Tailoring normal adhesion of arrays of thermoplastic, spring-like polymer nanorods by shaping nanorod tips.

The tip shape of contact elements in hairy adhesion systems is crucial for proper contact formation and adhesion enhancement. While submicrometer terminal contact elements show much better adhesion performance than their larger counterparts, shaping their tips so as to maximize normal adhesion has remained challenging. We prepared durable nanorod arrays consisting of stiff, highly entangled thermoplastic polymers with rationally shaped tips by replication of anodic aluminum oxide (AAO). Nanorod arrays with pancake-like tips showed pronounced normal dry adhesion already for small loading forces. For small loading forces, adhesion forces significantly exceeded the loading forces. Both the absence of hysteresis in force/displacement curves and the pronounced durability of the nanorods in series of repeated attachment/detachment cycles suggest that the nanorods behave like elastic springs. Experimental load-adhesion curves were reproduced with a modified Schargott-Popov-Gorb (SPG) model, assuming that contacts between probe and individual nanorods are sequentially formed with increasing indentation depth.