Adhesive contact mechanics of penta-twinned nanowires

Abstract

Penta-twinned nanowires, made of silver, gold, or copper, are envisioned to enable many applications, most notably stretchable and flexible electronics. In several instances of analysis of their behavior for device design, such as heat transfer due to current-induced heating, adhesion to substrates, or adhesion of nanowires to each other in conductive networks, knowledge of variables related to the mechanics of their contact is needed, for example the contact area (or radius). Due to the nanowires’ change in cross section, from circular to rounded-pentagonal as diameter increases, adhesive contact analysis is complex, and up to now has been simplified to assume either circular or perfect pentagonal cross sections. Here, we analyze the adhesive contact of the nanowires, fully considering the complexity of the cross section. We present equations to describe the geometry of the cross-section as it decreases in roundness with increasing diameter, finite element simulations that include elastic anisotropy and Lennard-Jones adhesive interactions, and observations on the adequacy of simplified contact models in describing the behavior. Calculations are presented for contact to typical stiff (silicon) and compliant (PDMS) substrates. The results reveal that, contrary to previous assumptions, the nanowires do not conform to the behavior of a perfect pentagon for the diameters typically encountered experimentally, and the roundness of the cross-section remains a factor that reduces the contact area. This reduction depends on the stiffness of the contact, being greater for stiff substrates.