Session V18: Function from Geometry: 3D Printing to Programable Matter II

High Fidelity Additive Manufacturing of Optically Transparent Glass Structures

Glass has been an integral part of human civilization with expressions across scales and disciplines: from the microscope to the telescope, from fiber optics to mobile interface, and from the petri dish to a building envelope. Such a diverse range of applications is enabled by the inherent material properties including mechanical strength, optical transparency and chemical inertness. Additive manufacturing provides opportunities for integrating the unique properties of glass to engineer novel structures that are functionary graded through precise spatiotemporal deposition of molten glass. This talk presents the Mediated Matter Group's latest development of a novel additive manufacturing platform, and related processes, for 3D Printing optically transparent glass for architectural scale applications.   

Drop footprints in a curing elastomer: Extreme rapid printing of an elastic solid

Interfaces between liquids that are static or in motion display a myriad of complex shapes and structures which can be both beautiful to the eye (e.g. water jets in fountains) and useful towards technological applications (e.g. fast inkjet printing ). We harness this idea to imprint shapes in an elastic solid formed by depositing water drops on a rapidly curing elastomer, which are then removed. The distorted water/elastomer interface can be controlled by the fluid's surface properties and the water volume. As the elastomer is cured, the drop shape is imprinted into the elastomer which can be computed from classical fluid mechanics, and confirmed by experimental measurements. We expand this notion to the rapid fabrication of drop shapes using the Rayleigh Plateau instability of a water column deposited on the curing elastomer's surface. The water column breaks into a line of drops via the instability, which allows us to control the volume of each drop and their separation, and thus the imprint into the elastomer.   

Direct Laser Writing of Single-Material Sheets with Programmable Self-Rolling Capability

Direct laser writing, a sub-class of two-photon polymerization, facilitates 3D-printing of single-material microstructures with inherent residual stresses. Here we show that controlled distribution of these stresses allows for fast and cost-effective fabrication of structures with programmable self-rolling capability. We investigate 2D sheets that evolve into versatile 3D structures. Precise control over the shape morphing potential is acquired through variations in geometry and writing parameters. Effects of capillary action and gravity were shown to be relevant for very thin sheets (thickness \textless 1.5um) and have been analytically and experimentally quantified. In contrast to that, the deformations of sheets with larger thickness (\textgreater 1.5um) are dominated by residual stresses and adhesion forces. The presented structures create local tensions up to 180MPa, causing rolling curvatures of 25E3m$^{\mathrm{-1}}$. A comprehensive analytical model that captures the relevant influence factors was developed based on laminate plate theory. The predicted curvature and directionality correspond well with the experimentally obtained data. Potential applications are found in drug encapsulation and particle traps for emulsions with differing surface energies.   

3D printed cat tongue is a self-cleaning, tangle-teasing brush

A cat's tongue is covered in an array of spines called papillae. These spines are thought to be used in grooming and rasping meat from bones of prey, although no mechanism has been given. We use high-speed video to film a cat grooming. We show that the spines on the tongue act as low pass filters for tangles in hair. The tongue itself is highly elastic, while the spines are rigid. We 3D print a cat tongue mimic and show that the nonlinear force applied by the spines helps to increase efficacy of grooming. The tongue also provides frictional anisotropy with backward-facing spines, allowing for self-cleaning properties post-groom.   

Viscous dipping, application to the capture of fluids in living organisms

Some insects, birds and mammals use flower nectar as their energy resources. For this purpose, they developed specific skills to ingest viscous fluids. Depending on the sugar content, {\it i.e.}, the viscosity, different strategies are observed {\it in vivo}. Indeed, butterflies use simple suction for low viscosity nectars; hummingbirds have a tongue made from two thin flexible sheets that bend to form a tube when immersed in a fluid; other animals exhibit in contrast complex papillary structures. We focus on this last method generally used for very viscous nectars. More specifically, bees and bats possess a tongue decorated with microstructures that, according to biologists, would be optimized for fluid capture by viscous dipping. In this talk, we will discuss this assumption by comparing physical models of viscous dipping to {\it in vivo} measurements. To mimic the tongue morphology, we used various rod shapes obtained by 3D printing. The influence of the type and size of lateral microstructures was then investigated and used to build a global framework describing viscous dipping for structured rods/tongues.   

High thermal conductivity in soft elastomers with elongated liquid metal inclusions.

Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrains thermal conductivity ($k)$ to decrease monotonically with decreasing elastic modulus ($E)$. This is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with a dielectric composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (E \textless 100kPa), and extreme deformations capability (\textgreater 600{\%} strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a \textasciitilde 25x increase in thermal conductivity (4.7 \textpm 0.2 W/m\textbullet K) over the base polymer (0.20 \textpm 0.01 W/m\textbullet K) under stress-free conditions and a \textasciitilde 50x increase (9.8 \textpm 0.8 W/m\textbullet K) when strained. This exceptional combination of thermal and mechanical properties is through the deformation of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer new possibilities for passive heat exchange in stretchable electronics and bio-inspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high power LED lamp and a swimming soft robot.   

