Measuring adhesion and friction of polymer nanofibers

MEMS nanofiber
A polymer nanofiber, smaller than one hundredth the size of a human hair, mounted on a MEMS mechanical testing device. The inset shows two devices positioned perpendicularly so that adhesion and friction forces could be simultaneously measured at the intersecting point of contact. Credit: University of Illinois at Urbana-Champaign

Nanofibers are tiny fibers with diameters measured in the nanometer range. They can be generated by a variety of methods, from various different polymers, therefore they can exhibit different physical properties with different application potentials. The diameters of nanofibers will depend on the variety of polymer used and the method of generation. The most common method of nanofiber generation is by electrospinning which enables mass production of continuous fibers from various polymers. Electrospinning gives the capability to generate ultrathin fibers with controllable parameters like diameter, orientation and composition.

Naturally occurring polymers include silk, keratin, gelatin and polysaccharides, collagen and cellulose. Synthetic polymers include compounds like polyurethane, poly(ethylene-co-vinylacetate) and polycaprolactone. Compared with microfibers, polymer nanofibers have a large surface area-to-volume ratio, are highly porous and exhibit high mechanical strength in a small footprint. Nanofibers have a wide range of commercial and technology applications, being invaluable in areas like tissue engineering and drug delivery. Other important applications include their use in cancer diagnosis, optical sensors, air filtration and even the production of seed coating material.

Studies conducted at the University of Illinois at Urbana-Champaign, were designed to further explore the properties of nanoscale polymer fibers. Their findings have implications for the design and manufacturing of materials and products generated from random networks of filaments, for example, to produce robust filters that are designed to protect our lungs from the introduction of foreign particles.

Debashish Das is a postdoctoral scholar in the Department of Aerospace Engineering at the University of Illinois at Urbana-Champaign. He explained that networks of interconnected filaments are everywhere in biological and bioengineered systems like spider webs, connective tissue and as scaffolds for tissue growth. They also play an important role in consumer products like air filters.

His research uncovered direct insights into the manner in which adhesion and friction work together at the nanometer length scale. Nanoscale fibers of similar materials strongly adhere to each other and this raises separation challenges. If they are forcefully separated, they will spontaneously stick together again. Gaining further insights into these phenomena provides direct implications on the design of tough, resilient networks of soft nanofibers.

The studies used a MEMS mechanical testing device, a device small enough to fit on the head of a pin, to test a single polymer nanofiber. The nanofiber was smaller than one hundredth the thickness of a human hair. Two tiny, micro-sized MEMS devices were placed perpendicular to each other to simultaneously measure the adhesion and friction forces at the intersecting point of contact. These tiny machines, designed and fabricated for the studies, are smaller than one millimeter in size.

The research revealed that as they examined the fibers and other surfaces from microscales to nanoscales, the landscape changed. As they went smaller, down to the nanometer length scales, the surface area of fibers decreased more slowly compared with the volume, and everything became stickier.

Das conducted experiments in a network of crisscrossing nanofibers with millions of junctions. The aim was to discover what happened at just one of the overlapping junctions. He measured the force required to separate two nanofibers at the junction where they crossed. This knowledge could then feasibly be applied to the network at the macroscale which can potentially be comprised of billions of nanofibers.

Das said, “In a previous study, we used a MEMS device to stretch a single collagen fiber. In this study, we coupled two MEMS devices oriented orthogonally to push two fibers together and then separated them by sliding. While doing so we were able to simultaneously measure the force due to adhesion and due to friction. This was the first time such complete measurements were made possible for nanoscale fibers.

“From our experimental measurements, we calculated the size of the contact area that is formed between the two nanofiber surfaces at their junction. As we applied a sliding force, the contact started peeling until the sliding force suddenly dropped and an instability occurred, which shows how strong adhesive properties can be at the nanoscale.”

He went on to report that a key finding arising from their experiments was that the critical sliding force divided by the contact area was equal to the shear yield stress of the polymer. As they pulled and stretched the polymer it began to deform plastically and wouldn’t return to its original configuration. The stress at which the plastic deformation occurs is called the yield stress of the polymer.

Further studies went on to test fibers of varying diameters and found that the sliding instability of stressed polymer nanofibers occurred at a specific value of the shear stress. This value was the tangential force divided by the contact size. They determined that it was equal to the shear strength of the polymer.

Further information: Debashish Das et al. Sliding of adhesive nanoscale polymer contacts, Journal of the Mechanics and Physics of Solids (2020). DOI: 10.1016/j.jmps.2020.103931

Keywords: polymer, nanofiber, nanoscale polymer fibers, adhesion, friction, sliding force, yield stress

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