Wettability: Physics and Importance

Introduction

Wettability refers to the ability of a liquid to maintain contact with a solid surface, which is influenced by the intermolecular interactions between the liquid and the solid. In physics, it is a key concept in understanding how liquids spread or bead on surfaces, whether for industrial processes, material science, or everyday phenomena like water droplets on a leaf. The degree of wettability is determined by the balance of forces between the cohesive forces within the liquid and the adhesive forces between the liquid and the solid surface. Understanding wettability is crucial for numerous applications in engineering, biology, chemistry, and environmental science.

Physics of Wettability:

1. Surface Tension and Interfacial Forces: The fundamental physics behind wettability involves surface tension, which is the force that acts at the interface between two different phases (liquid and air, for instance). Surface tension arises due to the cohesive forces between molecules in the liquid, leading to the minimization of the liquid’s surface area.

When a liquid droplet is placed on a solid surface, two forces come into play:

• Cohesive forces: The intermolecular forces within the liquid. These forces hold the liquid molecules together.
• Adhesive forces: The intermolecular forces between the liquid and the solid. These forces determine how the liquid spreads or sticks to the surface.

Wettability is essentially the competition between these forces. If the adhesive forces between the liquid and the solid are stronger than the cohesive forces within the liquid, the liquid will spread out over the surface, indicating good wettability. If the cohesive forces are stronger, the liquid will bead up, showing poor wettability.

2. Contact Angle and Young’s Equation: The degree of wettability is quantified by the contact angle (θ), which is the angle formed at the junction where the liquid, solid, and air meet. It describes how a liquid droplet interacts with a surface:

• High contact angle (>90°): Poor wettability; the liquid tends to form droplets and does not spread easily. This indicates that the cohesive forces within the liquid are stronger than the adhesive forces with the surface.
• Low contact angle (<90°): Good wettability; the liquid spreads out over the surface. This indicates that the adhesive forces between the liquid and the solid are stronger than the cohesive forces within the liquid.

A special case is when the contact angle is near zero, indicating that the liquid completely wets the surface, spreading into a thin film.

Young’s Equation is used to describe the balance of forces at the contact line of a droplet:

γSV=γSL+γLVcos⁡θ = \gamma_{SL} + \gamma_{LV} \cos \thetaγSV​=γSL​+γLV​cosθ

Where:

• γSV is the surface tension between the solid and the vapor.
• γSL​ is the surface tension between the solid and the liquid.
• γLV​ is the surface tension between the liquid and the vapor.
• θ is the contact angle.

Young’s equation helps to understand how the properties of the solid, liquid, and surrounding medium affect the wettability.

3. Wetting Regimes: Wettability is generally divided into two regimes:

• Complete Wetting: This occurs when the liquid completely spreads over the surface (θ = 0°). It happens when the adhesive forces between the liquid and the surface are so strong that the liquid prefers to stay in contact with the surface rather than forming droplets.
• Partial Wetting: In this regime, the liquid forms a finite contact angle (0° < θ < 180°), and a droplet shape is visible. This is the more common case seen in nature and industry.

4. Hysteresis in Wetting: Wettability can exhibit contact angle hysteresis, where the advancing contact angle (when a liquid is spreading) is different from the receding contact angle (when a liquid is retracting). This occurs due to surface roughness, heterogeneity, or chemical interactions and plays a significant role in understanding the dynamic behavior of liquids on surfaces, such as how droplets move or stick.

5. Surface Roughness and Wenzel/Cassie-Baxter Models: Surface roughness significantly affects wettability. There are two primary models that describe how roughness impacts the contact angle:

• Wenzel Model: This model assumes that the liquid completely penetrates the roughness grooves of the surface. According to the Wenzel model, surface roughness amplifies the intrinsic wettability of a surface—making hydrophobic surfaces more hydrophobic and hydrophilic surfaces more hydrophilic. The Wenzel equation is:

cos⁡θ_w=r*cos⁡θ 

where θw\theta_wθw​ is the Wenzel contact angle, $\theta$ is the contact angle on a smooth surface, and r is the roughness ratio (the ratio of the actual area of the rough surface to its projected area).

