Shear stress - Wikipedia
Are all materials either a solid or a fluid, all the time? Applied heat STRESS. SHEAR. STRESS. Stress: is the force acting on a surface, per unit area – may be A viscous fluid is defined by the relationship of stress to strain rate. L σ = 2με. Shear rate is the rate at which a fluid is sheared or “worked” during flow. The viscosity, η, is the relationship between the shear stress and the shear rate. the viscosity readings decrease and then plateau after some time. It is the time rate of change of strain. And we can find shear strain by the relation as τ =Gγ. But generally shear strain and shear strain rate is used in the.
This, in turn, must be considered when designing pumping, mixing, and spraying systems. It is important in designing various materials for consumer use, whether the products are pharmaceutical creams, ointments, or other.Viscosity and Shear Stress 1 - Fluid Mechanics - LetThereBeMath -
A high viscosity may help an ointment stay in place once it is applied, while a lower viscosity may help a cream spread more easily. The pharmaceutical industry manufactures and tests a wide variety of commercially important products. Material behavior is often non-Newtonian.
To reduce costs, many users try to test smaller quantities of materials. Cone-plate or cone-and-plate geometry may be used in this case, because it provides welldefined shear rates and typically requires small sample amounts. Figure 3 shows a cone-plate rheometer.
Figure 3 — Left: This particular geometry requires only 0. All experiments were run at conditions ensuring on-scale readings.
What is Shear Rate and Why is it Important?
Figure 4 presents data for the above samples. The cream data are in red, while the ointment data are in blue. The apparent viscosity of each sample decreases greatly with increasing shear rate.
Viscosity increases as the shear rate decreases again, but to values less than those measured in the initial, increasing-rate ramp. 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. In this case, the value of the exponent n will be greater than one.
Viscosity – The Physics Hypertextbook
Again, noticeable deviations can be expected when the Oswald equation is applied over a wider range of shear rates. Some other fluids require a threshold shear stress before they start to flow. This kind of fluid is called a plastic fluid and if the flowing liquid has a constant viscosity it is called a Bingham liquid. However, such behavior is not observed in ordinary polymer melts and solutions. The solid-like behavior at low shear stress can be explained by the formation of a silica network structure arising from attractive particle-particle interactions due to hydrogen bonding between silanol groups.
Using the standard definition for viscosity: Summary Viscosity is a measure of the internal friction of a fluid, or in other words, it is a measure for the resistance to shear flow. The viscosity of a polymer is always larger than that of the corresponding monomer and increases rapidly with increasing molecular weight number of repeat units.
Flow Properties of Polymers
This is due to entanglement and intermolecular forces between polymer chains. Measurements of the viscosity of polymer solutions and melts can provide information about the average molecular weight, and the molecular weight distribution. Rheological measurements provide important information about the flow and creep properties of polymeric materials. This information might aid in optimizing the processing conditions and the composition of plastics. Rheological measurements are helpful to understand the effect of aging on the viscoelastic properties.
They also help to assess the damage caused by heat, oxygen etc.