What is Wetting?

Wetting Definition

Wetting refers to the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Wetting is important in the bonding or adherence of two materials. Wetting and the surface forces that control wetting are also responsible for other related effects, including capillary effects.

Types of Wetting

There are two main types of wetting:

1. Spreading wetting: The liquid droplet spreads out over a large area of the surface.
2. Adhesional wetting: The liquid adheres to the surface without spreading out.

The degree of wetting is measured by the contact angle θ, which is the angle formed by a liquid at the three-phase boundary where the liquid, gas, and solid intersect. A low contact angle (< 90°) indicates high wettability, while a high contact angle (> 90°) means poor wettability. A zero contact angle represents complete wetting.

Factors Affecting Wetting

Several factors influence the wetting behavior of a liquid on a solid surface:

1. Surface tension: The cohesive forces among liquid molecules. Lower surface tension leads to better wetting.
2. Interfacial tension: The tension between the liquid and solid phase. Lower interfacial tension enhances wetting.
3. Surface roughness: Rough surfaces may hinder or enhance wetting depending on the liquid and surface properties.
4. Surface cleanliness: Contaminants or impurities on the surface can affect wetting behavior.
5. Temperature: Higher temperatures generally lead to better wetting due to reduced liquid viscosity and surface tension.

Measuring Wettability

Contact Angle Measurement

The most common method to quantify wettability is by measuring the contact angle formed by a liquid droplet resting on a flat solid surface. The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation.

The theoretical description of contact arises from the consideration of a thermodynamic equilibrium between the three phases: the liquid phase (L), the solid phase (S), and the gas/vapor phase (G) (which could be a mixture of ambient atmosphere and an equilibrium concentration of the liquid vapor). The “gaseous” phase could also be another (immiscible) liquid phase. If the solid–vapor interfacial energy is denoted by γSG, the solid–liquid interfacial energy by γSL and the liquid–vapor interfacial energy (i.e. the surface tension) by γLG, then the equilibrium contact angle θC is determined from these quantities by Young’s equation:

cosθC = (γSG – γSL) / γLG

Sessile Drop Method

The sessile drop technique is an optical contact angle method used to estimate wetting properties of a localized region on a solid surface. The angle between the baseline of the drop and the tangent at the drop boundary is measured. A high-resolution camera captures the profile of a pure liquid on a solid substrate. The image is then analyzed using computer software to calculate the contact angle.

Key advantages:
– Simple and inexpensive
– Relatively fast measurement (seconds to minutes)
– Can measure angles up to 180°

Limitations:
– Measurement inaccuracy for angles > 150°
– Static measurement, doesn’t account for contact angle hysteresis
– Evaporation effects can impact measurement for volatile liquids

Applications of Wetting

Adhesion

Wetting is a key factor in determining how well two materials will adhere together. Good wetting (low contact angle) leads to high adhesive forces between the materials, while poor wetting results in weak adhesion.

Some examples where wetting is important for adhesion:
– Applying paint or coatings to a surface
– Bonding of composite materials
– Soldering and brazing
– Sealing and waterproofing

Printing and Coating

Wetting plays a critical role in printing and coating processes. The ink or coating must sufficiently wet the substrate in order to form a uniform layer without defects.

Factors affecting wetting in printing and coating:
– Surface energy of substrate
– Surface tension of ink/coating
– Viscosity and rheology of ink/coating
– Application method (roll coating, spray coating, etc.)
– Drying/curing conditions

Challenges:
– Achieving good wetting on low surface energy substrates like plastics
– Preventing dewetting or beading up of ink/coating
– Controlling film thickness and uniformity
– Minimizing defects (air bubbles, pinholes, etc.)

Agriculture

Wetting is important in agriculture, influencing how well water and nutrients are absorbed by soil and taken up by plant roots. Soil wettability affects infiltration, runoff, and irrigation efficiency.

Soil wetting agents (surfactants) are often used to improve water penetration and distribution in soil, especially in hydrophobic or water-repellent soils. They work by reducing the surface tension of water and improving the wettability of soil particles.

Benefits of using soil wetting agents:
– Increased water infiltration and retention in soil
– Reduced runoff and erosion
– More uniform moisture distribution
– Enhanced nutrient uptake by plants
– Improved seed germination and plant growth

Oil Recovery

In the oil industry, wetting phenomena are important in both oil extraction and oil recovery processes. The wettability of reservoir rocks affects how fluids like water, oil, and gas flow through and interact with the rock surfaces.

