Capillarity-driven assembly of two-dimensional cellular carbon nanotube foams

Abstract

Capillary forces arising during the evaporation of liquids from dense carbon nanotube arrays are used to reassemble the nanotubes into two-dimensional contiguous cellular foams. The stable nanotube foams can be elastically deformed, transferred to other substrates, or floated out to produce free-standing macroscopic fabrics. The lightweight cellular foams made of condensed nanotubes could have applications as shock-absorbent structural reinforcements and elastic membranes. The ability to control the length scale, orientation, and shape of the cellular structures and the simplicity of the assembly process make this a particularly attractive system for studying pattern formation in ordered media.C ellular patterns arise frequently in nature on length scales ranging from microscopic to macroscopic as a result of spatially periodic and random perturbations (1-5); examples range from the morphogenesis of embryos to patterns in coffee stains. A film of aligned carbon nanotubes represents a unique, yet unstudied type of system in which pattern formation could arise from the collapse and reassembly of highly ordered, anisotropic, elastic, nanoscale rods with remarkable properties. We report the creation of intriguing two-dimensional cellular foams by the evaporation of liquids from such nanotube films (6, 7). Shrinkage and crack formation in the films caused by strong capillary forces during evaporation and strong van der Waals interactions between condensed nanotubes (8) result in the formation of visually striking, stable cellular patterns and contiguous foams. Patterns formed by nanotube aggregates differ significantly from other polygonal crack patterns (9-13) because of the inherent dimensions, strength, and flexibility of the nanotubes (14, 15). The length scale, orientation, and shape of the cellular structures can be controlled by varying the nanotube height and the rate of evaporation of liquid and by patterning the nanotube array. The nanotube foams also can be floated out to produce free-standing macroscopic films. The outstanding properties of the constituent nanotubes may lead to applications for these structures as shock-absorbent reinforcements and in nanofiltration devices. Materials and MethodsFabrication of Multiwalled Nanotube Arrays. Vertically aligned multiwalled nanotube (MWNT) arrays (Fig. 1a) were grown on rigid silica substrates by using a chemical vapor deposition process (7) based on the decomposition of ferrocene and xylene. Patterned MWNT arrays were fabricated by patterning silica (SiO 2 ) on Si(100) (6) and exposing these patterned substrates to a mixture of ferrocene and xylene at 800°C. Nanotubes grow selectively on the patterned silica regions (6).Formation of Cellular Carbon Nanotube Foams. The aligned nanotube arrays were oxidized in an oxygen plasma created in a glow discharge chamber (Harrick Scientific, Ossining, NY) at room temperature and 0.6 torr (1 torr ϭ 133 Pa) pressure for Ϸ10 min. Characterization of the oxidized MWNTs by Raman spectroscopy confirmed the preservation of the...

Pattern formation and coarsening dynamics in three-dimensional convective mixing in porous media

“…We are currently investigating how the detailed three-dimensional simulations and theory presented here may guide the development of non-equilibrium two-dimensional models of the pattern-forming process, in the spirit of surface-growth models [36,37]. This will inform our ability to model and predict the properties of other pattern-forming processes that lead to cellular structures [38], such as foams [39], elastocapillary assembly [40], desiccation cracks [41], columnar jointing [42,43] and mantle dynamics [44].…”

Elastocapillary coalescence of plates and pillars

“…The competition between capillarity and elasticity selects the structures formed, which can be used to construct substrates with tunable wetting and adsorption properties [11,12]. A variety of experimental systems that fall into this category include millimetre-scaled macroscopic brush hairs [8], micrometre-scaled mesoscopic polymeric surface mimicking gecko foot hairs [13], as well as nanometre-scaled carbon nanotube forests [14,15].…”

Deformation of a free interface pierced by a tilted cylinder: Variation of the contact angle

“…They appear in applications throughout biology and engineering, and include animal locomotion [1][2][3][4]; surface processing for nano-micro-applications, for example fiber coating and cleaning [5][6][7], the assembly of hairs, carbon nanotubes and biological filaments [8][9][10][11][12][13][14][15][16]; uses of AFM probes [6,17,18]; and impact of liquid jets [19].…”