Fluid-Assisted Sorted Assembly of Graphene on Polymer.

The significant size distribution of as-synthesized nanomaterials presents a challenge for reproducable and reliable applications. In this paper, we report a fluidic-assisted sorted assembly method in which nanomaterial sorting and enhanced assembly can be achieved simultaneously. As a proof of concept, a two-dimensional (2D) graphene flake, with a large size variation, was chosen as the target nanomaterial system. This study synergizes a novel fluidic assembly design, suspending a rotating disk over a polydimethylsiloxane (PDMS) substrate, and a computational fluid dynamics (CFD) model using Ansys CFX to disclose the mechanism of sorted assembly. By controlling the rotating speed and the gap between the disk and the substrate, the flow field is altered. In contrast to centrifugal sorting, where larger particles move outward, in this study, the size of assembled graphene flake (average lateral size, Xc) reduces significantly from the center (Xc = 3 μm) to the edge of the disk (Xc = 2 μm). The particle sorting process is dictated by the fluid shear-stress, with higher shear-stress leading to smaller particles, while the assembly process is mainly dominated by the pressure field with higher pressure magnitude leading to better assembly. Near the edge of the disk, enhanced particle sorting is coupled with an enhanced assembly where a continuous graphene film with smaller Xc can be formed. To prove the potential application of this method, an ultrasensitive strain sensor with one of the lowest detection limits, 0.02%, is demonstrated. This research presents a novel route toward large-scale and cost-effective manufacturing of nanomaterial-based flexible electronics.