Powder And Mixing - 14. What Is Powder Internal Friction Angle?

—Why Does It Determine Powder Flowability and Mixing Difficulty?

In powder engineering, many parameters are used to evaluate powder behavior, including:

• Particle size;

• Particle size distribution;

• Angle of repose;

• Bulk density;

• Tapped density;

• Flowability;

• Cohesion.

However, one of the most fundamental properties that determines how powders move, deform, and mix is:

Powder Internal Friction Angle (Internal Friction Angle)

Although it is not as commonly discussed as particle size or flowability, internal friction angle plays a critical role in:

• Powder flow;

• Storage stability;

• Feeding performance;

• Mixing efficiency;

• Segregation behavior.

Understanding internal friction angle helps explain why:

• Some powders flow easily like sand;

• Some powders behave like sticky soil;

• Some powders require strong mechanical action to achieve uniform mixing.

1. What Is Powder Internal Friction Angle?

Powder internal friction angle refers to:

The resistance generated when powder particles move relative to each other due to friction, interlocking, and particle interaction forces.

Simply speaking:

It describes How difficult it is for powder particles to slide past each other.

A powder with:

• Low internal friction angle → particles move easily;

• High internal friction angle → particles resist movement.

Basic Concept

Imagine a pile of powder.

When the pile is slowly tilted:

• At first, the powder remains stable.

• As the angle increases, particles begin to slide.

• The angle at which sliding starts reflects the resistance between particles.

This resistance is related to:

• Internal friction;

• Cohesion;

• Particle structure.

2. The Physical Meaning of Internal Friction Angle

Powder is not a continuous solid. It is a collection of many individual particles.

Between particles exist:

• Contact forces;

• Friction forces;

• Adhesion forces;

• Mechanical interlocking.

When an external force attempts to move the powder, particles must overcome these internal resistances.

The larger the resistance, the higher the internal friction angle.

3. Relationship Between Internal Friction Angle and Powder Flowability

Internal friction angle and flowability are closely related.

Generally:

Higher Internal Friction Angle = Poorer Powder Flowability

Lower Internal Friction Angle = Better Powder Flowability

3.1 Low Internal Friction Powder

Characteristics:

• Particles slide easily;

• Low resistance;

• Easy discharge.

Examples:

• Spherical glass beads;

• Coarse metal powders;

• Well-rounded particles.

Advantages:

• Easy conveying;

• Stable feeding;

• Good processing efficiency.

3.2 High Internal Friction Powder

Characteristics:

• Strong particle interaction;

• Difficult movement;

• High resistance.

Examples:

• Fine mineral powders;

• Fibrous powders;

• Ultrafine powders.

Problems:

• Bridging;

• Poor discharge;

• Uneven feeding;

• Difficult mixing.

4. Why Do Fine Powders Have High Internal Friction?

The main reason is increased particle interaction.

As particle size decreases, the number of particle contacts increases significantly.

At the same time, specific surface area increases.

This strengthens:

4.1 Van der Waals Forces

Small particles attract each other strongly.

4.2 Electrostatic Forces

Fine particles easily accumulate charges.

4.3 Surface Adhesion

Particles stick together more easily.

4.4 Mechanical Interlocking

Irregular particles lock together.

Therefore, ultrafine powders usually have:

• High internal friction;

• Poor flowability;

• Strong agglomeration tendency.

5. Internal Friction Angle and Powder Mixing Difficulty

Many people think, "Powder mixing is simply about moving particles."

However, successful mixing requires:

Particles to move, separate, and redistribute repeatedly.

Internal friction directly determines how easily this happens.

5.1 Case 1-Low Internal Friction Powder

Particles move easily.

Advantages:

• Fast mixing;

• Low energy consumption.

However, the problem is they can also separate easily.

This increases:

• Segregation risk;

• Re-separation during transportation.

5.2 Case 2-High Internal Friction Powder

Particles resist movement.

Problems:

• Poor mixing efficiency;

• Longer mixing time;

• Dead zones;

• Uneven distribution.

Therefore, both extremes create challenges.

6. Why High Internal Friction Causes Mixing Problems

During mixing, particles need to:

• Move;

• Collide;

• Separate;

• Redistribute.

