Corrugated Paraffin Nanocomposite Films as Large Stroke Thermal Actuators and Self-Activating Thermal Interfaces

High performance active materials are of rapidly growing interest for applications including soft robotics, microfluidic systems, and morphing composites. In particular, paraffin wax has been used to actuate miniature pumps, solenoid valves, and composite fibers, yet its deployment is typically limited by the need for external volume constraint. We demonstrate that compact, high-performance paraffin actuators can be made by confining paraffin within vertically aligned carbon nanotube (CNT) films. This large-stroke vertical actuation is enabled by strong capillary interaction between paraffin and CNTs and by engineering the CNT morphology by mechanical compression before capillary-driven infiltration of the molten paraffin. The maximum actuation strain of the corrugated CNT-paraffin films (∼0.02−0.2) is comparable to natural muscle, yet the maximum stress is limited to ∼10 kPa by collapse of the CNT network. We also show how a CNT–paraffin film can serve as a self-activating thermal interface that closes a gap when it is heated. These new CNT–paraffin film actuators could be produced by large-area CNT growth, infiltration, and lamination methods, and are attractive for use in miniature systems due to their self-contained design

Corrugated Paraffin Nanocomposite Films as Large Stroke Thermal Actuators and Self-Activating Thermal Interfaces

High performance active materials are of rapidly growing interest for applications including soft robotics, microfluidic systems, and morphing composites. In particular, paraffin wax has been used to actuate miniature pumps, solenoid valves, and composite fibers, yet its deployment is typically limited by the need for external volume constraint. We demonstrate that compact, high-performance paraffin actuators can be made by confining paraffin within vertically aligned carbon nanotube (CNT) films. This large-stroke vertical actuation is enabled by strong capillary interaction between paraffin and CNTs and by engineering the CNT morphology by mechanical compression before capillary-driven infiltration of the molten paraffin. The maximum actuation strain of the corrugated CNT-paraffin films (∼0.02−0.2) is comparable to natural muscle, yet the maximum stress is limited to ∼10 kPa by collapse of the CNT network. We also show how a CNT–paraffin film can serve as a self-activating thermal interface that closes a gap when it is heated. These new CNT–paraffin film actuators could be produced by large-area CNT growth, infiltration, and lamination methods, and are attractive for use in miniature systems due to their self-contained design

Synergetic Chemical Coupling Controls the Uniformity of Carbon Nanotube Microstructure Growth

Control of the uniformity of vertically aligned carbon nanotube structures (CNT “forests”), in terms of both geometry and nanoscale morphology (density, diameter, and alignment), is crucial for applications. Many studies report complex and sometimes unexplained spatial variations of the height of macroscopic CNT forests, as well as variations among micropillars grown from lithographically patterned catalyst arrays. We present a model for chemically coupled CNT growth, which describes the origins of synergetic growth effects among CNT micropillars in proximity. <i>Via</i> this model, we propose that growth of CNTs is locally enhanced by active species that are catalytically produced at the substrate-bound nanoparticles. The local concentration of these active species modulates the growth rate of CNTs, in a spatially dependent manner driven by diffusion and local generation/consumption at the catalyst sites. Through experiments and numerical simulations, we study how the uniformity of CNT micropillars can be influenced by their size and spacing within arrays and predict the widely observed abrupt transition between tangled and vertical CNT growth by assigning a threshold concentration of active species. This mathematical framework enables predictive modeling of spatially dependent CNT growth, as well as design of catalyst patterns to achieve engineered uniformity

Laser Printing of Nanoparticle Toner Enables Digital Control of Micropatterned Carbon Nanotube Growth

Commercialization of materials utilizing patterned carbon nanotube (CNT) forests, such as hierarchical composite structures, dry adhesives, and contact probe arrays, will require catalyst patterning techniques that do not rely on cleanroom photolithography. We demonstrate the large scale patterning of CNT growth catalyst via adaptation of a laser-based electrostatic printing process that uses magnetic ink character recognition (MICR) toner. The MICR toner contains iron oxide nanoparticles that serve as the catalyst for CNT growth, which are printed onto a flexible polymer (polyimide) and then transferred to a rigid substrate (silicon or alumina) under heat and mechanical pressure. Then, the substrate is processed for CNT growth under an atmospheric pressure chemical vapor deposition (CVD) recipe. This process enables digital control of patterned CNT growth via the laser intensity, which controls the CNT density; and via the grayscale level, which controls the pixelation of the image into arrays of micropillars. Moreover, virtually any pattern can be designed using standard software (e.g., MS Word, AutoCAD, etc.) and printed on demand. Using a standard office printer, we realize isolated CNT microstructures as small as 140 μm and isolated catalyst ″pixels″ as small as 70 μm (one grayscale dot) and determine that individual toner microparticles result in features of approximately 5–10 μm . We demonstrate that grayscale CNT patterns can function as dry adhesives and that large-area catalyst patterns can be printed directly onto metal foils or transferred to ceramic plates. Laser printing therefore shows promise to enable high-speed micropatterning of nanoparticle-containing thin films under ambient conditions, possibly for a wide variety of nanostructures by engineering of toners containing nanoparticles of desired composition, size, and shape

