In the process of selecting materials for wetting and leveling, it's often difficult to obtain comprehensive physical and chemical data for every material. However, when challenges arise, we are committed to offering practical and constructive solutions. Wetting and leveling of liquids on different substrates can be complex due to the many specific factors involved. While surface tension is commonly considered the main factor, real-world testing reveals that low-surface-tension additives may not always resolve the most challenging wetting issues. This makes material selection more time-consuming, and significant time costs can accumulate in the process.

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

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, but studies show that the impact of surface tension at the front is much less significant than at the contact line. For absorbent substrates, permeability is more about uniform and rapid penetration.

Tensile Stress

One of the most challenging aspects is that coating speed and substrate are often unmodifiable. The "shrinkage" time effect within holes is significant, closely related to applied force, particularly tensile stress. Whether it's thin, fast-drying films or thick, viscous wet films, tensile stress can help mitigate the "shrinkage" problem.

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

Viscous gravitational flow is characterized by horizontal fluid movement driven primarily by 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 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 pronounced.

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

The transition of liquid behavior from unconfined to confined is critical to wetting and leveling. The key factors affecting this transition can be understood through A.J. Hutchinson’s CGC (Confined Gravitational Flow) Model, which indicates that the ratio of liquid height to the height of the confined/wetting space is the primary factor governing wetting performance.

When 0<J<0.5 (where J is the ratio of unconfined gravitational flow height to confined space height), the liquid height approaches a critical point, and the effect of gravitational flow peaks. Once the liquid comes into contact with the substrate, the surface 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 aspect 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 substrate contact. 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, lubrication is nearly ineffective. This is when contact angle effects and tensile stress become crucial. Prioritizing "Lateral Flow" over "Surface Tension-Driven Spreading" is one of the approaches to solve difficult wetting issues and is 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!