9.2 Superhydrophobic and superhydrophilic surfaces

Superhydrophobic and superhydrophilic surfaces are all about extremes in water interaction. These surfaces either repel water like crazy or make it spread out super fast, thanks to clever combinations of surface chemistry and texture.

Nature's got some cool tricks up its sleeve when it comes to water-repelling or water-loving surfaces. By mimicking these, we can create materials with amazing properties for things like self-cleaning, anti-fogging, and even oil spill cleanup.

Surface Wetting Models

Contact Angle and Surface Tension

Top images from around the web for Contact Angle and Surface Tension

• 

  Surface tension - Wikipedia View original

  Is this image relevant?

• 

  Functional superhydrophobic surfaces made of Janus micropillars - Soft Matter (RSC Publishing) View original

  Is this image relevant?

• 

  Surface tension - Wikipedia View original

  Is this image relevant?

• 

  Functional superhydrophobic surfaces made of Janus micropillars - Soft Matter (RSC Publishing) View original

  Is this image relevant?

1 of 2

Top images from around the web for Contact Angle and Surface Tension

• 

  Surface tension - Wikipedia View original

  Is this image relevant?

• 

  Functional superhydrophobic surfaces made of Janus micropillars - Soft Matter (RSC Publishing) View original

  Is this image relevant?

• 

  Surface tension - Wikipedia View original

  Is this image relevant?

• 

  Functional superhydrophobic surfaces made of Janus micropillars - Soft Matter (RSC Publishing) View original

  Is this image relevant?

1 of 2

• Contact angle quantifies the wettability of a solid surface by a liquid
  • Measured as the angle formed between the liquid-solid interface and the liquid-vapor interface at the three-phase contact line
  • Depends on the interfacial tensions between the solid, liquid, and vapor phases (water on a hydrophobic surface like Teflon has a high contact angle)
• Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible due to cohesive forces between liquid molecules
  • Plays a crucial role in determining the shape of liquid droplets and their interaction with solid surfaces (mercury has a high surface tension, causing it to form nearly spherical droplets)

Wetting Models: Wenzel and Cassie-Baxter

• Wenzel model describes the wetting behavior of a liquid on a rough surface where the liquid completely penetrates the surface roughness
  • Assumes that the liquid follows the contours of the rough surface, increasing the actual surface area of the liquid-solid interface
  • Predicts that surface roughness amplifies the intrinsic wettability of the material (roughness increases hydrophobicity on hydrophobic surfaces and hydrophilicity on hydrophilic surfaces)
• Cassie-Baxter model describes the wetting behavior of a liquid on a rough surface where the liquid sits on top of the surface roughness, trapping air pockets underneath
  • Assumes a composite interface consisting of solid-liquid and liquid-air interfaces
  • Predicts that the apparent contact angle depends on the fraction of solid surface in contact with the liquid and the fraction of air trapped beneath the liquid (lotus leaf exhibits Cassie-Baxter wetting, allowing water droplets to easily roll off its surface)

Surface Structures and Properties

Hierarchical Structures and Water Repellency

• Hierarchical structures are multi-scale surface features that exhibit roughness at different length scales, from nano to micro levels
  • Found in many natural surfaces, such as lotus leaves, rose petals, and butterfly wings
  • Contribute to enhanced water repellency by trapping air pockets and reducing the contact area between the liquid and the solid surface (gecko feet have hierarchical structures of setae and spatulae, enabling them to adhere to surfaces)
• Water repellency, or hydrophobicity, is the ability of a surface to repel water droplets
  • Characterized by high contact angles (typically greater than 90°) and low contact angle hysteresis
  • Can be achieved through a combination of low surface energy materials and appropriate surface roughness (superhydrophobic surfaces like the lotus leaf have contact angles greater than 150°)

Water Spreading and Superhydrophilicity

• Water spreading refers to the ability of a liquid to spread out and form a thin film on a surface
  • Occurs when the contact angle is close to 0°, indicating complete wetting
  • Can be enhanced by surface roughness and high surface energy materials (titanium dioxide exhibits superhydrophilicity under UV light exposure)
• Superhydrophilicity is an extreme case of water spreading, where the contact angle is nearly 0° and water rapidly spreads across the surface
  • Often achieved through a combination of surface chemistry and surface roughness
  • Has applications in self-cleaning, anti-fogging, and oil-water separation (superhydrophilic coatings on solar panels can prevent dust accumulation and maintain high energy conversion efficiency)

Advanced Surface Coatings

Superhydrophobic Coatings

• Superhydrophobic coatings are engineered surfaces that exhibit extremely high water repellency, with contact angles greater than 150° and low contact angle hysteresis
  • Inspired by natural surfaces like the lotus leaf and water strider legs
  • Typically achieved through a combination of low surface energy materials (fluoropolymers, silicones) and hierarchical surface structures
  • Have applications in self-cleaning, anti-icing, and corrosion resistance (superhydrophobic coatings on aircraft wings can prevent ice accumulation and improve flight safety)

Superhydrophilic Coatings

• Superhydrophilic coatings are engineered surfaces that exhibit extremely high water affinity, with contact angles close to 0° and rapid water spreading
  • Often achieved through a combination of high surface energy materials (titanium dioxide, zinc oxide) and surface roughness
  • Can be triggered by external stimuli, such as UV light or electrical potential
  • Have applications in anti-fogging, self-cleaning, and oil-water separation (superhydrophilic coatings on bathroom mirrors can prevent fogging and maintain clear visibility)