Previously, researchers have had to rely on assumptions regarding the nature of fibre-fibre contacts when developing and manufacturing fibrous materials. Thanks to the work of two University of Queensland researchers, Dr Gleb Yakubov and Grace Dolan, they can now measure the actual forces at play.
Their dip-and-drag technique probes the interactive forces between individual nanofibres within a fibre network. The technique uses an atomic force microscope (AFM), a device that is used primarily to measure a material’s surface topology through a process akin to reading nanoscale braille.
The microscope has a sharp tip, measuring just a few atoms wide, attached to a cantilever that registers contact with a sample as it is scanned across it. The cantilever bends or twists when it encounters a force or obstacle on the surface of the sample. By reflecting a laser beam off the end of the cantilever, even tiny deflections can be registered, allowing for extremely accurate measurement of nanoscale structures. What’s more, the researchers can extrapolate from the twists and bends of the stylus, the micro- and nano- Newton forces acting on the tip.
Gleb and Grace used the device’s immense sensitivity as the basis of their technique. By setting the microscope in “lateral force” mode, i.e. dragging the AFM tip from side-to-side, the pair were able to measure the resistive force when the point came into contact with the fibres.
When the tip encounters a fibre, the cantilever begins to twist. If the tip comes across a fibre that’s easy to push, the change in registered position of the light is small, but if a strongly bonded fibre gets in the way, the cantilever is heavily warped and the change is much bigger.
The dip-and-drag technique was validated using equipment at ANFF-Q and individual electrospun nanofibres from ANFF-Vic. “We successfully measured the adhesion forces arising from van der Waals interactions between the model electrospun fibres in networks of varying density,” Grace explained.
As well as providing realistic results, the technique offers other advantages when measuring these inter-fibre forces. Existing equivalent methods require the isolation of bonded fibres for accurate testing, but the dip-and-drag method eliminates the need for this.
Testing larger numbers of fibres at the same time also means engineers and scientists can get a truer representation of the material’s average properties. Also, bonds between neighbouring fibres are able to form naturally, as they would during a realistic production process, so the strength of the bonds is more reflective of a real-world result.
“Our approach can be readily employed in the design and evaluation of advanced materials that are increasingly incorporating nanofibres,” Grace said.
The pair envisages the technique being used to test the performance of nanofibres that have been added to improve the toughness of composite materials. The team’s next step is to broaden the application to extract information about the mechanics and tensile strength of individual nanofibres.