Switchable origami adhesives.

Creating a reusable adhesive that can hold objects on a wall and can yet be easily removed al for researchers in the adhesives community for many years. Geckos and other climbing organisms demonstrate just this ability: to hold large loads (on-state) due to specialized digits and microstructures, yet they are also able to quickly peel their feet from a surface while climbing (off-state). Inspired by the simplicity of the gecko's geometric switching mechanism, we have investigated the use of origami design methods to create geometries that can transition from a stiff configuration to a more flexible and easily peeled configuration. Specifically, we examined three different origami designs (Kresling, Waterbomb and Ron Resch) fabricated in polycarbonate and supplemented with 3D printed structures. Although the polycarbonate could be coated with a commercial adhesive, we investigated the devices in contact with polydimethylsiloxane adhesive pads in order to chemically control interfaces and create a range of differing adhesion levels. We show that the devices are capable of moderate switching ratios (Fon/Foff up to ∼50). We give a simple model to aid design and provide many options for scaling design performance through size, adhesive strength or through repetition of the pattern beyond a single unit cell.

Kuttsukigami: sticky sheet design.

Shaping 3D objects from 2D sheets enables form and function in diverse areas from art to engineering. Here we introduce kuttsukigami, which exploits sheet-sheet adhesion to create structure. The technique allows thin sheets to be sculpted without requiring sharp folds, enabling structure in a broad range of materials for a versatile and reconfigurable thin-sheet engineering design scheme. Simple closed structures from cylindrical loops to complex shapes like the Möbius loop are constructed and modeled through the balance between deformation and adhesion. Importantly, the balance can be used to create experimental measurements of elasticity in complex morphologies. More practically, kuttsukigami is demonstrated to encapsulate objects from the kitchen to micro scales and to build on-demand logic gates through sticky electronic sheets for truly reusable, reconfigurable devices.

Energy renormalization for temperature transferable coarse-graining of silicone polymer.

The bottom-up prediction of thermodynamic and mechanical behaviors of polymeric materials based on molecular dynamics (MD) simulation is of critical importance in polymer physics. Although the atomistically informed coarse-grained (CG) model can access greater spatiotemporal scales and retain essential chemical specificity, the temperature-transferable CG model is still a big challenge and hinders widespread application of this technique. Herein, we use a silicone polymer, i.e., polydimethylsiloxane (PDMS), having an incredibly low chain rigidity as a model system, combined with an energy-renormalization (ER) approach, to systematically develop a temperature-transferable CG model. Specifically, by introducing temperature-dependent ER factors to renormalize the effective distance and cohesive energy parameters, the developed CG model faithfully preserved the dynamics, mechanical and conformational behaviors compared with the target all-atomistic (AA) model from glassy to melt regimes, which was further validated by experimental data. With the developed CG model featuring tremendously improved computational efficiency, we systematically explored the influences of cohesive interaction strength and temperature on the dynamical heterogeneity and mechanical response of polymers, where we observed consistent trends with other linear polymers with varying chain rigidity and monomeric structures. This study serves as an extension of our proposed ER approach of developing temperature transferable CG models with diverse segmental structures, highlighting the critical role of cohesive interaction strength on CG modeling of polymer dynamics and thermomechanical behaviors.

Silicone elastomers and the Persson-Brener adhesion model.

Many modern anti-icing and anti-fouling coatings rely on soft, low surface energy elastomeric materials such as polydimethylsiloxane for their functionality. While the low surface energy is desirable for reducing adhesion, very little work considers the larger contribution to adhesive failure caused by the viscoelastic nature of elastomers. Here we examine several different siloxane elastomers using a JKR adhesion test, which was operated over a range of different speeds and temperatures. Additionally, we characterize the dynamic mechanical modulus over a large range of frequencies for each material. We note that surface energies of the materials are all similar, but variation in adhesion strength is clear in the data. The variation at low speeds is related to elastomer architecture but the speed dependence itself is independent of architecture. Qualitative correlations are noted between the JKR adhesion measurements and the dynamic moduli. Finally, an attempt is made to directly compare moduli and adhesion through the recent Persson-Brener model. Approximations of the model are shown to be inaccurate. The full model is found to be accurate at low speeds, although it fails to precisely capture higher speed behaviour.

Crumpling Defective Graphene Sheets.