Most reservoir rocks are preferentially water-wet, meaning water occupies the small pores and coats the rock surfaces. This condition is favorable for oil extraction because the oil can flow through the larger pores and channels while water remains trapped by capillary forces. However, some reservoir rocks can be oil-wet or mixed-wet, which can hinder oil extraction.

Wettability alteration techniques are used in enhanced oil recovery to shift the wettability of reservoir rocks from oil-wet to water-wet. This is typically done by injecting surfactants or other chemicals that adsorb onto the rock surface and make it more water-wet. The increased water wettability helps displace oil from the rock pores and improve oil recovery.

Factors affecting wettability in oil reservoirs:
– Mineralogy and surface chemistry of reservoir rocks
– Composition of crude oil and brine
– Temperature and pressure conditions
– Injection of chemicals or gases

Controlling Wettability

There are various ways to modify the wettability of surfaces depending on the desired application. Some common methods include:

Surface Treatment

• Cleaning: Removing contaminants or unwanted residues from the surface
• Etching: Using chemicals or plasma to roughen the surface and change its chemistry
• Priming: Applying a thin layer of primer to enhance adhesion and wetting of subsequent coatings

Surface Coatings

• Painting: Applying a layer of paint to change the surface properties
• Printing: Depositing inks or other functional materials onto the surface
• Thin films: Coating the surface with a thin layer of material like metals, polymers, or ceramics

Surface Modification

• Plasma treatment: Exposing the surface to ionized gases to alter its chemistry and wettability
• Chemical grafting: Covalently bonding molecules or polymers to the surface
• Surfactant adsorption: Adsorbing surfactant molecules onto the surface to change its wetting behavior

The choice of surface modification method depends on factors such as:
– Substrate material and geometry
– Desired surface properties (hydrophobicity, adhesion, durability, etc.)
– Compatibility with the liquid or coating to be applied
– Cost and scalability of the modification process

Frequently Asked Questions (FAQ)

What is a hydrophobic surface?

A hydrophobic surface is one that has a low affinity for water. Water droplets tend to bead up on hydrophobic surfaces rather than spreading out, resulting in a high contact angle (> 90°). Some examples of naturally hydrophobic surfaces include lotus leaves, butterfly wings, and duck feathers. Artificial superhydrophobic surfaces can be created by combining micro/nanoscale surface roughness with low surface energy coatings.

What is surface tension?

Surface tension is the elastic tendency of a fluid surface to minimize its surface area due to cohesive forces between the liquid molecules. It is responsible for phenomena like the beading up of water droplets, the formation of bubbles, and the ability of some insects to walk on water. The surface tension of a liquid depends on factors such as temperature, solute concentration, and the presence of surfactants.

How does temperature affect wettability?

In general, higher temperatures lead to better wettability due to decreased liquid viscosity and surface tension. As temperature increases, the cohesive forces between liquid molecules decrease, making it easier for the liquid to spread out on a surface. However, the effect of temperature on wettability can vary depending on the specific liquid-solid system and other factors like surface roughness and chemistry.

What is the difference between adhesional and spreading wetting?

Adhesional wetting occurs when a liquid adheres to a surface without spreading out significantly, resulting in a high contact angle. This type of wetting is characterized by strong adhesive forces between the liquid and solid, but weak cohesive forces within the liquid.

In contrast, spreading wetting occurs when a liquid spreads out over a large area of the surface, resulting in a low contact angle. This type of wetting is favored when the adhesive forces between the liquid and solid are stronger than the cohesive forces within the liquid.

What are surfactants and how do they affect wetting?

Surfactants (surface active agents) are compounds that lower the surface tension of a liquid and improve its wetting and spreading properties. They have a hydrophobic tail and a hydrophilic head, allowing them to adsorb at interfaces and reduce the interfacial tension.

When added to a liquid, surfactants can:
– Lower the surface tension and contact angle, enhancing wettability
– Facilitate spreading and penetration of the liquid into porous surfaces
– Promote foam and emulsion formation by stabilizing the liquid-gas or liquid-liquid interfaces
– Aid in dispersion and solubilization of hydrophobic substances

Surfactants are commonly used in applications such as cleaning, coatings, oil recovery, agriculture, and pharmaceuticals to improve wetting and other interfacial properties.