🦎Biomimetic Materials Unit 9 – Biomimetic Materials: Surface Properties

Key Concepts and Definitions

• Biomimetics involves studying and emulating biological systems to develop innovative solutions and technologies
• Surface properties encompass characteristics such as wettability, adhesion, friction, and self-cleaning abilities
• Hierarchical structures consist of multi-scale features that contribute to unique surface properties (lotus leaf)
• Superhydrophobicity refers to surfaces with water contact angles greater than 150° and low contact angle hysteresis
  • Achieved through a combination of surface chemistry and micro/nanoscale roughness
• Superhydrophilicity describes surfaces with water contact angles close to 0°, allowing water to spread completely
• Omniphobicity extends beyond water repellency to include resistance to various liquids (oils, alcohols)
• Bioadhesion involves the attachment of organisms to surfaces through specialized mechanisms (gecko feet, mussel byssus threads)
• Antifouling properties prevent the accumulation of unwanted substances or organisms on surfaces (shark skin)

Natural Surface Structures and Functions

• Lotus leaf exhibits superhydrophobicity and self-cleaning due to hierarchical micro/nanoscale structures and epicuticular wax
  • Water droplets easily roll off the surface, collecting dirt particles in the process
• Gecko feet possess millions of microscopic hair-like structures called setae, enabling strong dry adhesion
  • Each seta further divides into hundreds of nanoscale spatulae, maximizing contact area and van der Waals forces
• Shark skin comprises dermal denticles arranged in a specific pattern, reducing drag and preventing biofouling
• Butterfly wings display structural coloration and directional wettability through precise micro/nanostructures
  • Scales on the wings create intricate photonic crystals that selectively reflect light
• Desert beetles (Stenocara gracilipes) harvest water from fog using hydrophilic bumps and hydrophobic troughs on their back
• Pitcher plants secrete a slippery liquid layer on their peristome surface to trap insects
  • The anisotropic microstructure of the peristome causes insects to lose traction and fall into the pitcher
• Namib desert grass (Stipagrostis sabulicola) collects water from fog using longitudinal grooves and microstructures on its leaves

Biomimetic Surface Design Principles

• Understand the underlying mechanisms and structures responsible for desired surface properties in natural systems
• Identify key micro/nanoscale features and their arrangement that contribute to specific functionalities
• Consider the role of hierarchical structures in enhancing surface properties and performance
  • Combine multiple length scales (micro and nano) to achieve synergistic effects
• Optimize surface chemistry and topography to tailor wettability, adhesion, and other properties
  • Modify surface energy through chemical functionalization or coatings
• Ensure the stability and durability of biomimetic surfaces under various environmental conditions
• Incorporate responsive or adaptive elements to create dynamic and smart surfaces
  • Respond to external stimuli such as temperature, pH, or light
• Simplify and streamline designs for scalability and manufacturing feasibility
• Conduct thorough characterization and testing to validate the performance of biomimetic surfaces

Fabrication Techniques for Biomimetic Surfaces

• Lithography methods (photolithography, soft lithography) create precise micro/nanopatterns on surfaces
  • Involve the use of masks or templates to selectively expose and modify surface regions
• Etching processes (plasma etching, chemical etching) remove material from surfaces to create desired textures and structures
• Self-assembly techniques leverage the spontaneous organization of molecules or particles to form ordered structures
  • Examples include block copolymer self-assembly and colloidal self-assembly
• Sol-gel processing enables the formation of porous and hierarchical structures through the controlled hydrolysis and condensation of precursors
• Electrospinning produces micro/nanofibers by applying a high electric field to a polymer solution
  • Fibers can be collected and arranged to mimic natural fibrous structures
• 3D printing offers the ability to fabricate complex geometries and hierarchical structures layer by layer
• Chemical vapor deposition (CVD) deposits thin films of functional materials onto surfaces
  • Allows precise control over composition and thickness
• Atomic layer deposition (ALD) enables the conformal coating of surfaces with ultrathin layers of materials
  • Provides excellent control over film thickness and uniformity

Characterization Methods for Surface Properties

• Contact angle measurements quantify surface wettability by measuring the angle formed between a liquid droplet and the surface
  • Static contact angle indicates the equilibrium wetting behavior
  • Dynamic contact angle (advancing and receding angles) provides information about contact angle hysteresis
• Scanning electron microscopy (SEM) captures high-resolution images of surface morphology and topography
  • Reveals micro/nanoscale features and their spatial arrangement
• Atomic force microscopy (AFM) maps surface topography and measures local surface properties (adhesion, friction) with nanoscale resolution
  • Provides quantitative data on surface roughness and force interactions
• X-ray photoelectron spectroscopy (XPS) analyzes the chemical composition of surfaces
  • Identifies the presence of specific elements and their bonding states
• Fourier-transform infrared spectroscopy (FTIR) detects the presence of functional groups on surfaces
• Optical profilometry measures surface roughness and texture non-destructively
  • Generates 3D surface maps and extracts roughness parameters
• Wetting dynamics studies observe the behavior of liquids on surfaces (droplet impact, spreading, and bouncing)
  • High-speed imaging captures the time-dependent wetting processes
• Biofouling assays assess the antifouling performance of surfaces by exposing them to biological organisms (bacteria, algae)
  • Quantifies the attachment and growth of fouling species on the surface

