What are non-Newtonian fluids?

Short Answer:

Non-Newtonian fluids are those fluids that do not obey Newton’s law of viscosity. In such fluids, the shear stress is not directly proportional to the rate of shear strain (velocity gradient). This means that their viscosity changes with the rate of shear or deformation.

In simple words, the viscosity of a non-Newtonian fluid is not constant; it may increase or decrease depending on how fast it is stirred or moved. Examples of non-Newtonian fluids include toothpaste, blood, paints, ketchup, and mud, which behave differently under different flow conditions.

Detailed Explanation :

Non-Newtonian Fluids

Non-Newtonian fluids are a class of fluids that do not follow Newton’s law of viscosity, which states that shear stress (τ) is directly proportional to the velocity gradient (du/dy) for a constant viscosity (μ). In non-Newtonian fluids, this linear relationship does not hold true because their viscosity is dependent on the rate of shear strain or shear stress.

Mathematically, Newton’s law is expressed as:

For Newtonian fluids, μ (viscosity) is constant.
However, for non-Newtonian fluids, μ varies depending on shear rate, temperature, or time.

These fluids show complex flow behavior and are commonly found in natural and industrial applications. They are very important in industries like food processing, pharmaceuticals, cosmetics, and petroleum engineering.

Characteristics of Non-Newtonian Fluids

  1. Variable Viscosity:
    The viscosity of non-Newtonian fluids changes with the rate of shear strain. It can either increase (thickening) or decrease (thinning).
  2. Non-linear Relationship:
    The relationship between shear stress and velocity gradient is non-linear. The graph of τ vs. du/dy is a curve instead of a straight line.
  3. Time Dependence:
    Some non-Newtonian fluids show time-dependent behavior — their viscosity changes with time when subjected to continuous shear.
  4. Complex Molecular Structure:
    These fluids generally consist of large, complex molecules (polymers, colloids, suspensions) that rearrange under stress.
  5. Cannot be Described by a Single Constant:
    Unlike Newtonian fluids, their behavior cannot be defined by a single viscosity value; mathematical models are used instead.

Types of Non-Newtonian Fluids

Non-Newtonian fluids can be classified based on how their viscosity changes with shear rate and time. The major types are:

  1. Shear-Thinning (Pseudoplastic) Fluids:
    In these fluids, viscosity decreases with an increase in shear rate. The fluid becomes thinner when stirred or agitated.

    • Examples: Paints, blood, polymer solutions, ketchup.
    • Explanation: When shear is applied, molecular chains align in the direction of flow, reducing resistance.
  2. Shear-Thickening (Dilatant) Fluids:
    In these fluids, viscosity increases with an increase in shear rate. The fluid becomes thicker when stirred or shaken rapidly.

    • Examples: Mixture of cornstarch and water, wet sand, concentrated sugar solution.
    • Explanation: At high shear rates, particles crowd together and resist motion.
  3. Bingham Plastic Fluids:
    These fluids behave like a solid until a certain minimum stress, called yield stress, is applied. Once the yield stress is exceeded, they start flowing like a fluid.

    • Examples: Toothpaste, butter, grease, clay, mayonnaise.
    • Explanation: Internal structure resists small stresses but breaks down under larger stresses.
  4. Thixotropic Fluids:
    These fluids decrease in viscosity with time when subjected to constant shear stress. Once the stress is removed, viscosity gradually increases again.

    • Examples: Some paints, printing inks, gels.
    • Explanation: Molecular structure breaks down with continuous stirring and rebuilds at rest.
  5. Rheopectic Fluids:
    These are the opposite of thixotropic fluids. Their viscosity increases with time under constant shear stress.

    • Examples: Some lubricants, printer inks, gypsum paste.
    • Explanation: Continuous shearing causes molecular networks to build up, making the fluid thicker.

Graphical Representation

If we plot shear stress (τ) against shear rate (du/dy):

  • For Newtonian fluids, the graph is a straight line passing through the origin (constant viscosity).
  • For non-Newtonian fluids, the graph is a curved line, showing variable viscosity behavior depending on shear rate.

This graphical distinction helps identify the type of fluid experimentally.

Mathematical Models for Non-Newtonian Fluids

Non-Newtonian fluids are represented mathematically using empirical models such as:

  1. Power Law Model (Ostwald-de Waele Model):

where,

    • K = flow consistency index,
    • n = flow behavior index.
    • If n = 1, the fluid is Newtonian.
    • If n < 1, the fluid is shear-thinning (pseudoplastic).
    • If n > 1, the fluid is shear-thickening (dilatant).
  2. Bingham Plastic Model:

where,

    • τ_y = yield stress,
    • μ_p = plastic viscosity.

These models help engineers predict flow behavior in industrial processes.

Examples of Non-Newtonian Fluids

  • Toothpaste: Behaves as a Bingham plastic; it flows only after sufficient pressure is applied.
  • Ketchup: Shear-thinning fluid; flows more easily when shaken.
  • Blood: Shear-thinning; viscosity decreases with increasing flow rate.
  • Cornstarch and Water (Oobleck): Shear-thickening; becomes solid-like when struck.
  • Paint: Thixotropic; becomes thinner when brushed and thickens when left to rest.

These examples show how non-Newtonian fluids behave differently under varying flow conditions.

Applications of Non-Newtonian Fluids

  1. Lubrication:
    Used in greases and gels, which provide stable lubrication even under high loads.
  2. Food Processing:
    Used in handling sauces, syrups, and dairy products where viscosity control is important.
  3. Pharmaceuticals and Cosmetics:
    Creams, lotions, and gels are designed to exhibit thixotropic or Bingham plastic behavior for easy application.
  4. Paints and Coatings:
    Thixotropic paints allow smooth application and prevent dripping.
  5. Industrial Processes:
    In drilling muds, non-Newtonian fluids are used to carry rock cuttings and provide pressure balance.
  6. Biomedical Engineering:
    Blood, a non-Newtonian fluid, is studied to understand flow behavior in arteries and artificial organs.

Importance in Engineering

Understanding non-Newtonian fluids is essential in designing systems involving variable viscosity flow, such as pipelines, mixers, and extrusion devices. Engineers must consider their time-dependent and shear-dependent behavior to ensure proper equipment performance, energy efficiency, and product quality.

Conclusion

In conclusion, non-Newtonian fluids are those whose viscosity changes with the rate of shear strain or time, meaning they do not obey Newton’s law of viscosity. Their behavior can be complex, as seen in types such as pseudoplastic, dilatant, thixotropic, rheopectic, and Bingham plastic fluids. These fluids are common in industries and daily life, including food, paint, cosmetics, and lubrication. Understanding non-Newtonian fluids helps engineers design efficient systems and processes that handle materials with variable flow properties.