Mechanics of Capillary Forming of Aligned Carbon Nanotube Assemblies

Abstract

Elastocapillary self-assembly is emerging as a versatile technique to manufacture three-dimensional (3D) microstructures and complex surface textures from arrangements of micro- and nanoscale filaments. Understanding the mechanics of capillary self-assembly is essential to engineering of properties such as shape-directed actuation, anisotropic wetting and adhesion, and mechanical energy transfer and dissipation. We study elastocapillary self-assembly (herein called "capillary forming") of carbon nanotube (CNT) microstructures, combining in situ optical imaging, micromechanical testing, and finite element modeling. By imaging, we identify sequential stages of liquid infiltration, evaporation, and solid shrinkage, whose kinetics relate to the size and shape of the CNT microstructure. We couple these observations with measurements of the orthotropic elastic moduli of CNT forests to understand how the dynamic of shrinkage of the vapor-liquid interface is coupled to the compression of the forest. We compare the kinetics of shrinkage to the rate of evporation from liquid droplets having the same size and geometry. Moreover, we show that the amount of shrinkage during evaporation is governed by the ability of the CNTs to slip against one another, which can be manipulated by the deposition of thin conformal coatings on the CNTs by atomic layer deposition (ALD). This insight is confirmed by finite element modeling of pairs of CNTs as corrugated beams in contact and highlights the coupled role of elasticity and friction in shrinkage and stability of nanoporous solids. Overall, this study shows that nanoscale porosity can be tailored via the filament density and adhesion at contact points, which is important to the development of lightweight multifunctional materials.

Hydrothermal Coating of Patterned Carbon Nanotube Forest for Structured Lithium‐Ion Battery Electrodes

“…Figure a,b shows the CNT scaffold before and after the microwave‐assisted hydrothermal reaction. The aspect ratio of the pores is over 17, and their structure is maintained during the harsh hydrothermal processing conditions, with just a slight reshaping into a honeycomb‐like structure due to capillary aggregation when drying the sample (close‐ups in Figure S2, Supporting Information, and large‐area images in Figure S3a–c, Supporting Information) . The height evolution of the structure during these processing steps is also shown in Figure S3d.…”

Ultrathin high-resolution flexographic printing using nanoporous stamps

“…CNT forests are typically hydrophobic ( 40 ), but oxygen plasma etching creates surface defects and promotes attachment of oxygen-containing surface groups, rendering the CNT forest hydrophilic ( 41 ). As a result, upon liquid infiltration, the CNT microstructures may shrink slightly, and during liquid evaporation, the capillary force exerted by the contracting meniscus can overcome the elastic restoring forces of the deformed CNTs, causing significant densification and mechanical damage (video S1) ( 42 ). Notably, elastocapillary densification upon recession of solvent has been used to densify and shape CNT forests and other nanostructures into a variety of complex architectures ( 43 , 44 ).…”

“…We expect the resistance to elastocapillary densification is due to the reinforcement of individual CNTs by the pPFDA coating and development of pPFDA nanowelds at CNT-CNT contact points. Together, these reinforcements increase the lateral rigidity of the CNT forest, which has been found to be essential to prevent shrinkage upon evaporation ( 42 ). The pPFDA coating and the subsequent plasma treatment also certainly change the internal surface area per unit volume of the forests and the adhesion between CNTs and CNT bundles; the influence of these effects on the mechanics and stability of the CNT structures deserves further study.…”

Carbon Nanotube Fiber Pretreatments for Electrodeposition of Copper

“…CNT material consists mainly of multiwalled carbon nanotubes (MWCNTs) with a small percentage of double-walled (DWCNTs) and single-walled carbon nanotubes (SWCNTs). Yarn samples consist of a plethora of bers rolled together and treated with acetone to densify and bundle them together to form yarns [13]. e CNT samples were produced by dry spinning directly from a CVD reactor.…”

“…As made, CNT structures are typically improved by procedures that clean the CNT sidewalls of unwanted impurities and add additional reactivity by modification and introduction of new functionalities, such as oxygen-containing functional groups [11,12]. Other methods include densifying the CNT material, either by organic solvents [13] or mechanical application [14], effectively bridging the distance between individual nanotubes to decrease contact resistances. Doping with halogen gases and organic and ionic species have also been explored to alter the carrier density, and thus electrical resistance and activity of CNT material [15].…”