flow means that the substance moves in imaginary thin layers. Molecules do not move from one layer to another, their movement forms a regular pattern.
Turbulent flow is not structured because molecules move at random. This leads to eddies and vortices and causes erroneous results during measurement. For example, submitting a fluid to a too high shear rate during the test can result in turbulent flow. That could happen if a glass capillary viscometer is too wide for the tested substance i. This inversely proportional relation applies to all substances. Any change in temperature always influences viscosity, but for different fluids, the size of this influence varies.
Pressure has less influence on viscosity than temperature. However, fluids are not dramatically affected if the applied pressure is low or medium: Most liquids react to a significantly altered pressure from 0.
In case the pressure goes up from 0. This applies to most low-molecular liquids. Highly viscous mineral oils react with a viscosity increase of times under identical circumstances.
For synthetic oil this pressure change can even result in a viscosity increase by a factor of up to 8 million. For example, lubricants in cogwheels or gears can be submitted to pressures of 1 GPa and higher. Conversion of pressure units. In most liquids, pressure reduces the free volume in the internal structure, and thus limits the movability of molecules.
Consequently, internal friction and viscosity increase. How does water behave under pressure? Another anomaly affects the flow behavior of water under pressure. The pressure destroys the structure of the three-dimensional network of hydrogen bridges. How to derive dynamic viscosity. SI International System of Units  units: As density is defined as mass per volume, gravity comes into the equation via the quantity mass. Kinematic viscosity is dynamic viscosity divided by density. Gravitational influence is introduced through the mass, which is contained in density.
Other commonly used units: Relative viscosity Relative viscosity is a vital parameter when measuring dissolved polymers . This response is thixotropy. Structure in the material breaks down over time and needs time to recover.
The non-Newtonian behavior, in these cases, is useful—the viscosity decreases as the materials are rubbed on the skin, for example, easing application. Stopping the rubbing, and ceasing the shearing, allows the viscosity to increase, helping the ointment or cream to stay on the area. One example is the use of cone-plate geometry to test a sample at one speed, waiting a few minutes before taking the data point.
Shearing the material for some time helps to break down the structure; the viscosity readings decrease and then plateau after some time. Therefore, more consistent readings may be taken from sample to sample, after a suitable shearing time. Representative data are shown for the ointment in Figure 5. The data show that the viscosity plateaus within about 2 minutes for this material. Therefore, an appropriate, single-point QC test may be to shear each ointment sample at 1 rpm for 2 minutes before recording a reading.
Figure 5 — Thixotropy: Conclusion Shear rate is important because it may significantly affect the viscosity and therefore the processability and applicability of various materials.
Basics of viscometry :: Anton Paar Wiki
In the example given here, higher shear rates during rubbing result in lower viscosities and thereby allow ointments and creams to be more easily spread by the user. Viscosity measurements at various shear rates help quantify similarities or differences between various pharmaceutical ointments and creams, for example. The cone-plate rheometer provides a convenient tool for testing small amounts of pharmaceutical products at different shear rates. Proper equipment selection may allow significantly different materials to be tested with one instrument and geometry, merely by changing the testing speed or shear rate.
This dependency means that more extended molecules increase the viscosity to greater extents at low concentrations than more compact molecules of similar molecular weight. Generally, less-flexible links between sequential monomers in the polymeric chains give rise to more extended structures but the linkage spacing, direction and charge density are all important factors.relation between bending moment ,shear force and intensity of load
Where residues are negatively charged, the repulsion between similar charges increases molecular extension but this can be reduced at higher ionic strength or below the pKa's of the anionic groups and this reduction is particularly noticeable for polymers with high molecular mass. The lack of much change in viscosity of such molecules with ionic strength is indicative of an inflexible rod-type conformation.
It should be noted that although attaching short sugar units as branch-points to linear polysaccharides does increase their rigidity into an extended structure, this is at the cost of greatly increased molecular mass.
The extended nature of the molecules has an extreme effect on the molecular mass dependency of the viscosity. This is as the hydrodynamic volumes and hence viscosities of compact highly flexible but poorly hydrated molecules increase approximately as the cube root of their molecular mass, whereas those of more-extended well-hydrated molecules such as alginate and xanthan gum increase approximately linearly with molecular mass.
Amylosecarboxymethylcellulosearabinoxylansand guar all have exponents a of about 0.
What is Shear Rate and Why is it Important?
Knowledge of these constants allows the viscosity-averaged molecular mass to be calculated from viscosity data. The viscosity increases with concentration until the shape of the volume occupied by these molecules becomes elongated under stress causing some overlap between molecules and a consequent reduction in the overall molecular volume with the resultant effect of reducing the amount that viscosity increases with concentration under stress.
At higher concentrations, the viscosity increases up to about the fifth power of the concentration, and this can cause apparently synergic behavior of hydrocolloid mixtures, particularly if they cause phase separation with its inherent concentration increases. At high shear strain rate and sufficient concentrationmolecules may become more ordered and elastic.
After release from such conditions, the molecules relax back with time the relaxation time.
As G'' depends on the frequency but G' depends on the square of the frequency, G' becomes more important at higher frequencies. At higher concentrations in viscous solutions, G' is generally greater than G'' throughout a wide frequency range.