Writing Polymer Nanofibers on Superhydrophobic Surfaces for the Directed Deposition of 3D Nanowire Surface Structures

Proceedings to be published in electronic-only format (see MRS Online Proceedings Library at www.mrs.org/opl)International audienceAn aqueous droplet on a superhydrophobic nanopost array is only exposed to the very tips of the post structures, and therefore material deposition from solution can be limited to those selective areas. Herein we demonstrate a novel method for the directed writing of polymer nanofibers from a droplet solution of polyvinyl alcohol (PVA) on the posts of a superhydrophobic array of Si posts (300 nm diameter, 6 - 8 um tall, pitch spacing 1-4 um), etched by deep reactive ion etching [1]. Highly uniform, parallel strands of PVA nanofibers with diameter ~ 50 nm have been 'drawn' from solution to connect the posts, in a rapid, room temperature process, by translating a droplet over the non-wetting surface. A motorized XY stage was used to control the movement of a droplet at the end of a syringe tip along specific directions. The fiber formation depends on the polymer concentration, droplet speed, and surface functionalization, to form stable fibers at the receding edge of a mobile droplet in the Cassie (non-wetting) superhydrophobic state, under conditions of high shear rates. We have developed a model for the formation mechanism as a balance of the viscosity, surface tension, the pulling speed, the rate of solvent evaporation and the post array geometry. We show that this approach is general and is widely applicable to the directed deposition of a range of fibrous materials. In particular, it has been applied to the deposition of multi-walled carbon nanotubes, and collagen fibers. Since this method provides a means to connect specific posts together with nanofibers along controlled directions, it could be applied to the design of nanoelectronic structures and 3D nanostructures. In addition, such polymer nanofibers could be used as a template for vapor deposition, inorganic mineral growth from solution and directed adhesion of biological cells