Liquid manipulation using engineered carbon nanotube surfaces

Abstract

The understanding and control of liquid-surface interactions is of fundamental interest and practical relevance for applications including self-cleaning surfaces, microfluidic devices, and phase change energy conversion. Carbon nanotubes (CNTs) are well known for their outstanding properties, and can be manufactured over large areas by scalable chemical vapor deposition methods. In this thesis, the mechanical properties of CNT microstructures, and their wettability and porosity via coating and functionalization, are explored as a platform for engineering liquid-surface interactions. First, I study the capillary-driven liquid imbibition in nanoporous ceramic-coated vertically aligned CNT films. Deposition of a conformal ceramic coating prevents capillary-induced deformation of the CNTs, and tailors the nanoporosity. A model based on Darcy's law is found to accurately relate the effective pore size and surface wettability to the imbibition dynamics. I then demonstrate the use of ceramic-coated CNT microstructured surfaces for enhanced pool boiling. A critical heat flux of 235 W-cm 2 is measured on the nanoporous microstructure surface, which is 9% and 57% greater than measured on solid microstructures and smooth ceramic surfaces, respectively. Via in situ infrared imaging, faster bubble nucleation and departure are observed on the nanoporous microstructures; the additional imbibed volume contributes to the liquid supply for vaporization and delays the boiling crisis. Further enhancement could be achieved by optimizing the microstructure pattern and nanoscale porosity and wettability. Third, I present a compliant CNT-based scale surface architecture for tuning anisotropic droplet adhesion on hydrophobic surfaces. This was inspired by the superhydrophobicity and anisotropic droplet roll-off on Morpho aega butterfly wings. By video microscopy, we reveal that extreme deflections of the individual scales are responsible for directional adhesion. Inspired by this finding, I demonstrate a synthetic scale surface with stiffness-tunable anisotropic droplet adhesion, fabricated from curved CNT microstructures. A model considering the contact line forces and scale stiffness predicts the scaling of adhesion anisotropy for the natural and synthetic surfaces. The findings in this thesis demonstrate the versatility of CNT-based surfaces for manipulating liquids. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex 3D microarchitectures, suggest further opportunities for future work.