However, high internal friction prevents these actions.

Consequences include:

6.1 Poor Particle Rearrangement

Particles cannot easily exchange positions.

The mixture remains locally concentrated.

6.2 Agglomerate Formation

Strong internal resistance promotes:

• Particle clustering;

• Stable agglomerates;

• Poor dispersion.

6.3 Longer Mixing Time

More energy and time are required to overcome particle resistance.

6.4 Uneven Additive Distribution

Especially in systems containing:

• Conductive additives;

• Nano materials;

• Fine chemicals.

Poor movement prevents uniform distribution.

7. Internal Friction Angle and Powder Segregation

Interestingly, internal friction affects both:

• Mixing difficulty;

• Segregation tendency.

This creates another powder engineering contradiction.

7.1 Low Internal Friction

Powders mix easily. But, Particles also move easily.

Therefore, they may separate according to:

• Size;

• Density;

• Shape.

Example:

A highly free-flowing mixture of large particles and small particles may quickly segregate.

7.2 High Internal Friction

Powders resist movement.

Segregation may be reduced. But, mixing becomes difficult.

Therefore, the ideal powder system requires:

Controlled Internal Friction.

8. Internal Friction Angle in Light and Heavy Powder Systems

Many modern materials involve:

• Light powders;

• Heavy powders;

• Nano additives.

Examples:

• Battery electrode materials;

• Composite materials;

• Functional fillers.

A large difference in internal friction between components can cause:

• Uneven movement;

• Different residence times;

• Poor distribution.

For example:

A fine conductive additive may have:

• High internal friction;

• Strong cohesion.

The main active material may have:

• Lower internal friction;

• Better flowability.

During mixing, the two materials behave completely differently.

This is one reason why conventional mixers often struggle with advanced materials.

9. How Is Internal Friction Angle Measured?

Common testing methods include:

Shear Cell Testing

Widely used in powder engineering.

It measures:

• Shear stress;

• Normal stress;

• Flow resistance.

Jenike Shear Testing

Developed by: Arthur Jenike

It is widely used for:

• Hopper design;

• Powder flow evaluation;

• Industrial powder handling.

The results are used to determine:

• Internal friction angle;

• Cohesion;

• Flow function.

10. Why Traditional Mixers Struggle with High Internal Friction Powders

Traditional mixers such as:

• V-Type Mixers;

• Double Cone Mixers;

• 3D Mixers;

• 2D Mixers;

mainly rely on gravity diffusion mixing

They are effective for:

• Free-flowing powders;

• Similar particle systems.

However, for powders with:

• High internal friction;

• Strong cohesion;

• Agglomeration tendency;

gravity-driven movement is insufficient.

Problems include:

• Incomplete dispersion;

• Long mixing cycles;

• Residual agglomerates.

11. Modern Powder Mixing Requires More Than Simple Movement

Advanced powder processing requires:

11.1 Controlled Shear

To overcome internal resistance.

11.2 Effective Dispersion

To break agglomerates.

11.3 Multi-Directional Movement

To continuously redistribute particles.

11.4 Anti-Segregation Capability

To maintain uniformity after mixing.

12. Conclusion

Powder internal friction angle is one of the fundamental properties controlling powder behavior.

It determines:

• How easily powders flow;

• How easily particles rearrange;

• How difficult mixing becomes;

• How likely powders are to segregate.

A high internal friction angle can cause:

• Poor flowability;

• Agglomeration;

• Difficult mixing.

A very low internal friction angle can cause:

• Easy segregation;

• Poor mixture stability.

Therefore, the goal of advanced powder processing is not to eliminate internal friction, but to control it.

The ideal powder mixing system must achieve a balance between:

Particle Movement + Dispersion Ability + Mixing Uniformity + Segregation Prevention

This balance is becoming increasingly important in advanced industries such as:

• Lithium batteries;

• Pharmaceuticals;

• New energy materials;

• Fine chemicals;

• High-performance composites.

For next-generation powder mixing equipment, the challenge is no longer simply "How to mix powders faster?"

The real question is:

"How to control particle behavior and achieve stable micro-uniform mixing?"