What Are the Differences Between Wettability, Suspension Rate, and Dispersibility in Pesticide Formulations?

Wettability, dispersibility, and suspension rate are all important physical indicators for evaluating the water-mixing performance of pesticide formulations, but they do not describe the same process.

The entire process can be understood as:

Formulation comes into contact with water → Is wetted by water → Disperses in water → Forms a suspension system → Particles remain suspended

In which:

• **Wettability** focuses on whether water can quickly wet the surface of the formulation;
• **Dispersibility** refers to whether the formulation can disperse rapidly and uniformly in water after wetting;
• **Suspensibility** refers to whether insoluble particles can remain fully suspended within a specified time after a suspension system has been formed.

These three properties are related on a continuum but are not interchangeable.

A WP formulation may wet very quickly but exhibit poor suspension due to particle size, dispersants, or particle-particle interactions; a WG formulation may disintegrate and disperse rapidly, but the particles may subsequently settle quickly.

Therefore, when evaluating the water-mixing performance of pesticide formulations, one should not rely on just a single parameter.

What are the main differences between wettability, dispersibility, and suspension rate?

The most fundamental difference among these three lies in the fact that they evaluate different stages of the formulation after it enters water.

This is why a pesticide product may perform well on one parameter but exhibit problems on another.

Good water-mixing performance is a continuous process, not a single numerical value.

What is the wettability of a pesticide formulation?

Wettability describes whether water can quickly cover and penetrate the surface of a solid pesticide formulation upon contact.

Many pesticide active ingredients inherently possess a certain degree of hydrophobicity.

If the surface of WP powder or WG granules is difficult to wet with water, the following may occur after adding water:

• floating on the water’s surface;
• forming clumps of dry powder;
• failing to disperse into the water for an extended period fully;
• localized clumping;
• subsequent uneven dispersion.

Therefore, solid water-dispersible formulations typically require an appropriate wetting system to reduce the wetting resistance between water and the solid surface.

Why is surface wetting important?

In WP formulations, if the powder cannot be rapidly wetted by water, subsequent dispersion and suspension will be compromised.

In WG formulations, water must first come into contact with the particle surface before it can penetrate the interior of the particles and promote their disintegration.

Therefore, wetting can be understood as the first step in the entire water-mixing process:

If water cannot effectively contact the formulation surface, the subsequent dispersion process will be difficult to carry out normally.

However, a fast wetting rate does not necessarily mean the product has a good suspension rate.

These two indicators evaluate different stages.

What is the dispersibility of pesticide formulations?

Dispersibility describes whether a formulation, after entering water, can form a relatively uniform system through disintegration, separation, and particle distribution.

This parameter is particularly important in WG / WDG.

WG does not truly “dissolve” into a molecular state in water.

Under normal conditions, it must undergo:

Particle wetting → Particle disintegration → Release of small particles → Uniform dispersion

If WG has poor dispersibility, the following may occur:

• Particles fail to disintegrate for an extended period;
• Large clumps remain;
• Localized agglomeration;
• Uneven distribution in water;
• Subsequent suspension performance is affected.

Dispersibility is not the same as solubility

This is a very important distinction.

Dispersibility ≠ Solubility.

The insoluble active ingredients in WP and WG are typically dispersed in water in the form of solid particles.

These particles do not actually dissolve.

For example:

• WP → forms a suspension;
• WG / WDG → form a suspension after disintegration;
• SP / SG → more typically form a solution.

Therefore, just because WG “appears to have dispersed” in water does not mean the active ingredient has completely dissolved.

If you need to compare the product forms of WP and WDG and understand the practical selection criteria, please refer to POMAIS’s Guide to Selecting WP and WDG Pesticide Formulations.

What is the suspension rate of a pesticide formulation?

The suspension rate describes the ability of insoluble particles to remain suspended in water after entering the water and forming a suspension system, under specified test conditions.

It does not answer the question:

Can the product enter the water?

Rather, it answers:

Once in the water, will the particles settle to the bottom too quickly?

This is also the key difference between suspension rate and wettability and dispersibility.

Factors affecting the suspension rate may include:

• particle size and size distribution;
• particle density;
• dispersant system;
• interparticle interactions;
• water quality;
• formulation concentration;
• test temperature;
• sedimentation behavior.

For example, a WP can be completely wetted by water within a few seconds, but if its particles are too coarse or its dispersion stability is poor, it may still settle rapidly after being mixed with water.

