Plastic flowliquidOnly whenstressStart when τ 0 is exceededflow。The flow characteristic curve of plastic flow does not pass through the origin.Plastic flow is divided intoBingham flow And non Bingham mobility.Bingham flow means that when the stress exceeds τ 0, the flow characteristics conform toNewtonian fluidRegular flow.Non Bingham flow refers to flow that does not conform to Newton's flow law.The liquid with these two flow characteristics is called Bingham fluid or non Bingham fluid respectively.Concentrated gravy in food is a typical Bingham fluid.Kasson studied the grid structure andShear rateThe relationship between shear stress and shear rate is as follows: σ 1/2=σ 01/2+η a ε 1/2.
The flow law of some non Bingham liquid food conforms to the Casson formula, such as tomato sauce, chocolate, etc.
Chinese name
Plastic flow
Foreign name
plastic flow
Features
Flow starts when the stress exceeds τ 0
Curve
The flow characteristic curve does not pass through the origin
For fluids with different viscosity, there is a functional relationship between stress and strain rate.Newtonian fluid is a fluid whose viscosity does not change with the change of shear rate, and the shear rate is proportional to the shear stress.The flow state equation of Newtonian fluid is as follows:
τ=ηγ
Wherein, η refers to viscosity, which reflects the flow resistance, represents the proportional coefficient between shear stress and shear rate, γ represents the shear rate, and τ represents the shear stress.For Newtonian fluid, the change of shear rate does not affect its viscosity.The ideal Newtonian fluid is isotropic, incompressible and inelastic.Fluid that basically conforms to Newton's law within a certain range is treated as Newtonian fluid in rheology.For example, water, ordinary honey, oil, wine, liquid sugar, corn syrup, filtered fruit juice, etc. in food are usually analyzed and calculated according to Newtonian fluid because complete Newtonian fluid does not exist in nature.
Non Newtonian flow
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In the nature of food, the majority of fluids do not conform to Newton's fluid law.A large number of foods, including concentrated fruit juice, jam, whole eggs, vegetable puree, concentrated milk, chocolate syrup and other solid liquid suspensions are non-newtonian fluids. The following empirical formula is often used to express the relationship between the shear stress and the shear rate of these fluids:
τ=k(γ)n(1<n<∞，0<n<1)（1）
Where,
N is the flow characteristic index, and k is the consistency coefficient.If it is Newtonian fluid formula, then n=1, where k is viscosity.In the above formula, if η a=k (γ) n-1, thenNon Newtonian fluidThe state equation of can be written as:
τ=ηaγ（2）
From this formula, it can be concluded that η and η a represent the same physical properties and have the same dimensions, that is, η a is the apparent viscosity.The apparent viscosity η a is the sum of the internal resistance of the fluid.However, unlike η, η a is a function of γ, which is related to k and n.In other words, η a refers to the viscosity of non Newtonian fluid at a certain flow rate.
For many non Newtonian fluids, T can flow only when T is greater than a certain value T 0 (that is, after the fluid obtains energy to overcome a yield stress value).The expression formula proposed by Bulkey and Hershel is as follows:
Τ=Τ0+k(γ)n（3）
0 indicates yield stress.Due to the different ranges of τ 0 and n in the formula, the non Newtonian flow is divided into the following five categories:
Pseudo plastic flow
When the viscosity decreases with the increase of shear rate or shear stress, the flow corresponding to 0<n<1 in formula (1) is called pseudoplastic flow.This flow is also called shear thinning flow, that is, the viscosity decreases due to the increase of flow velocity.
