Scalable fabrication of microstructures with freeform geometries, in particular curved and re-entrant shapes, is limited by the constraints of fabrication methods that present a tradeoff between structural complexity and throughput. We present a scalable fabrication technique to produce freeform Carbon Nanotube (CNT) microstructures with exceptional uniformity by controlling the growth rate of aligned CNTs via engineered catalyst film stack. The growth rate of CNTs can be controlled by introducing a growth retardant layer underneath the regular catalyst film stack. Offset patterning of the CNT growth catalyst and growth retardant layer with varying thicknesses is used to locally modulate the CNT growth rate at inter- and intra-structural level which enables multi-height scale structures and curved structures whose trajectories depend on the strain mismatch within the structures. The final shape of the curved CNT microstructures can be designed via finite element modeling, and compound catalyst shapes produce microstructures with multi-directional curvature and unusual self-organized patterns. Conformal coating of the CNTs enables tuning of the mechanical properties independently from the microstructure geometry, representing a versatile principle for design and manufacturing of complex microstructured surfaces. This principle can be applied toward applications demanding uniform arrays of complex structures over large areas, such as micro-architectured composite materials for maximized stiffness and damping properties, and nature-inspired superhydrophobic surfaces with intricate micro- and nanoscale features for directional wetting properties.
University of Michigan, Massachusetts Institute of Technology
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