What Controls the Friction on Soft Surfaces that have Periodic Structures?

Introduction

Advancements in tribology increasingly depend on understanding how surface structures influence friction, especially in soft materials used in modern technologies such as wearable devices, tactile systems, and biomedical interfaces. In a recent webinar hosted by TriboNet, Alison presented collaborative research with Shelby Hutchins’ group, focusing on how periodic soft surfaces can be characterized and how their structure affects frictional behavior.

The session offered a comprehensive exploration of experimental techniques and data interpretation. For readers who want to revisit the foundational concepts behind these findings, TriboNet provides a helpful overview of What Tribology Is, explaining how friction, wear, and lubrication interact in different systems.

Engineering Soft Surfaces with Controlled Properties

The research focused on polydimethylsiloxane (PDMS)-based materials, engineered into three types of surfaces: smooth, periodic, and rough. By adding 10% oil, the researchers adjusted the elastic modulus, making the material more representative of soft, real-world applications.

Periodic surfaces were the primary focus because of their predictable structure. Unlike random roughness, periodic patterns allow researchers to systematically study how geometry influences friction. This is particularly relevant in applications like tactile sensing, where consistent surface response is critical.

To better understand how surface structure affects performance, you can explore Wear in Tribology, which explains how surface features contribute to friction and material degradation.

Measuring Surface Topography Accurately

Characterizing soft surfaces requires careful measurement techniques. In this study, optical surface profilometry was used to scan large samples at the centimeter scale. Because of the sample size, multiple scans were stitched together, and polynomial filtering was applied to isolate fine surface features.

One key takeaway was the importance of magnification. Low magnification scans can exaggerate surface features, leading to misleading conclusions. In contrast, higher magnification (up to 20X) provides more accurate measurements of parameters such as RMS roughness and developed area ratio.

This reinforces a core principle in tribology: measurement conditions significantly influence results. Understanding these limitations is essential for producing reliable and reproducible data.

Quantifying Periodicity in Surface Structures

To analyze periodic surfaces, the researchers used several techniques, including peak detection, power spectral density, and height–height correlation. Among these, the height–height correlation method proved most effective, revealing dominant periodic features around 600 microns.

This method is particularly valuable because it captures spatial relationships across the surface, rather than focusing only on isolated peaks. As a result, it provides a clearer understanding of how surface patterns repeat and influence performance.

Friction Testing and Experimental Setup

Friction measurements were conducted using a microtribometer under controlled conditions. Steel probes with different radii were used, with normal loads ranging from very small values to higher levels, and sliding speeds between 0.1 and 5 mm/s.

These testing approaches are consistent with standard methods used in tribology research. For additional context, you can refer to Friction Basics in Tribology, which explains how controlled experiments are designed to evaluate friction behavior under different conditions.

How Load Influences Friction Behavior

One of the most important findings from the webinar was the relationship between normal load and friction. At high loads, all surfaces—smooth, periodic, and rough—showed similar friction coefficients of approximately 0.8. This suggests that surface features are flattened under pressure, reducing their influence.

However, at lower loads, the differences became much more pronounced. Smooth surfaces exhibited lower friction, while periodic and rough surfaces showed higher values. This indicates that surface texture plays a dominant role when contact pressure is low.

This behavior aligns with fundamental tribology principles, where friction depends not only on material properties but also on surface geometry and contact conditions.

Shear Stress as a More Reliable Metric

Instead of relying solely on the friction coefficient, Alison introduced characteristic shear stress as a more robust metric. By incorporating the Hertzian contact area, shear stress accounts for both material properties and surface structure.

Unlike the friction coefficient, which can vary significantly with load, shear stress remained relatively stable across different conditions. This makes it a more reliable parameter for comparing different surfaces.

Creating Periodic Surfaces Through Controlled Cutting

The webinar also introduced a fabrication method based on controlled cutting. In this process, elastomeric samples are pushed over a razor blade, and the balance between cutting and tearing determines the resulting surface structure.

When cutting dominates, periodic features are formed. When tearing becomes more significant, the surface becomes rough and irregular. Interestingly, the force signal during the cutting process itself showed periodic patterns that directly corresponded to the resulting surface features.

This establishes a strong link between fabrication method, surface topography, and frictional performance.

Conclusion

This TriboNet webinar offers valuable insight into how periodic soft surfaces can be accurately characterized and how their structure directly influences frictional behavior. The study highlights the importance of surface texture at low loads, where small differences in topography can lead to significant changes in performance.

It also emphasizes the need for more reliable evaluation methods, such as shear stress, instead of relying only on friction coefficients. Alongside this, factors like measurement scale and probe size play a crucial role in ensuring accurate and meaningful results.

Overall, the webinar reinforces that understanding the relationship between surface design, measurement techniques, and friction is essential for developing more effective and optimized materials across various applications.