Pseudoplastic fluidIt conforms to the law of pseudoplastic flow, and most liquid foods are pseudoplastic liquids.grossNon Newtonian fluidAll are pseudoplastic fluids.Most foods with pseudoplasticity have macromolecular colloidal particles composed of huge chain molecules. At low flow rate or at rest, they are intertwined with each other and have high viscosity, so they appear viscous.However, when the flow rate increases, these scattered chain particles will be affected by the shear stress between the flow layers, reducing their hooking, and will roll and rotate to shrink into clusters, which is a phenomenon of shear thinning.For most fluids, the shear thinning phenomenon is reversible, although it will lag for a little time.The initial high viscosity state of the fluid will recover when the shear rate decreases or even stops shearing.From a microscopic point of view, Brownian motion makes the aggregate re form, that is, the chain like colloidal molecules return to their unoriented natural position, and the deformed droplets return to spherical shape.The processing behavior is directly affected by the pseudoplasticity of polymer fluid.Such as vegetable soup, soy sauce, concentrated sugar water, tomato juice, apple jam, starch paste and other polymer solutions, emulsions and suspensions are pseudoplastic fluids[1]。
Dilatant flow
The flow whose viscosity increases with the increase of shear rate is also called shear thickening flow.In the formula τ=k (γ) n (1<n<∞, 0<n<1), if 1<n<∞, it is called dilatational plastic flow.A fluid that exhibits dilatational plastic flow is calledExpanding plastic fluid。There are not many swelling plastic fluids in food materials, but raw corn starch paste is a typical one.When water is added to the starch and mixed into a paste, slowly tilt the container, and the starch paste will flow like a liquid.However, if a greater shear stress is applied, such as stirring the starch vigorously and quickly, the starch will become "hard" and lose its flowing property.If the chopsticks are stirred quickly, even the resistance will break the chopsticks.
Plastic flow
The liquid only starts to flow when the stress exceeds τ 0.The flow characteristic curve of plastic flow does not pass through the origin.Bingham flow refers to the flow whose flow characteristics conform to Newtonian fluid law when the stress exceeds τ 0.Non Bingham flow refers to flow that does not conform to Newton's flow law.The liquid with these two flow characteristics is called Bingham fluid or non Bingham fluid respectively.Concentrated gravy in food is a typical Bingham fluid.After studying the relationship between the grid structure of paint flow and the shear rate, Kasson found that the shear stress and the shear rate have the following relationship:
σ1/2=σ01/2+ηaε1/2
The flow law of some non Bingham liquid food conforms to the Casson formula, such as tomato sauce, chocolate, etc.
Thixotropy
Thixotropy refers to the phenomenon that when the liquid is vibrated, stirred and shaken, the fluidity of the liquid increases and the viscosity decreases. After standing, it becomes difficult to flow after a period of time.Also called shake soluble flow.For example, tomato ketchup, mayonnaise, etc., when placed in containers for a long time, become difficult to flow when dumped.But as long as the container is shaken violently or stirred vigorously for a while, they become very easy to flow.If they are placed for a longer time, they will become difficult to flow.The occurrence of thixotropic flow is due to the combination structure formed between particles, which is destroyed with the increase of shear stress, resulting in the reduction of viscosity.However, when these inter particle bonding structures stop the stress action, it takes a period of time to recover and gradually form.Therefore, the curve when the shear rate slows down is below the curve when the shear rate increased last time, forming a hysteresis loop related to the flow time.The greater the structural damage of the material, the larger the surrounding area of the hysteresis loop.The impact of thixotropy on the taste is reflected in the refreshing and soft feeling.
Rheopexy flow
As the flow time increases, the liquid becomes more and more viscous in contrast to the thixotropic flow.Gelatinized flowing food gives people a sticky taste.When the flow rate increases and reaches the maximum value, the flow rate will be reduced, and the flow curve when the flow rate decreases will be above the curve when the flow rate increases.This phenomenon is also called reverse thixotropy.This is because the flow promotes the formation of the structure between liquid particles.
Constitutive model
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Guo Weiguo established a plastic flow constitutive model, systematically tested five typical aluminum alloys, Al3003-H12, 2219-T87, 7050-T7451, 2024-T351 and LY12-cz, under the conditions of strain rate from 10 − 4/s to 8000/s, initial temperature from 77K to 800K and real strain exceeding 0.50, and analyzed the plastic flow behavior.The results show that these aluminum alloys have strain rate effect;The strain rate sensitivity of aluminum alloy can be attributed to the effect of short-range barrier on thermally activated dislocation motion;At 200~600K, these materials have the third typeDynamic strain agingPhenomenon.Based on the mechanism of thermally activated dislocation motion, a constitutive model of physical concepts is proposed. The predicted results of the model are consistent with the experimental results, which can be easily used in engineering applications[2]。