Similarly, a WG may disintegrate quickly, but the resulting suspension of fine particles may still have a poor suspension rate.

POMAIS further explains the relationship between suspension stability, sedimentation, and redispersibility in Why Is Suspension Stability Important for SC Pesticides?.

How Do Wettability, Dispersibility, and Suspension Rate Occur Continuously?

The simplest way to understand these three parameters is to consider the sequence in which the pesticide enters the water.

The WP Process

For WP:

Powder is added to water

↓

Wetted by water

↓

Powder particles enter the aqueous phase

↓

A suspension is formed

↓

Particles remain suspended

Therefore, issues with WP can occur at different stages.

If wetting is poor:

The powder floats and agglomerates.

If wetting is good but particle dispersion stability is poor:

Settling occurs rapidly.

Therefore, the fact that “the powder sinks quickly in water” does not necessarily indicate good final suspension performance.

The WG / WDG Process

WG involves an additional, distinct particle disintegration process compared to WP.

The basic sequence is:

Particles enter water

↓

Wetting

↓

Particles absorb water and disintegrate

↓

Fine particles disperse

↓

Formation of a suspension

↓

Maintenance of suspension

Therefore, WG focuses on:

• wettability;
• dispersibility;
• suspensibility.

These three parameters are used to evaluate different stages, respectively.

The SC Process

SC is, in itself, a liquid suspension concentration system.

Therefore, unlike WP or WG, it does not require the complete wetting process of a dry formulation to be addressed first.

What is more important for SC is:

• dispersion after water addition;
• suspensibility;
• particle size;
• wet sieving;
• sedimentation;
• redispersibility.

This is also why the same set of QC criteria cannot be mechanically applied to all formulation types.

If wetting is good, why might the suspension rate still be poor?

Because wetting only addresses:

whether water can penetrate and cover the surface of the formulation.

It does not determine how the particles will subsequently move in water.

Suppose a WP uses a highly effective wetting agent t.

After adding water:

• is quickly and completely wetted;
• no powder floats;
• no obvious dry clumps.

This indicates excellent wettability.

However, if the following conditions exist:

• larger particles;
• a broad particle size distribution;
• an inadequate dispersant system;
• severe flocculation;
• a significant density difference between larger particles and the aqueous phase;

rapid sedimentation may still occur.

Therefore:

Good wettability does not equate to good suspensibility.

The two should be evaluated separately.

If dispersibility is good, why might the suspension rate still be poor?

The same logic applies to WG.

A WG may completely disintegrate within a very short time after being added to water.

This indicates that its:

disintegration/dispersibility

performs well.

However, whether the particles produced after disintegration can remain uniformly suspended over the long term depends on another set of factors.

For example:

• particle size;
• dispersant;
• particle interaction;
• water quality;
• tendency to settle.

Therefore, the following scenario may occur:

A WG disperses very well within 1 minute, but after standing for a period of time, a large number of particles settle to the bottom.

This illustrates that:

Good dispersibility does not necessarily imply good Suspensibility; for this reason, in actual product specifications, dispersibility and Suspensibility are evaluated as two separate parameters.

What is the difference between a wetting agent and a dispersant?

These two formulation components are also easily confused.

Wetting Agent

The core function of a wetting agent is:

to help water more easily come into contact with hydrophobic solid surfaces.

It primarily addresses:

• powder floating on water;
• difficulty in wetting particles;
• difficulty in water penetrating dry formulations.

Therefore, it primarily affects wettability.

Dispersant

The core function of a dispersant is:

to help particles that have already entered the water remain dispersed and to reduce uncontrolled particle re-aggregation.

It can affect:

• particle separation;
• flocculation;
• agglomeration;
• suspension behavior.

Therefore, although wetting agents and dispersants are often present together in a formulation, they serve different functions.

A good wetting system cannot replace a good dispersion system.

What is the difference between dispersibility and redispersibility?

These two English terms are very similar, but they actually evaluate completely different processes.

Dispersibility

Description:

Whether a product can disperse rapidly after being added to water for the first time.

It primarily focuses on the initial mixing process.

Redispersibility

Description:

Whether a material that has settled or been left to stand can be restored to a properly dispersed or suspended state.

Redispersibility is particularly important in suspension-based formulations such as SCs.

For example, an SC may form a small amount of soft sediment during storage.

If it can be shaken to restore a uniform system, this is entirely different from the formation of a hard cake.

