The hydrodynamic flow patterns inside high aspect ratio evaporating cylindrical droplets have utility for parallel alignment, positioning and placement of nanoparticles. We successfully create ~200 nm diameter cylindrical droplets for the first time using alternating hydrophobic and hydrophilic SAM layers made by a micro-contacting printing technique. We find that as the diameter of the droplet is reduced below 1000 nm (950-175 nm), a reversal of the internal flow is evident with highly accurate parallel alignment and placement of single walled carbon nanotubes along the centerline with 95 % precision. In contrast deposition is along the edges for droplets larger than 3 um in diameter. This flow reversal is a direct consequence of the evaporation mode switching for the narrow droplets. A model is developed to describe the transition from radially outward to inward flow. This technique should be useful for precisely aligning and placing individual or bundled anisotropic nanoparticles (i.e. carbon nanotubes) with arbitrary surface chemistries.

References:

1. Sharma, R.; Lee, C. Y.; Choi, J. H.; Chen, K.; Strano, M. S. Nano Letters 2007, 7,2693-2700, "Nanometer Positioning, Parallel Alignment, and Placement of Single Anisotropic Nanoparticles Using Hydrodynamic Forces in Cylindrical Droplets".

2. Sharma, R.; Strano, M. S.;"Centerline Placement and Alignment of Anisotropic Nanotubes in High Aspect Ratio Cylindrical Droplets of Nanometer Diameter" (submitted).