(1) Plastic fluid
Plastic fluid is also called Bingham fluid, and its flow curve is a straight line that does not pass through the origin. When the shear stress is less than σy , the liquid does not flow, and the apparent viscosity is equivalent to infinity. Only when the shear stress is greater than σy , can the flow be initiated, and with the increase of the shear stress or shear rate, The apparent viscosity gradually decreases. σy is called the yield stress. For plastic fluids, the following formula can be used to describe its characteristics:
In the formula, ηp is the slope of the straight line, which is called plastic viscosity or structural viscosity.
For a dispersed system with a continuous network structure inside, a small shear stress cannot destroy this structure, so gravity generally does not trigger flow, and flow can only start after a certain shear stress is reached. When the concentration of the suspension reaches the point of contact with each other, a three-dimensional space structure can be formed, and it becomes a plastic fluid. The interaction of the particles in the network structure reflects the strength of this structure.
Ink, paint, toothpaste, etc. are all plastic bodies. For paints with organic solvents, in order to prevent flocculation during storage and prevent sagging after construction, the yield value should be greater than 0.40Pa; while the yield value of latex paint before construction should be greater than 1.0Pa. Immediately drop below 2.5Pa, and need to return to above 5.0Pa after construction.
(2) Pseudoplastic fluid
Flow curves for pseudoplastic fluids. At low shear rate, its flow curve is similar to Newtonian fluid; at higher shear rate, the tangent line can intersect the longitudinal axis at σ' y , which is similar to plastic fluid, so it is called pseudoplastic fluid. Its apparent viscosity gradually decreases with the increase of shear rate. The relationship between shear stress and shear rate can be expressed as:
In the formula, K and n are empirical constants related to the properties of the liquid, K is the consistency, the larger the K value, the thicker the liquid, n is the non-Newtonian index, and the more the n value deviates from 1, the more significant the non-Newtonian behavior is. Then the apparent viscosity plus η a can be expressed as:
There may be particle aggregation in this kind of fluid system, which can be destroyed when the shear rate is increased, and at the same time, the asymmetric particle of the system can be changed from random orientation to directional arrangement, both of which can reduce the apparent viscosity .
Most polymer melts, polymer solutions and emulsions are pseudoplastic fluids. Under the action of shear force, the polymer chains in the polymer melt and polymer solution are oriented, which reduces the entanglement between the chains, and the viscosity of the system decreases accordingly.
(3) dilatant
Contrary to the pseudoplastic fluid, the apparent viscosity of the dilatant fluid increases gradually with the increase of the shear rate, and the relationship between the shear stress and the shear rate can also be expressed by the above formula, but n>1.
The concentration of the dispersed phase of the dilatant is quite large and limited to a narrow range, about 42%~45%. The particle particles are dispersed. When the shear stress is increased, the dispersed particles will be stirred together to increase the flow resistance. , increasing the viscosity of the fluid.
Starch suspensions, suspensions of inorganic solid powders in organic solvents, etc. all have the property of dilatant, and the ground paint in coatings is also often formulated as dilatant.
When the dilatant is in a static state, the particles are dispersed in the medium and the interaction is small. When agitation is applied, the probability of collision between particles increases. If the interaction between particles is strong, it will cause the rearrangement of particles and form a flocculation structure, which surrounds the dispersion medium in the suspension, and the free-flowing liquid Decrease, the resistance increases, so that the apparent viscosity increases. The changes in the structure of the expansive substance before and after stirring are shown in the figure.
The dilatant generally needs to meet the following two conditions: one is the concentration. The system is not dilatant in all concentration ranges. For example, starch paste exhibits obvious dilatant properties in the concentration range of about 40% to 50%. When the concentration is low, it is a Newtonian fluid; when the concentration is high, it is a plastic fluid. Because the concentration of the dispersed phase is too small, the distance between the particles is too far, and it is difficult to form the structure shown in the figure above when stirring; and when the concentration is too high, the distance between the particles is too close to contact, and the original structure will be dismantled when stirring. A plastic or pseudoplastic fluid form will appear. The minimum concentration required for the system to exhibit dilatation is called the critical concentration, which is related to factors such as the shape and size of the particles and the properties of the dispersion medium. The more asymmetric particle shape, the lower the critical concentration for dilatation. The second is dispersion. Particles need to be dispersed in the system without agglomeration, so it is often necessary to add a dispersant, and the nature and quantity of the dispersant affect its critical concentration.