Liquid Imbibition in Ceramic-Coated Carbon Nanotube Films

Understanding of the liquid imbibition dynamics in nanoporous materials is important to advances in chemical separations, phase change heat transfer, electrochemical energy storage, and diagnostic assays. We study the liquid imbibition behavior in films of ceramic-coated vertically aligned carbon nanotubes (CNTs). The nanoscale porosity of the films is tuned by conformal ceramic coating via atomic layer deposition (ALD), enabling stable liquid imbibition and precise measurement of the imbibition dynamics without capillary densification of the CNTs. We show that the imbibition rate decreases as the ceramic coating thickness increases, which effectively changes the CNT-CNT spacing and therefore decreases the permeability. We derive a model, based on Darcy's law, that incorporates an expression for the permeability of nanoscale post arrays, and we show that the model fits the experimental results with high accuracy. The tailorable porosity, along with controllable surface wettability and mechanical stability of coated CNTs, suggest their suitability for application-guided engineering, and for further investigation of imbibition behavior at finer length scales

<i>In situ</i> photopolymerization of microstructures resulting in physical confinement of a <i>C</i>. <i>elegans</i>.

<p><b>(a)</b> Three exemplary assays built sequentially (from left to right): an open frame, an array of micropillars (100 μm diameter), and a rippled microchannel (approx. 200 μm wide). <b>(b)</b> Tracking of the worm motion over a time period of 200s, within the pillar array. <b>(c)</b> Box-whisker plots of velocity in each configuration, showing that sequential confinement increases the maximum velocity at which the worm pushes against the surface of features. <b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0145935#pone.0145935.s011" target="_blank">S1 Video</a></b> shows the experiment.</p

Dynamic photopatterning method and implementation.

<p><b>(a)</b> Sequence, including real-time image capture and decision-based projection of the next fabrication step. <b>(b)</b> Schematic of the dynamic photopatterning system and configuration for patterning agar plates during <i>C</i>. <i>elegans</i> culture. Photo of the system is shown in <b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0145935#pone.0145935.s002" target="_blank">S1 Fig</a></b>. <b>(c)</b> An example of a framed micropillar array formed around a <i>C</i>. <i>elegans</i> worm <i>in situ</i>.</p

Interaction between worms and a simple hinge-pin mechanism fabricated within a millimeter-scale area.

<p><b>(a)</b> Sequential frames show the lower worm contacting the hinge and extending its body, causing the hinge to rotate around the encapsulated pin. <b>(b)</b> Angle of the hinge with respect to vertical position plotted against time. Over six seconds the hinge rotates from 6° to 27°. <b>(c)</b> Schematics of worm motion as observed in the experiment, which is also shown in <b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0145935#pone.0145935.s012" target="_blank">S2 Video</a></b>. When the worm contacts two points spanning from the frame to the hinge, it extends its body, exerting force on the hinge and causing it to rotate. The motion cycle is repeated, rotating the hinge clockwise a small amount with each cycle.</p

Tablet-based method for real-time assay modification by hand drawing.

<p><b>(a)</b> Schematic of sequence, resulting in projection of manual tablet input to create PEG-DA features on the substrate. In this case, the scale of the hand drawing is reduced 50X. <b>(b)</b> Video frames of the photopatterning of a series of dots drawn by the researcher to confine a worm inside a spiral frame (<b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0145935#pone.0145935.s013" target="_blank">S3 Video</a></b>). The system projects each feature drawn by the researcher after a delay of 0.25 seconds.</p

Fabrication of maze assays as a test case of <i>C</i>. <i>elegans</i> decision-making behavior.

<p><b>(a)</b> Array of T-shaped mazes. The observed ripples are part of the agar surface. <b>(b)</b> T-shaped PEG-DA microchannel, with dot sequence indicating the centroid position of the worm during a 30 second period. <b>(c)</b> Percentage of worms that ended at each leg of the maze after being inserted into the entrance of the maze. See <b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0145935#pone.0145935.s014" target="_blank">S4 Video</a></b> for video of a nematode solving a maze.</p