Failure modes of microstructured fibers with sacrificial bonds made by instability-assisted 3D printing

A simple modification by increasing the deposition height on a commercially available 3D printer makes it a “mechanical sewing machine” due to the fluid mechanical instability. A variety of stitches-like patterns can be produced, similar to those by the Newtonian “fluid mechanical sewing machine”, but with more interesting characteristics in the additional third dimension, which creates weakly fused bonds in some patterns. With these bonds, the fabricated fibers exhibit improved toughness in uniaxial tensile test. The toughening mechanism is found to be similar to the one in spider silk --- the breaking of sacrificial bonds and the releasing of hidden length contribute significant dissipated energy to the system. However, the mechanical performance of these microstructured fibers is restricted by early fiber breakage as the number of sacrificial bonds increases. Here, we seek to understand the failure mechanisms of the microstructured fibers through tensile tests and finite element simulations. Static and dynamic failure are both found to cause early fiber breakage. These findings are helpful for the design optimization of microstructured fibers with high toughness and ductility, which can find potential use in impact protection and safety-critical applications.   

Optical Characterization of Polymeric Waveguides Fabricated by Thermocapillary Replication

Demand for increasing bandwidth has generated interest in optoelectronic circuits which incorporate polymeric components such as optical waveguides. Such components are usually cast into planar single- and multimode waveguides structures using techniques such as embossing, photolithography, dip coating or direct laser writing. In this talk, we describe an alternative technique based on thermocapillary replication for fabricating optical waveguides. A chilled preform mask held in close proximity to the surface of a polymer nanofilm draws fluid toward the mask by thermocapillary forces. The ensuing fluid shape, which replicates the pattern set by the mask, is then solidified in situ. We have used various mask patterns to enforce film surface thermal distributions purposely designed to pool liquid into ribbon-like shapes for use as rectilinear waveguides. As expected, capillary effects imbue the structures with ultrasmooth rounded and not rectangular shapes. We discuss measurements of the polarization extinction ratio and coupling efficiency for these modified shapes. For waveguide structures with large aspect ratios, there is evidence of strong geometrical birefringence between the TE and TM polarizations including shapes which altogether exclude TM modes.   

Fabrication of Converging and Diverging Polymeric Microlens Arrays By Spatiotemporal Control of Thermocapillary Forces

Spatiotemporal control of the surface tension of liquid films allows methods for patterning films into myriad 3D shapes. We demonstrate how thermocapillary forces arising from local control of surface temperature deform a flat nanofilm into a microlens array then solidified in situ. An array of chilled, prefabricated slender pins placed in close proximity to the film provide thermal control via thermal conduction. By varying the pin width, pin pitch and evolution time, we have fabricated plano-convex, plano-concave, caldera-like and hierarchical microlens arrays with ultrasmooth surfaces. For demonstration, the diverging arrays were used in a Shack-Hartmann geometry for imaging wavefronts disturbed by bursts of cooled spray. Characterization of the arrays by scanning white light interferometry shows that the resulting microlenses resemble aspheric paraboloids. The aspherical nature is likely caused by unintended lateral thermal flow due to non-parallelism of the heated and cooled substrates. We discuss results of finite element simulations showing how an initial flat film evolves in time through various shapes which can be affixed by controlling the processing time.   

Focused Laser Dewetting of Metallic Thin Films

Focused Laser Spike (FLaSk) Annealing generates extreme thermal gradients from a microscale laser spot, which in turn initiate thermocapillary dewetting. Through this controllable mobility and driving force spike, FLaSk has shown the ability to pattern polymer thin films, resulting in direct write of submicron-resolution trench-ridge structures. Due to the lower viscosity and higher surface tension of metallic melts, the gradient-induced dewetting occurs simultaneously with Rayleigh droplet formation, resulting in a trench-ridge-dot feature. When lines are overlapped, the moving instead of removing of metal will result in sub-spot features at a sub-micron scale. Shrinking the line distance coalesces droplets into continuous chains of small islands, reconstructing a trench-ridge structure, which generates visible plasmonic effects. Additionaly, gratings with similar features were generated both on the smooth and roughened films, indicating that to obtain highly uniform final structures does not require defect-free starting materials. Further, continuous samples were generated from an initially discontinuous starting condition, opening the possibility for building conductive pathways.   

Thermocapillary Multidewetting of Thin Films

Thermocapillary dewetting of liquids and molten films has recently emerged as a viable alternative to conventional microprocessing methods. As this thermal gradient-induced mechanism is universal, it can be applied to any material. This work explores the sequential dewetting of materials with varying melting points, including polymers and metals, to create aligned morphologies. The variation in melting point allows for the dewetting of single layers at a time or mobility-limited simultaneous dewetting. As a result, a variety of multimaterial structures can be produced with built-in alignment, such as arrays of concentric circles, lines with periodic segmentation, or islands on holes. This approach employs photothermal methods to induce the necessary thermal gradient, with several variables being manipulated in order to influence the consequent structures. Adjusting laser power and light intensity allows for the control of temperature for selective dewetting of films; altering beam size and exposure time affects the extent of dewetting in terms of diameter size; overlap effects and simultaneous dewetting can result in complex architectures.