| Experiments by several groups during the past decade have demonstrated that molten nanofilms whose surface is exposed to a large and uniform transverse thermal gradient undergo spontaneous formation of nanopillar arrays with a pitch on the order of tens of microns. These 3D arrays adopt various symmetries and shapes depending on local initial and boundary conditions. Once the thermal gradient is removed, the structures rapidly solidify in place resulting in nanostructures with extraordinarily smooth surfaces, particularly advantageous for optical and photonic applications. Control over structure formation, however, requires identification of the dominant physical mechanism which establishes the minimum lateral feature size as well the material and geometric properties affecting the growth rates. There are currently two prevailing explanations for this observed phenomenon: (i) electrostatic attraction between the molten film and overlying substrate due to induced surface image charge (Chou et al. 2002) and (ii) interface radiation pressure from coherent reflections of acoustic phonons (Schäffer et al. 2003). In this talk, we demonstrate instead that the fluid elongations are likely caused by a long-wavelength deformational instability caused by extremely large thermocapillary forces ("nano-Bénard-Marangoni flow") which rapidly outweigh stabilization by capillary forces. Linear stability and Lyapunov analyses of the governing interface equation for the parameter range relevant to experiments indicate that there is no critical number for instability and no steady states. If not mass limited, the nanopillars grow continuously until contact with the cooler substrate is achieved. We compare these predictions with ongoing experimental measurements in our laboratory of in-situ array formations. Although measurements indicate closest agreement with the thermocapillary model, there remain quantitative differences with theory. Time permitting, we will also examine resonant wavelength phenomena in these systems, which suggests alternative methods for achieving perfectly uniform arrays. In total, these results demonstrate the tremendous potential of this patterning technique for 3D lithography based on interface modulation. |
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