Upon crumpling, graphene sheets yield intriguing hierarchical structures with high resistance to compression and aggregation, garnering a great deal of attention in recent years for their remarkable potential in a variety of applications. Here, we aim to understand the effect of Stone-Wales (SW) defects, i.e., a typical topological defect of graphene, on the crumpling behavior of graphene sheets at a fundamental level. By employing atomistically informed coarse-grained molecular dynamics (CG-MD) simulations, we find that SW defects strongly influence the sheet conformation as manifested by the change in size scaling laws and weaken the self-adhesion of the sheet during the crumpling process. Remarkably, the analyses of the internal structures (i.e., local curvatures, stresses, and cross-section patterns) of crumpled graphene emphasize the enhanced mechanical heterogeneity and "glass-like" amorphous state elicited by SW defects. Our findings pave the way for understanding and exploring the tailored design of crumpled structure via defect engineering.

Crumpled Kirigami.

When a thin sheet is confined to a volume much smaller than its length (or width), it forms a complex state of sharp bends, point-like developable cones (d-cones) and extended ridges known as crumpled matter. One interesting feature of this state, is its high resistance to compression given its light weight. While the origins of this strength still remain a matter of debate, much has been learned through simple experiments and models. Very little work has explored how crumpling is affected by the sheet's topology, which is curious given the close relation between geometry and mechanics. In this work, we couple confocal microscopy with simple force experiments and coarse-grained molecular dynamics (CG-MD) simulations to explore how adding cuts to a sheet alters its behavior in the crumpled state. We find that cutting does not significantly alter the overall compressive behaviour: force scales as a power law irrespective of cuts and magnitudes are only slightly reduced by cutting. Remarkably, when examining regions of high curvature in the crumpled sheets we see evidence of significant changes in the distribution of curvature in cut sheets.

Roughness tolerant pressure sensitive adhesives made of sticky crumpled sheets.

If an adhesive is meant to be temporary, roughness often poses a challenge for design. An adhesive could be made soft so that it can deform and increase surface contact but a softer material will in general hold a smaller load. Bioinspired adhesives, made with numerous microscale posts, show promise as roughness tolerant adhesives but are complicated to fabricate. In this work, we show how thin polymer sheets, when crumpled into a roughly spherical shape, form a very simple and roughness tolerant adhesive system. We use micro and macro-scale experiments to measure adhesion forces between various substrates and crumpled polydimethylsiloxane sheets. We find the force-displacement curves resemble probe-tack experiments of traditional pressure sensitive adhesives and that moderate tensile forces are required to initiate interfacial failure. Notably, we see that sticky crumples often perform better on long wavelength roughness than they do on smooth substrates. In order to improve the peak pull-off forces, we create a sticky crumple from a thin sheet of a glassy polymer, polycarbonate, coated with an adhesive layer. This elasto-plastic sticky crumple achieves high pull-off forces even on the rough surface of a landscaping brick.

Adhesion directed capillary origami.

Capillary origami takes advantage of the surface forces of a liquid drop to assemble thin film structures. After a structure is assembled, the drop then evaporates away. The transient nature of the liquid drop means that the creation of dry and stable structures is impossible. Work presented in this paper shows that adhesion is, in fact, a key tool that enables the creation of stable, complex, capillary assembled origami structures, rather than a problem to be avoided. Here, polydimethylsiloxane thin films were used in several simple experiments designed to identify the balance between substrate-film adhesion and film-film adhesion in the context of capillary assembly. We then demonstrate how directional adhesion can be used to direct film peeling in order to create non-trivial patterned folds after a fluid drop is deposited. A minimal complex structure, a "double-fold" was created to demonstrate how adhesion uniquely facilitates multiple-step capillary assembly. Finally, a familiar "origami airplane" was created with these methods, demonstrating that adhesion aided capillary origami can be used to assemble complex, functional structures.

Understanding the Role of Self-Adhesion in Crumpling Behaviors of Sheet Macromolecules.