This is why POMAIS clearly distinguishes between soft settling and hard caking in Causes of Settling in SC Pesticides.

Therefore:

Dispersibility = How easily the system disperses the first time.

Redispersibility = Whether the system can be redispersed after settling.

Do not confuse these two concepts.

How does the wet sieve test relate to these three parameters?

The wet sieve test is also part of the evaluation of a formulation’s physical quality, but it addresses a different question:

Are there excessive large particles, crystals, or agglomerates in the system?

It is not directly equivalent to:

• wettability;
• dispersibility;
• suspensibility.

For example, the Suspensibility of an SC formulation can be affected by particle size, and the wet sieve test can help identify whether there is any abnormally coarse material present.

POMAIS explains in detail the difference between wet sieve testing and complete particle size distribution in “How Particle Size Affects SC Pesticide Performance.”

Therefore, formulation QC should be understood as multiple parameters collectively describing product performance, rather than a single parameter serving as a substitute for all others.

How are these properties typically tested?

Different formulation types require testing methods suited to their specific characteristics.

Common CIPAC method systems include:

Note:

There is no single set of pass values applicable to all pesticide formulations.

Specific requirements depend on:

• formulation type;
• active ingredient;
• concentration;
• product specification;
• applicable regulatory or FAO specification.

Therefore, the following should not be used simplistically:

Wettability < X seconds
Suspensibility > X%
Dispersibility > X%

as a universal criterion for all pesticide products.

Why can’t we rely on just one QC parameter?

Because the actual performance of a pesticide formulation in water is a continuous system.

For example, a WG may exhibit:

• excellent wettability;
• excellent dispersibility;
• poor suspension:

• wet slightly more slowly;
• but ultimately form a more stable suspension.

Comparing only one of these parameters could lead to incorrect conclusions about quality.

A more reasonable evaluation method is to take a comprehensive view based on the formulation type:

This is why formulation quality must be evaluated holistically.

FAQ

What is the difference between wettability and dispersibility?

Wettability focuses on whether water can rapidly wet the surface of the formulation; dispersibility focuses on whether the formulation can further disintegrate and distribute uniformly in water after wetting. Wetting typically occurs before dispersion.

What is the difference between dispersibility and Suspensibility? Dispersibility primarily describes the ability of a formulation to disperse after entering water, while Suspensibility is the ability of already-formed suspension particles to remain suspended for a specified period of time.

Can a pesticide have excellent wettability but poor suspension stability?

Yes. A good wetting agent can allow the product to enter water quickly, but if the particle size, dispersant, or particle interactions are not ideal, the resulting suspension may still settle rapidly.

Why does WG need to consider both dispersibility and suspension stability?

Because WG formulations must first disintegrate and disperse in water before forming a suspension of insoluble particles, dispersibility evaluates the former stage, while Suspensibility tests the latter.

Are Dispersibility and Dissolution the same thing?

No. Dispersibility refers to the uniform distribution of solid particles in water; Dissolution refers to a substance actually dissolving into molecular or ionic forms.
WP and WG typically form suspensions rather than complete solutions.

Do all pesticide formulations need to be tested for wettability, dispersibility, and Suspensibility? Different formulation types correspond to different key physical properties. The testing focus for WP, WG, and SC is not entirely fixed; evaluation should be conducted according to specific product specifications and applicable methods.

How should these three performance indicators for pesticide formulations be properly understood?

Wettability, Dispersibility, and Suspensibility should not be viewed as three interchangeable terms.

They describe the behavior of water-dispersible pesticides at different stages—from the moment they are added to water until a stable, usable system is formed:

Wettability

Determines whether water can effectively come into contact with the formulation.

↓

Dispersibility

Determines whether the formulation can further disintegrate and distribute uniformly.

↓

Suspensibility

Determines whether the insoluble particles formed can maintain sufficient suspension under specified conditions.

Therefore:

Good wettability does not guarantee good dispersibility, nor does good dispersibility guarantee good Suspensibility. Reliable formulation quality requires a comprehensive evaluation of these parameters in conjunction with particle size, wet sieve analysis, sedimentation, redispersibility, and storage stability.

For a further comparison of the differences between WP and WDG in actual product development, please refer to POMAIS’s WP and WDG Formulation Selection Guide; POMAIS also offers commercial pesticide formulations for various applications, covering multiple formulations such as WP, WG/WDG, SC, EC, and FS.