Dip-and-Drag Lateral Force Spectroscopy for Measuring Adhesive Forces between Nanofibers

Abstract

Adhesive interactions between nano-fibers strongly influence the mechanical behavior of soft materials comprised of fibrous networks. We use atomic force microscopy (AFM) in lateral force mode to drag a cantilever tip through fibrous networks, and use the measured lateral force response to determine the adhesive forces between fibers of the order of 100 nm diameter. The peaks in lateral force curves are directly related to the detachment energy between two fibers; the data is analyzed using the Jarzynski equality to yield the average adhesion energy of the weakest links. The method is successfully used to measure adhesion forces arising from van der Waals interactions between electrospun polymer fibers in networks of varying density. This approach overcomes the need to isolate and handle individual fibers, and can be readily employed in the design and evaluation of advanced materials and biomaterials which, through inspiration from nature, are increasingly incorporating nano-fibers. The data obtained with this technique may also be of critical importance in the development of network models capable of predicting the mechanics of fibrous materials.

Probing adhesion between nanoscale cellulose fibres using AFM lateral force spectroscopy: The effect of hemicelluloses on hydrogen bonding

“…The recorded lateral force-distance curves, an example of which is shown in Figure 2B, feature force peaks that consistently rise above the baseline. Following the methods established in our previous work (Dolan et al, 2016), the observed sharp increase…”

“…In this work we aim to probe the interactive forces between nanoscale cellulose fibres and explore the effect of non-cellulosic components (arabinoxylan and xyloglucan) on inter-fibre adhesion (Dolan, 2017). To enable such nano-scale characterisation, we adapted and further advanced our recently developed dip-and-drag lateral force spectroscopy (DnD-LFS) technique (Dolan et al, 2016), which uses an AFM cantilever tip to pull fibres out of a network and measure forces associated with detachment events at fibre contacts. Building on previous developments (Lopez-Sanchez, Cersosimo, Wang, Flanagan, Stokes & Gidley, 2015; Martinez-Sanz, Whitney, Gothard, Mitchell & Gidley, 1999), BC networks are used as a model system and are self-assembled to give a random distribution of fibre orientations and contact configurations.…”

“…The DnD-LFS technique, originally developed to probe adhesion between electrospun fibres (Dolan et al, 2016), has been advanced to make it applicable for probing inter-fibre adhesion in the BC systems. First, we have performed in-situ imaging of BC hydrogels and identify protruding fibre loops around the edge of the micropellicle.…”

“…in force (above the baseline) is attributed to the AFM tip engaging with a cellulose fibre and dragging it until the latter is in tension a . This is followed by a detachment event at a fibre contact point (Dolan et al, 2016), when the fibre being pulled by the AFM tip is no longer in tension, which results in the cantilever deflection signal returning back to the baseline. For very low density networks, the friction force baseline (flat baseline) is anticipated to reflect the friction force between the glass substrate and the AFM tip.…”

“…A a There is a chance that the cantilever engages several fibres at once. This scenario, however, accounts only for the second order correction to the measured pull-off forces as elaborated in (Dolan et al, 2016). To assist in interpreting DnD-LFS results, a force balance across a section of a hypothetical network during a pulling experiment is considered, as illustrated in Figure 3.…”