Applications in Engineering and Technology

• Self-cleaning surfaces inspired by lotus leaves for easy maintenance and reduced cleaning costs (building materials, solar panels)
  • Superhydrophobic coatings repel water and contaminants, allowing them to be easily removed
• Drag-reducing surfaces modeled after shark skin for enhanced hydrodynamic efficiency (ships, swimsuits)
  • Microstructured surfaces minimize turbulence and flow separation, leading to reduced drag forces
• Dry adhesives based on gecko feet for reversible and strong adhesion without the need for chemical glues (robotics, climbing gear)
  • Micro/nanoscale arrays of high aspect ratio structures enable high adhesion strength and easy release
• Anti-icing surfaces that prevent or delay ice formation and accretion (aircraft wings, power lines)
  • Superhydrophobic and low-adhesion surfaces reduce ice nucleation and adhesion strength
• Water harvesting surfaces inspired by desert beetles and plants for sustainable water collection in arid regions
  • Patterned surfaces with alternating hydrophilic and hydrophobic regions guide water droplets to collection points
• Structural color surfaces that produce vibrant and iridescent colors without the use of pigments (displays, packaging)
  • Precise micro/nanostructures interact with light to create angle-dependent color effects
• Antifouling surfaces that resist the attachment of bacteria, algae, and other organisms (medical devices, marine equipment)
  • Microstructured surfaces and chemical modifications disrupt the settling and adhesion of fouling species
• Biosensors with enhanced sensitivity and specificity enabled by biomimetic surface functionalization
  • Micro/nanostructured surfaces increase the surface area and improve the immobilization of biorecognition elements

Challenges and Future Directions

• Scalable and cost-effective manufacturing methods for large-area fabrication of biomimetic surfaces
  • Develop high-throughput and roll-to-roll compatible techniques
• Long-term stability and durability of biomimetic surfaces under real-world conditions
  • Address issues of mechanical wear, chemical degradation, and environmental factors
• Integration of multiple functionalities on a single surface for multifunctional performance
  • Combine self-cleaning, antifouling, and sensing capabilities on a single platform
• Adaptive and responsive surfaces that dynamically adjust their properties based on external stimuli
  • Incorporate smart materials and active elements for real-time adaptation
• Theoretical modeling and simulation tools to predict and optimize the performance of biomimetic surfaces
  • Develop multiscale models that bridge molecular interactions with macroscopic behavior
• Exploration of new biological systems and mechanisms for novel surface functionalities
  • Investigate lesser-known organisms and their unique surface properties
• Sustainable and eco-friendly materials and fabrication processes for biomimetic surfaces
  • Prioritize the use of renewable resources and minimize environmental impact
• Collaborative research efforts across disciplines (biology, materials science, engineering) to accelerate progress
  • Foster interdisciplinary knowledge exchange and integrated approaches to biomimetic surface design

Case Studies and Real-World Examples

• Lotusan paint by Sto Corp. mimics the self-cleaning properties of lotus leaves for building facades
  • Superhydrophobic coating keeps surfaces clean and dry, reducing maintenance requirements
• Sharklet micropattern by Sharklet Technologies inspired by shark skin for antifouling applications
  • Microscale diamond-shaped pattern disrupts the attachment of bacteria and other microorganisms
• Geckskin adhesive by UMass Amherst based on gecko feet for strong and reversible adhesion
  • Soft elastomeric pad with stiff fibers achieves high adhesion strength on various surfaces
• Slips technology by SLIPS Technologies Inc. inspired by pitcher plants for liquid-repellent surfaces
  • Lubricant-infused porous surface creates a stable and slippery interface that repels various liquids
• Mirasol display by Qualcomm utilizing structural color principles from butterfly wings
  • Microelectromechanical systems (MEMS) with interferometric modulation produce vivid and low-power displays
• Fog nets inspired by desert beetles and plants for water harvesting in arid regions
  • Mesh structures with hydrophilic and hydrophobic patterns collect water droplets from fog
• Bioinspired underwater adhesives developed by researchers at Purdue University based on mussel adhesion
  • Catechol-functionalized polymers form strong and reversible bonds in wet environments
• Salvinia effect coatings inspired by the Salvinia molesta plant for oil-water separation
  • Hydrophobic and oleophilic microhairs trap oil while repelling water, enabling efficient separation