In the process of selecting wetting and leveling materials, it is often challenging to obtain comprehensive physical and chemical data for each material. However, when facing such difficulties, we are committed to providing practical and constructive solutions. Wetting and leveling of liquids on different substrates are often complex, as they are affected by multiple specific factors (e.g., substrate porosity, coating viscosity). Although surface tension is commonly regarded as a key influencing factor, real-world tests show that low-surface-tension additives do not always solve the most difficult wetting problems—this indicates that surface tension is not the only decisive factor, making the material selection process more time-consuming and leading to significant accumulated time costs.

These Three Key Factors Address Over 95% of Common Wetting and Leveling Issues:

1. Capillary Action

Capillary action wetting range on absorbent substrates < 30%

When liquid is applied to a substrate, the leading edge is driven by capillary forces, pushing the liquid forward, much like how liquid rises in a straw. This is particularly relevant for absorbent substrates. The surface tension at the leading edge prevents radial diffusion; however, studies show that the impact of surface tension at the front is significantly less than at the contact line. For absorbent substrates, permeability is more about uniform and rapid penetration.

2. Tensile Stress

One of the most challenging scenarios in coating processes is that coating speed and substrate type are often unmodifiable. For porous substrates, the "shrinkage" time effect in their pores is significant, and this effect is closely associated with the applied force—especially tensile stress, which can mitigate the shrinkage phenomenon. Whether dealing with thin, fast-drying films or thick, viscous wet films, tensile stress plays a key role in alleviating the "shrinkage" problem.

In practical coating processes, applying high tensile stress to porous substrates enables rapid downward wetting of the liquid. As the liquid spreads laterally during coating, the tensile stress gradually weakens. After the coating is completed, this approach can significantly improve the film’s aesthetics and uniformity.

3. Prioritizing "Lateral Flow" over "Surface Tension-Driven Spreading"

Viscous gravitational flow is characterized by horizontal fluid movement, which is primarily driven by gravity (rather than buoyancy differences). Driven by its own weight, the fluid spreads radially outward as a thin layer, where the vertical gradient of shear stress balances the horizontal gradient of hydrostatic pressure. In most studied cases, the free surface of the fluid is not restricted by any external vertical boundaries, and such gravitational flow is defined as unconfined. For ultra-thin coatings, the influence of gravity becomes particularly pronounced.

For porous substrates, the relationship between film thickness, drying time, and wetting results is clear: after coating, if the film thickness is normal, the wetting process can be completed within the drying time; if the film is ultra-thin, wetting failure will occur due to insufficient time for liquid spreading.

The Relationship Between Gravity and Stress: Reducing Surface Tension May Lead to Wetting Errors

The transition of liquid behavior from unconfined to confined flow is critical to wetting and leveling performance. According to A.J. Hutchinson’s Confined Gravitational Flow (CGC) Model, the key factor affecting this transition is the ratio of the liquid height to the height of the confined space (e.g., pores in porous substrates)—this ratio is the primary determinant of wetting performance.

When 0 < J < 0.5 (where J represents the ratio of unconfined gravitational flow height to confined space height), the liquid height approaches a critical value, and the effect of gravitational flow reaches its peak. Once the liquid comes into contact with the substrate, the interfacial tension at the contact line (liquid-solid interface) increases significantly. This explains why high-viscosity coatings with greater film thickness exhibit better wetting properties: the tensile stress generated by high-viscosity liquids effectively suppresses crater formation.

An interesting characteristic of the CGC Model is the liquid slippage at the contact line. When J = 0.5, the meniscus at the liquid front pulls the contact line forward upon contact with the substrate—this mechanism is analogous to the slippage of outer wheels when a train navigates a curve.

Opt for 'Spreading' over 'Extension': Balancing Uniformity and Wettability

During the wetting process, viscous fluids displace air, and near the contact line, the lubrication effect is nearly ineffective. At this stage, contact angle effects and tensile stress become crucial factors. Prioritizing "Lateral Flow" over "Surface Tension-Driven Spreading" is an effective approach to solving difficult wetting issues and a common method for preventing wetting failure.

We hope this article proves helpful. For more technical support, please feel free to contact us. We welcome your inquiries!