Wetting and contact angle measurements are key to understanding how liquids interact with surfaces. They reveal crucial information about surface properties, influencing everything from paint adhesion to oil recovery. These concepts are fundamental to grasping the broader principles of surface thermodynamics.

Contact angles help quantify wetting behavior, with various measurement techniques providing insights into surface energy and wetting parameters. Factors like surface roughness and treatments can dramatically alter wetting, leading to special behaviors like superhydrophobicity, which has wide-ranging applications in materials science.

Wetting in Surface Science

Fundamentals of Wetting

• Wetting interaction between a liquid and a solid surface determines how the liquid spreads or adheres to the surface
• Contact angle characterizes the degree of wetting, formed between the liquid-solid interface and the liquid-vapor interface at the three-phase boundary
  • Complete wetting: contact angle is 0°
  • Partial wetting: contact angle is between 0° and 90°
  • Non-wetting: contact angle is greater than 90°
• Wetting behavior determined by the balance of adhesive forces (between the liquid and solid) and cohesive forces (within the liquid)

Importance of Wetting in Applications

• Crucial in various applications (adhesion, coating, printing, oil recovery) as it affects the performance and quality of materials and processes
• Examples:
  • Adhesion of paint or coating to a surface
  • Spreading of ink on paper in printing processes
  • Efficiency of oil recovery from porous rocks

Contact Angle Measurement

Sessile Drop and Wilhelmy Plate Methods

• Sessile drop method: placing a liquid droplet on a solid surface and measuring the contact angle using a goniometer or image analysis software
• Wilhelmy plate method: measures the force exerted on a thin plate partially immersed in a liquid, related to the contact angle through the Wilhelmy equation
  • Advancing and receding contact angles can be measured by increasing or decreasing the volume of the liquid droplet
  • Provides information on surface heterogeneity and hysteresis

Other Measurement Techniques

• Captive bubble method for hydrophobic surfaces: an air bubble is placed beneath the solid surface immersed in a liquid, and the contact angle is measured
• Tilting plate method: solid surface is tilted until the liquid droplet starts to move
  • Advancing and receding contact angles are measured at the front and back of the droplet, respectively
• Examples:
  • Measuring the contact angle of water on a superhydrophobic surface using the sessile drop method
  • Determining the advancing and receding contact angles of an oil droplet on a polymer surface using the tilting plate method

Interpreting Contact Angle Data

Surface Energy Determination

• Young's equation relates the contact angle to the interfacial tensions between the solid, liquid, and vapor phases, allowing the calculation of surface energy
• Owens-Wendt method and van Oss-Chaudhury-Good (OCG) method: commonly used to determine the surface energy components (disperse and polar) from contact angle measurements with multiple liquids
• Zisman plot: represents the cosine of the contact angle versus the surface tension of various liquids, used to estimate the critical surface tension of a solid

Wetting Parameters and Graphical Representations

• Work of adhesion: work required to separate the liquid-solid interface, calculated from the contact angle and the liquid surface tension using the Young-Dupré equation
• Wetting envelope: graphical representation of the wetting behavior of a solid surface, showing the range of surface tensions and polarities of liquids that can wet the surface
• Examples:
  • Calculating the surface energy of a polymer using the Owens-Wendt method with contact angle data from water and diiodomethane
  • Constructing a wetting envelope for a metal surface to determine the range of liquids that can wet it

Factors Influencing Wetting

Surface Roughness and Heterogeneity

• Surface roughness affects wetting by altering the actual surface area and creating air pockets between the liquid and the solid
  • Wenzel state: homogeneous wetting, where the liquid fills the surface asperities
  • Cassie-Baxter state: heterogeneous wetting, where the liquid sits on a mixture of solid and air pockets
• Wenzel equation relates the apparent contact angle to the intrinsic contact angle and the roughness factor (ratio of the actual surface area to the projected surface area)
• Cassie-Baxter equation describes the apparent contact angle on a composite surface consisting of solid and air fractions
• Chemical heterogeneity (hydrophobic and hydrophilic domains on a surface) can lead to contact angle hysteresis and pinning effects

Surface Treatments and Special Wetting Behaviors

• Surface treatments (plasma treatment, chemical functionalization, self-assembled monolayers) can modify the surface energy and wetting behavior by altering the chemical composition or structure of the surface
• Superhydrophobic surfaces: water contact angles greater than 150° and low contact angle hysteresis, achieved by combining surface roughness and low surface energy materials
  • Mimicking natural surfaces like lotus leaves
• Examples:
  • Creating a superhydrophobic coating on a fabric surface by applying a rough, low surface energy material
  • Modifying the wetting behavior of a glass surface by applying a self-assembled monolayer of a hydrophobic silane