Understanding the crumpling behavior of two-dimensional (2D) macromolecular sheet materials is of fundamental importance in engineering and technological applications. Among the various properties of these sheets, interfacial adhesion critically contributes to the formation of crumpled structures. Here, we present a coarse-grained molecular dynamics (CG-MD) simulation study to explore the fundamental role of self-adhesion in the crumpling behaviors of macromolecular sheets having varying masses or sizes. By evaluating the potential energy evolution, our results show that the self-adhesion plays a dominant role in the crumpling behavior of the sheets compared to in-plane and out-of-plane stiffnesses. The macromolecular sheets with higher adhesion tend to form a self-folding planar structure at the quasi-equilibrium state of the crumpling and exhibit a lower packing efficiency as evaluated by the fractal dimension of the system. Notably, during the crumpling process, both the radius of gyration Rg and the hydrodynamic radius Rh of the macromolecular sheet can be quantitatively described by the power-law scaling relationships associated with adhesion. The evaluation of the shape descriptors indicates that the overall crumpling behavior of macromolecular sheets can be characterized by three regimes, i.e., the less bent, intermediate, and highly crumpled regimes, dominated by edge-bending, self-adhesion, and further compression, respectively. The internal structural analysis further reveals that the sheet transforms from the initially ordered state to the disordered glassy state upon crumpling, which can be facilitated by greater self-adhesion. Our study provides fundamental insights into the adhesion-dependent structural behavior of macromolecular sheets under crumpling, which is essential for establishing the structure-processing-property relationships for crumpled macromolecular sheets.

Adhesion of a tape loop.

In this work, we revisit experimentally and theoretically the mechanics of a tape loop. Using primarily elastic materials (polydimethylsiloxane, PDMS, or polycarbonate, PC) and confocal microscopy, we monitor the shape as well as the applied forces during an entire cycle of compression and retraction of a half-loop compressed between parallel glass plates. We observe distinct differences in film shape during the cycle; points of equal applied force or equal plate separation differ in shape upon compression or retraction. To model the adhesion cycle in its entirety, we adapt the 'Sticky Elastica' of [T. J. W. Wagner et al., Soft Matter, 2013, 9, 1025-1030] to the tape loop geometry, which allows a complete analytical description of both the force balance and the film shape. We show that under compression the system is generally not sensitive to interfacial interactions, whereas in the limit of large separation of the confining parallel plates during retraction the system is well described by the peel model. Ultimately, we apply this understanding to the measurement of the energy release rate of a wide range of different cross-linker ratio PDMS elastomer half-loops in contact with glass. Finally, we show how the model illuminates an incredibly simple adhesion measurement technique, which only requires a ruler to perform.

The compressive strength of crumpled matter.

Crumpling a sheet creates a unique, stiff and lightweight structure. Use of crumples in engineering design is limited because there are not simple, physically motivated structure-property relations available for crumpled materials; one cannot trust a crumple. On the contrary, we demonstrate that an empirical model reliably predicts the reaction of a crumpled sheet to a compressive force. Experiments show that the prediction is quantitative over 50 orders of magnitude in force, for purely elastic and highly plastic polymer films. Our data does not match recent theoretical predictions based on the dominance of building-block structures (bends, folds, d-cones, and ridges). However, by directly measuring substructures, we show clearly that the bending in the stretching ridge is responsible for the strength of both elastic and plastic crumples. Our simple, predictive model may open the door to the engineering use of a vast range of materials in this state of crumpled matter.

Microscopic details of a fluid/thin film triple line.

In recent years, there has been a considerable interest in the mechanics of soft objects meeting fluid interfaces (elasto-capillary interactions). In this work we experimentally examine the case of a fluid resting on a thin film of rigid material which, in turn, is resting on a fluid substrate. To simplify complexity, we adapt the experiment to a one-dimensional contact geometry and examine the behaviour of polystyrene and polycarbonate films directly with confocal microscopy. We find that the fluid meets the film in a manner consistent with the Young-Dupré equation when the film is thick, but transitions to what appears similar to a Neumann-like balance when the thickness is decreased. However, on closer investigation we find that the true contact angle is always given by the Young construction. The apparent paradox is a result of macroscopically measured angles not being directly related to true microscopic contact angles when curvature is present. We model the effect with an Euler-Bernoulli beam on a Winkler foundation as well as with an equivalent energy-based capillary model. Notably, the models highlight several important lengthscales and the complex interplay of tension, gravity, and bending in the problem.

Localization in an idealized heterogeneous elastic sheet.

