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CURATOR

Postdoctoral Researcher, Columbia University

PINBOARD SUMMARY

This pinboard is a collection of articles that have been useful and inspiring in my postdoc research

Hybrid soft materials where one component exhibits well-controlled anisotropy are ideal go-to candidates for engineering fine microstructures in a material with an impact on its macroscopic properties. Among these systems, nematic liquid crystalline polymer networks are distinct given their imprinted orientational order in an otherwise elastic polymer network. These materials are formed by physically cross-linking elongated liquid crystal molecules (a.k.a. mesogens) to a rubbery network. A variety of out-of-plane actuation behavior – including curling, bending and accordion-like folding – can be encoded in these materials by implementing complex nematic director microstructures. On my actuator research, we model a rich morphing behavior via finite element elastodynamics simulations with a unique in-house-developed algorithm. These studies provide insight on how morphology at the macroscopic level and its stability depend on liquid crystalline director micro-structure, sample aspect ratio as well as the strength of the stimulus-response.

Another approach to engineer complex microstructures in hybrid-materials includes structural templating of particle dispersions via solidification (a process that includes nucleation and grow). A dispersed suspension of particles in water or any other melt can adopt the geometry and morphology of the growing crystals. Via numerical experiments on a simple model for particle kinetics at the melt/crystal interface, we have investigated tunable structural templating given by a single parameter: crystallization speed. We evaluate the threshold crystallization velocity for structural templating and show its dependence on particle size, solvent size, melt viscosity and surface tension. Overall, these simulations studies show exceptional agreement with experimental observations and provide light for further development of advanced hybrid soft materials and applications.

In addition, to pointing out some of my research articles (some self-promotion). I am glad and honored to share with you the outstanding research by soft matter colleagues that have inspired me while carrying out my research.

18 ITEMS PINNED

Modeling Defects, Shape Evolution, and Programmed Auto-origami in Liquid Crystal Elastomers

Abstract: Liquid crystal elastomers represent a novel class of programmable shape-transforming materials whose shape change trajectory is encoded in the material's nematic director field. Using three-dimensional nonlinear finite element elastodynamics simulation, we model a variety of different actuation geometries and device designs: thin films containing topological defects, patterns that induce formation of folds and twists, and a bas-relief structure. The inclusion of finite bending energy in the simulation model reveals features of actuation trajectory that may be absent when bending energy is neglected. We examine geometries with a director pattern uniform through the film thickness encoding multiple regions of positive Gaussian curvature. Simulations indicate that heating such a system uniformly produces a disordered state with curved regions emerging randomly in both directions due to the film's up-down symmetry. By contrast, applying a thermal gradient by heating the material first on one side breaks up-down symmetry and results in a deterministic trajectory producing a more ordered final shape. We demonstrate that a folding zone design containing cut-out areas accommodates transverse displacements without warping or buckling; and demonstrate that bas-relief and more complex bent-twisted structures can be assembled by combining simple design motifs.

Pub.: 14 Feb '16, Pinned: 31 Aug '17

Tunable Multiscale Nanoparticle Ordering by Polymer Crystallization

Abstract: The multiscale assembly of nanoparticles is achieved by leveraging the hierarchical structure of lamellar polymer crystals and the kinetics of crystallization. This NP ordering increases the Young’s modulus but without sacrificing fracture toughness.While ∼75% of commercially utilized polymers are semicrystalline, the generally low mechanical modulus of these materials, especially for those possessing a glass transition temperature below room temperature, restricts their use for structural applications. Our focus in this paper is to address this deficiency through the controlled, multiscale assembly of nanoparticles (NPs), in particular by leveraging the kinetics of polymer crystallization. This process yields a multiscale NP structure that is templated by the lamellar semicrystalline polymer morphology and spans NPs engulfed by the growing crystals, NPs ordered into layers in the interlamellar zone [spacing of (10–100 nm)], and NPs assembled into fractal objects at the interfibrillar scale, (1–10 μm). The relative fraction of NPs in this hierarchy is readily manipulated by the crystallization speed. Adding NPs usually increases the Young’s modulus of the polymer, but the effects of multiscale ordering are nearly an order of magnitude larger than those for a state where the NPs are not ordered, i.e., randomly dispersed in the matrix. Since the material’s fracture toughness remains practically unaffected in this process, this assembly strategy allows us to create high modulus materials that retain the attractive high toughness and low density of polymers.

Pub.: 07 Jun '17, Pinned: 31 Aug '17

Making waves in a photoactive polymer film.

Abstract: Oscillating materials that adapt their shapes in response to external stimuli are of interest for emerging applications in medicine and robotics. For example, liquid-crystal networks can be programmed to undergo stimulus-induced deformations in various geometries, including in response to light. Azobenzene molecules are often incorporated into liquid-crystal polymer films to make them photoresponsive; however, in most cases only the bending responses of these films have been studied, and relaxation after photo-isomerization is rather slow. Modifying the core or adding substituents to the azobenzene moiety can lead to marked changes in photophysical and photochemical properties, providing an opportunity to circumvent the use of a complex set-up that involves multiple light sources, lenses or mirrors. Here, by incorporating azobenzene derivatives with fast cis-to-trans thermal relaxation into liquid-crystal networks, we generate photoactive polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination, with a feedback loop that is driven by self-shadowing. We explain the mechanism of wave generation using a theoretical model and numerical simulations, which show good qualitative agreement with our experiments. We also demonstrate the potential application of our photoactive films in light-driven locomotion and self-cleaning surfaces, and anticipate further applications in fields such as photomechanical energy harvesting and miniaturized transport.

Pub.: 29 Jun '17, Pinned: 31 Aug '17