3.7 The effects of materials' hierarchical structure

In their exploration of smart materials with tunable friction, scientists have made interesting observations regarding the structure of materials. In experiments they found that friction forces are the result of various types of complex multi-scale interactions between sliding surfaces.
Many natural and man-made materials exhibit structures on more than one length scale; in some materials, the structural elements themselves have structure. This structural hierarchy can play a major part in determining the bulk material properties.
Hierarchical structure of the Eiffel Tower
Hierarchical structure of the Eiffel Tower. As a whole, the tower is composed of pig iron and stands over 300 meters tall, with a square base of 100 meters per side. The larger structure is composed of a latticework where diagonal girders connect elements together. These units, with dimensions on the order of tens of meters, are composed of girders on the order of meters in length. Each individual girder has cross-sectional dimensions of 10-2 to 10-1 meters composed of an iron material with an atomic structure made up primarily of iron, carbon, silicon and manganese ions in a lattice structure on the Angstrom scale (10-10 meters). (Image: Bone Biology and Mechanics Lab, Indiana University-Purdue University at Indianapolis)
Hierarchical structures are very common in nature; but only recently have they been systematically studied in materials science, in order to understand the specific effects they can have on the mechanical properties of various systems.
Take animals that cling to walls and walk on ceilings. As we have seen before, they owe this ability to micro- and nanoscale attachment elements.
The highest adhesion forces in the animal kingdom are encountered in geckos; they have evolved one of the most versatile and effective dry adhesives known to man. A gecko is the heaviest animal that can ‘stand’ on a ceiling, with its feet over its head. This is why scientists are intensely researching the adhesive system of the tiny hairs on its feet.
The gecko paw's hierarchical structure – its branches become increasingly smaller over three levels – has attracted much research interest.
Nevertheless, to illustrate how ingenious Nature's design is, researchers haven't yet managed to come up with a simple recipe for fabricating synthetic gecko adhesives.
In 2000, scientists discovered (PNAS, "Evidence for van der Waals adhesion in gecko setae") that van der Waals’ interactions were the main contributors to gecko adhesion force: the nanostructures at the end of the gecko's hairs interact at the atomic level with molecules on the surface the gecko is trying to grip. This interaction, powered by van der Waals forces, causes the gecko’s toes to easily attach and detach as needed.
Since then, many research groups around the world have spent considerable effort in trying to fabricate synthetic gecko adhesives, i.e. artificial materials that reproduce its peculiar properties of adhesion and friction.
Hierarchical structure of the gecko's paw
Hierarchical structure of the gecko's paw (click image to enlarge). (Creative Commons Attribution-ShareAlike 3.0)
Understanding the effects of hierarchical structure and how they influence macroscopic mechanical properties (such as friction) can guide the synthesis of new materials with physical properties, which are tailored for specific applications.
This means that structural hierarchy can provide a way to tune and optimize the tribological properties of a surface. Experimental results by U.S. scientists indicate that by exploiting hierarchical structure, the global friction properties of a surface can be tuned arbitrarily. To achieve this, it is essential to provide structuring at various different length scales (see: Static and dynamic friction of hierarchical surfaces).
Soft robotic gripper with gecko inspired adhesives
Science notes: Smart materials with tunable friction
Key takeaways
• Researchers are working on a new class of 'smart' materials with the ability to modulate the friction of a surface.
• There are several approaches, quite different from each other, to fabricate these smart materials.
• The hierarchical structure of materials affects their mechanical properties. Understanding how they influence macroscopic mechanical properties (such as friction) can guide the synthesis of new materials with physical properties, which are tailored for specific applications.
Vocabulary
Smart material: A functional material that can reversibly respond to changes in its environment via external stimuli such as temperature, light, pressure or electricity.
Photochromic molecules: Molecules whose properties can be changed reversibly under the action of light as an external stimulus.
Hydrogels: Swollen polymer networks, class of materials closely resembling biological tissues in their physical and chemical properties.
Hierarchical materials: Materials that exhibit structures on more than one length scale.
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