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Research

Our group seeks to produce new function in scalable engineering materials via the nuanced, programmable control of their structural and compositional features.  We work at the intersection of advanced manufacturing, applied mechanics, and materials processing to produce heterogeneous, multiscale, and tunable architectures, exploring fundamental and applied questions.

See below for a few examples of past work:

Multistability and controlled storage/release of elastic energy

With Katia Bertoldi (Harvard), Dennis M. Kochmann (Caltech), Jennifer A. Lewis (Harvard), and Chiara Daraio (Caltech)

strainenergy

When beams undergo loading they can exhibit unstable responses, which vary significantly depending on the details of the loading and boundary conditions.  We produced soft beam-based architectures using a 3D-printable elastomeric ink.  By constraining these beams using a second material of higher stiffness, the beams could be designed to show a bistable response, in which either of two geometric configurations could be maintained even after the loading was removed.  One of these configurations was associated with a straight beam, and the other with a buckled beam, the latter of which possesses an elevated strain energy relative to the former.  Despite (due to boundary conditions) being able to maintain its configuration without an applied load, when a beam is in this second configuration it stores elastic energy in the strain of the material itself.  Fascinating mechanical responses can be designed by making use of this simple motif.  In one study, we designed and fabricated a new type of architected material, or metamaterial, which acts as an impact energy absorber, converting incident mechanical energy into internal elastic energy.multistablestruct2

Unlike conventional energy absorbing materials, which either absorb energy through permanent destruction of bonds or else are highly rate dependent due to making use of viscous processes for damping, our energy-absorbing material is reversible since the energy absorption is entirely elastic.  It therefore has the same mechanical response under repeated loading.  It also is based on a rapid instability (the beam buckling and snap-through), which typically occurs more rapidly than the applied loading, resulting in an independence of the mechanical response to the loading conditions.

In a second study, rather than absorbing energy (going from the straight beam to the buckled configuration), we explored the transition in the opposite direction.  Here, by starting with the beams in the higher energy state, we examined how transition waves propagated through many units of beams, sequentially snapping from their higher energy to lower energy state.  The release of the elastic energy was sufficient to cancel out the energy lost due to damping in these highly-dissipative soft materials.  The movement of the transition waves can be highly tuned by locally varying the geometry of the beams, which can be easily accomplished due to the soft nature of the architecture.  We made use of this to design soft mechanical logic devices, allowing a single system to switch between OR and AND gate behavior.

transitionwaves

Our publications related to this topic:

 

Bioinspired, lightweight structural composites

With Jennifer A. Lewis (Harvard), Brett G. Compton (University of Tennessee), Lorna J. Gibson (MIT), and Kristina Shea (ETH Zurich)

Natural lightweight materials such as wood, bone, and shells consist of complicated multiscale features, such as heterogeneous compositions and spatially-varying fiber alignments.  By careful material design and simultaneous hardware development, we have developed 3D printable fiber-reinforced composite inks that allow lightweight but rigid structures to be 3D printed.  By designing novel 3D printing nozzles we are able to control fiber alignment and compositional heterogeneity to achieve enhanced mechanical performance, following approaches and designs inspired by nature.

Our publications related to this topic:

 

Hierarchical carbon nanotube arrays as protective foams

With Chiara Daraio (Caltech)

cnthierarchy

We developed centimeter-scale structures comprised almost entirely of nominally-aligned carbon nanotubes (CNTs).  Structural integrity is maintained by extensive entanglements among nanotubes.  When subjected to compression, the system behaves macroscopically like an open cell foam, dissipating on the order of 1000 times more energy than standard polymeric foams of comparable density.  Another difference between these and traditional polymeric foams is that the compression of CNT arrays is very insensitive to the rate of compression.  The mechanical energy is thought to be dissipated by the rapid formation and breaking of van der Waals interactions as many CNTs slide against and away from one another, a process that occurs at a time scale much more rapid than the rate of external loading.  We model these interactions using a multiscale spring model based on a series of bistable elements, each of which can snap from one stable configuration to another when deformed.  We also developed many different processes for synthesizing arrays of CNTs with controlled changes to structure at different length scales.  For example, increasing hydrogen concentration during synthesis leads to smaller diameter CNTs with a narrow diameter distribution (at the scale of nanometers).  Starting and stopping the flow of reactants leads to microstructural heterogeneities in CNT alignment and density (at the scale of microns to tens of microns).

Partial list of our publications related to this topic: