When are rough surfaces sticky? (1311.1178v1)

Abstract: At the molecular scale there are strong attractive interactions between surfaces, yet few macroscopic surfaces are sticky. Extensive simulations of contact by adhesive surfaces with roughness on nanometer to micrometer scales are used to determine how roughness reduces the area where atoms contact and thus weakens adhesion. The material properties, adhesive strength and roughness parameters are varied by orders of magnitude. In all cases the area of atomic contact rises linearly with load, and the prefactor rises linearly with adhesive strength for weak interactions. Above a threshold adhesive strength, the prefactor changes sign, the surfaces become sticky and a finite force is required to separate them. A parameter-free analytic theory is presented that describes changes in these numerical results over up to five orders of magnitude in load. It relates the threshold strength to roughness and material properties, explaining why most macroscopic surfaces do not stick. The numerical results are qualitatively and quantitatively inconsistent with classical theories based on the Greenwood-Williamson approach that neglect the range of adhesion and do not include asperity interactions.

• The paper demonstrates how simulated contact area increases linearly with load until a threshold adhesive strength induces a sticky transition.
• It details how geometrical features, like rms slope and curvature, critically predict adhesive interactions beyond classical models.
• The findings provide actionable insights for engineering surfaces in applications such as gecko-inspired adhesives and flexible electronics.

Understanding Adhesive Interactions of Rough Surfaces

The paper by Pastewka and Robbins addresses the complex issue of adhesion between rough surfaces, a significant area of paper within the field of contact mechanics. The central question posed is when do rough surfaces become sticky, given the ubiquitous attractive forces at the molecular scale, such as van der Waals forces, yet the apparent lack of stickiness in most macroscopic surfaces.

Overview of Methodology

The authors utilized extensive simulations of adhesive contact between surfaces, varying in roughness from nanometer to micrometer scales. The focus was to understand how surface roughness affects the real area of atomic contact, consequently weakening adhesion. The paper varied material properties, adhesive strengths, and roughness parameters extensively to establish a comprehensive understanding. A critical development from this work is a parameter-free analytic theory correlating numerical results and predicting adhesion effects based on these complex dynamics.

Key Findings

1. Linear Relationship Between Load and Contact Area: Regardless of the roughness scales examined, the area of atomic contact increased linearly with applied load. This finding contradicts classical theories, such as the Greenwood-Williamson model, which assumed spherical asperities and neglected significant factors like asperity interactions.
2. Role of Adhesive Strength: For weak interactions, the contact area increased linearly with the adhesive strength. However, beyond a specific threshold of adhesive strength, the surfaces transitioned into sticky behavior requiring finite force for separation.
3. Simulations vs. Classical Theories: The numerical results indicated inconsistencies with classical models, especially in factors such as the treatment of the range of the adhesive forces and interactions between asperities.
4. Geometric Characterization: The paper provided detailed insights into geometrical features critical to adhesive interactions, such as the fractal nature of contact areas, characterized by statistical properties like the root mean square (rms) slope and curvature. These properties were significant predictors of adhesive behavior.
5. Implications for Macroscopic Adhesion: The theory articulates why macroscopic surfaces do not exhibit the stickiness predicted by molecular-scale forces – the interplay between roughness and adhesive pressure plays a critical role, with the adhesive interaction's range being a decisive factor.

Theoretical and Practical Implications

The paper's outcomes have significant theoretical implications, challenging traditional theories on rough-surface adhesion, which often place undue emphasis on parameters such as rms roughness without fully accounting for the microscopic interactions that govern macroscopic behavior.

From a practical perspective, the results can guide the design of engineered surfaces where control over adhesion is crucial, such as in the development of gecko-inspired adhesives or in industries involving flexible electronics and medical devices.

Future Directions

The paper paves the way for future investigations into tailoring surface interactions by manipulating roughness and material properties to achieve desired adhesive properties. This could involve exploring further the parameter spaces outlined in this paper, especially the interrelation between surface energies, elastic moduli, and surface topographies.

Conclusion

Pastewka and Robbins' paper provides a rigorous and quantitative exploration of adhesion in rough surfaces, emphasizing the importance of contact geometry and providing a robust theoretical framework to address the adhesion paradox. This work not only deepens our understanding of surface interactions but also extends practical insights into the creation of new materials and surfaces with customizable adhesive properties.