• Cassie-Baxter Model: This model applies when the liquid rests on top of the roughness features, trapping air pockets beneath the droplet. This leads to a composite interface. The Cassie-Baxter equation is:

cos⁡θ_CB=f_s(cos⁡θ+1)−1

where f_s​ is the fraction of the solid surface in contact with the liquid.

These models help explain why some surfaces, like lotus leaves, exhibit superhydrophobicity (extremely high contact angles, often >150°), where droplets roll off easily due to minimal contact with the solid surface.

Importance of Wettability in Various Fields:

1. Industrial Applications: Wettability plays a critical role in many industrial processes:
   • Coatings and Paints: The ability of paints and coatings to spread and adhere to surfaces depends on their wettability. Poor wettability leads to uneven coatings and potential flaking.
   • Adhesives: Adhesives require good wettability to form strong bonds with surfaces. Inadequate wettability can cause weak or incomplete bonding.
   • Textile Manufacturing: In processes like dyeing or waterproofing fabrics, controlling the wettability of textile surfaces is crucial. Fabrics with hydrophobic surfaces resist water, while hydrophilic surfaces absorb it, influencing the textile’s functionality.
2. Microfluidics: In microfluidic devices, which manipulate small amounts of fluids in channels, controlling wettability is vital. The behavior of liquids in such small-scale environments depends heavily on surface interactions. Wettability affects capillary action, droplet movement, and mixing, all of which are critical for lab-on-a-chip technologies used in diagnostics and chemical analysis.
3. Environmental Science:
   • Soil-Water Interaction: Wettability is critical in understanding how water interacts with soils and sediments. Hydrophobic soils (those with poor wettability) resist water infiltration, leading to issues like water runoff and erosion. Conversely, hydrophilic soils absorb water more easily, aiding in groundwater recharge and plant growth.
   • Oil Spill Cleanup: In environmental disasters like oil spills, understanding the wettability of oil and water on various surfaces (such as rocks or water) helps design effective cleaning strategies. Surfactants are often used to alter wettability, making it easier to remove oil from contaminated surfaces.
4. Biology and Medicine: Wettability is crucial in biological systems, where it influences how cells interact with surfaces, such as medical implants or tissue engineering scaffolds:
   • Medical Implants: Implants that promote cell adhesion (good wettability with cells) are more likely to integrate successfully with surrounding tissues. In contrast, implants designed to repel biological fluids (poor wettability) may resist biofouling, where proteins or bacteria accumulate on the surface.
   • Drug Delivery: Wettability affects how drugs interact with biological tissues. For example, hydrophobic drug formulations may pass through cell membranes more easily, while hydrophilic formulations might dissolve better in biological fluids.
5. Energy and Oil Recovery:
   • Enhanced Oil Recovery (EOR): Wettability is a key parameter in oil extraction processes. The ability of oil to move through porous rock formations depends on the wettability of the rock surface. In water-wet (hydrophilic) rocks, water tends to displace oil more efficiently, while oil-wet (hydrophobic) rocks trap oil, making it harder to extract. Adjusting the wettability of reservoir rocks is a strategy used in EOR techniques.
   • Solar Panels: The efficiency of solar panels can be impacted by surface contamination, such as dust or water droplets. Hydrophobic coatings with low wettability are used to prevent water and dust from adhering to solar panel surfaces, maintaining their performance.
6. Food Industry: Wettability plays a role in food packaging and processing. For instance, hydrophobic coatings on food packaging prevent moisture buildup, preserving the product’s shelf life. In food processing, wettability influences how liquids interact with machinery and packaging materials, impacting the quality and safety of food products.

Conclusion:

Understanding the physics of wettability is crucial for various scientific and industrial applications. By examining the balance between cohesive and adhesive forces, surface tension, and contact angles, researchers and engineers can manipulate surfaces to achieve desired outcomes, whether it’s promoting liquid spreading or preventing it. From industrial processes like painting and adhesives to microfluidics, biology, environmental science, and energy applications, wettability is a key factor in material performance and functionality. The ability to control and engineer wettability has led to innovations in fields ranging from medical implants to enhanced oil recovery, demonstrating the wide-reaching importance of this physical phenomenon.