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Tribological processes determine the lifespan and functional properties of devices and machinery across almost every industry. This realization has led researchers to further investigate tribological effects beyond the macroscopic level, which has led to the continuously advancing field of nanotribology.
What is Nanotribology?
Tribology encompasses the science, technology and subjects related to interactive surfaces in motion.
Through the use of analytical instruments such as scanning tunneling microscopy (STM), atomic force microscopy (ATM), and other computational techniques, nanotribology emerged to further understand the mechanisms and dynamics that arise when two surfaces collide at both the atomic and molecular level.
Some of the most widely studied interfacial processes studied in nanotribology include those which occur during adhesion, friction, scratching, wear and indentation on solid surfaces.
The Importance of Nanotribology
Regardless of how smooth a surface might appear or how precise the machining method used to create the surface is, every surface will contain some form of irregularities at the atomic level.
These microscopic differences can, therefore, determine the tribological processes that will arise when the surfaces of these materials come in contact with each other. Any alterations in the physical properties of a surface, regardless of the scale in which they occur, can contribute to several important tribological processes such as wear and friction.
Over the past several years, numerous micro-electromechanical systems (MEMS) have been developed for biomedical applications, including biosensors and micro implants.
Although silicon has the potential to cause undesirable immune responses in humans, it is the most common substrate found in these bio-MEMS devices.
To reduce the occurrence of adverse immune reactions and ultimately prevent rejection, protein coatings are often used on silicon-based surfaces.
In addition to its biocompatibility properties, protein coatings are also widely used for their functional specificity and affinity for specific antigens.
When these bio-MEMS devices are used in vivo, there is a potential for adhesion to occur between the surface coating, which can have harmful effects on its sensing reliability. As a result, researchers have found that enhancing the wear resistance of biosensor surfaces, particularly that which exists on the protein substrate, is crucial to ensuring its consistent performance.
Human hair is a nanocomposite biological fiber that is often subject to a wide range of natural weathering processes, as well as a variety of mechanical and chemical processes that can damage and weaken these fibers.
Some mechanical processes that alter human hair include combing, cutting and blow drying, whereas common chemical processes include chemical dying and permanent wave treatments.
Tribology at the macro-, micro- and nanoscale play important roles in enhancing the appearance and overall health of human hair. For example, conditioners often contain cationic surfactants, silicones, fatty alcohols and water, which, taken together, forms a positively charged network of molecules.
During the application of a conditioner product, its positive molecules are immediately attracted to the negatively charged particles present on hair fibers. Many of the desirable features that conditioners provide, such as a smooth feel in both wet and dry environments, are a direct result of tribological attributes such as reduced friction and adhesion between the hair and skin.
Advancements in Nanotribology Techniques
Nanomechanical properties, which can include nanoindentation hardness and the elastic modulus at shallow depths, are often measured through depth-sensing nano- and picoindentation systems.
Although atomic force microscopy (AFM) can be used for nanoindentation measurement purposes, quasi-static nanoindentation has emerged as the standard technique.
The basic principle of any quasi-static nanoindentation test involves the application and immediate removal of a load from a sample, both of which are conducted in a highly controlled environment with a geometrically well-defined probe.
More specifically, the transducer of the nanoindentation system applies a force to the surface of a sample. As the transducer moves across the surface, the displacement of the probe is continuously measured, creating a force vs. displacement curve that serves as the ‘mechanical fingerprint’ of the sample material. Other quantitative information provided by this technique includes toughness, stiffness, delamination force and film thickness.
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US-based Bruker produces a wide range of high-performance scientific instruments and offers a line of Hysitron standalone nanoindentation systems capable of in-situ scanning probe microscopy (SPM) imaging.
Bruker’s hybrid nanoindentation imaging techniques are capable of in-situ SPM, as well as transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman and fluorescence microscopy.
As compared with the standalone nanoindentation techniques, Bruker’s hybrid nanoindentation systems are unique in their ability to perform nanomechanical and nanotribiological testing in non-ambient environmental conditions. This environmental characterization capability allows these hybrid systems to effectively characterize how the presence of temperature, humidity, atmospheric gasses, electrochemical reactions, and hydration can determine the nanotribological properties of tested samples.
References and Further Reading
Bhushan, B. (2020) Frontiers in nanotribology: Magnetic storage, bio/nanotechnology, cosmetics, and bioinspiration. Journal of Colloid and Interface Science 577; 127-162. doi:10.1016/j.jcis.2020.05.040.
Bhushan, B. (2010) Chapter 7: Surface Potential Studies of Human Hair Usign Kelvin Probe Microscopy. In: Biophysics of Human Hair: Structural, Nanomechanical and Nanotribological.
Bruker. Quasi-Static Nanoindentation: An Overview. [Online] Available at: https://www.bruker.com/products/surface-and-dimensional-analysis/nanomechanical-test-instruments/landing-pages/nanoindentation.html (Accessed on 16 June 2020).
Bruker. Hybrid Nanoindentation – Techniques and Properties. [Online] Available: https://www.bruker.com/products/surface-and-dimensional-analysis/nanomechanical-test-instruments/techniques-and-properties.html (Accessed on 16 June 2020).
Bruker. Environmental Characterization. [Online] Available at: https://www.bruker.com/products/surface-and-dimensional-analysis/nanomechanical-test-instruments/techniques-and-properties/environmental-characterization.html (Accessed on 16 June 2020).