Localized deformation is ubiquitous in many natural and engineering materials as they approach failure, and a significant effort has been made to understand localization processes with simple continuum models. Real materials are much more commonly heterogeneous but it is unclear exactly how heterogeneity affects outcomes. In this work we study the response of an idealized heterogenous elastic sheet on a soft foundation as it is uniaxially compressed. The patterned surface layers are created by selective ultraviolet/ozone treatment of the top surface of a polydimethylsiloxane (PDMS) sample using a TEM grid as a mask. By controlling the exposure time of UV/O3, samples ranging from continuous thin films to sets of isolated small plates were created. We find that patterned regions noticeably localize while bulk regions appear as uniform wrinkles, and that local and global strains depend on the pattern pitch, exposure levels and the treatment protocol. Remarkably, various responses can be modeled using well-understood theory that ignores pattern details aside from the small distance between the adjacent boundaries and the local value of strain.

Late stage drainage of block copolymer stabilized emulsion drops.

Polymer covered emulsion droplets have a considerable number of applications ranging from active cosmetics to advance drug delivery systems. In many of these systems the emulsion droplets do not exist in isolation but interact with other drops, surfaces and particles. In a step towards understanding how these complex mechanical interactions take place, we examine the interaction between a block copolymer covered emulsion droplet (polystyrene-b-poly(ethylene oxide) (PS-PEO) covered toluene) and a flat mica interface. As buoyancy pushes the droplet upwards, it buckles in as it nears the mica and traps a droplet of the surrounding fluid. The trapped outer fluid (water/glycerine in our experiment) drains out through an annular region of PEO brush. This study focuses on the late stage drainage, unique to large molecule surfactants, and examines the effects of the polymer and droplet size on the drainage rate. We introduce a scaling model of the drainage which highlights the importance of three lengthscales in the problem - the brush height, the slip length along the emulsion drop interface and the width of the annular contact region.

Using the sessile drop geometry to measure fluid and elastic block copolymer interfaces.

There is considerable interest in the fabrication and mechanics of soft spheres and capsules because of their use in a large number of applications ranging from targeted drug delivery to cosmetically active agents. Many systems, such as lipid and block copolymer vesicles, are already finding considerable industrial use where the performance of soft spheres depends intimately on their mechanics. New advanced features such as fast cargo delivery can be realized only if they fit into the existing mechanical niche of the system in question. Here we present a model system to demonstrate how a capsule structure can be fundamentally changed while maintaining its overall mechanical response as well as a simple, universal method to measure the resulting capsule material properties. Specifically, we use confocal microscopy to adapt the sessile drop geometry to a measurement of the static properties of an ensemble of polystyrene-b-poly(ethylene oxide) (PS-PEO)-stabilized oil droplets. We then synthesize a polystyrene-b-poly(acrylic acid)-b-polystyrene (PS-PAA-PS) elastic-shell-coated emulsion drop that shows an identical deformation to the fluidlike PS-PEO droplets. Both systems, in sessile geometry, can be related to their basic material properties through appropriate modeling. We find that the elastic shell is dominated by its surface tension, easily enabling it to match the static response of a purely fluid drop.

Localization and length-scale doubling in disordered films on soft substrates.

Wrinkling and folding are examined experimentally for three distinct types of disordered films on polydimethylsiloxane (PDMS) substrates; diblock copolymers, glassy polymers, and single-wall carbon nanotubes. All three of these systems exhibit localization and length-scale doubling at small strains, and we qualitatively account for these observations with a simple physical argument related to the width of the stress correlation function and the interaction of localization sites. Our results have relevance to wrinkling and folding in a diverse array of disordered films on soft substrates, and the insights offered here should help guide the development of theoretical models for the influence of structural disorder on thin-film wrinkling instabilities.

Onset of plasticity in thin polystyrene films.

Polymer glasses have numerous advantageous mechanical properties in comparison to other materials. One of the most useful is the high degree of toughness that can be achieved due to significant yield occurring in the material. Remarkably, the onset of plasticity in polymeric materials is very poorly quantified, despite its importance as the ultimate limit of purely elastic behavior. Here, we report the results of a novel experiment which is extremely sensitive to the onset of yield and discuss its impact on measurement and elastic theory. In particular, we use an elastic instability to locally bend and impart a local tensile stress in a thin, glassy polystyrene film, and directly measure the resulting residual stress caused by the bending. We show that plastic failure is initiated at extremely low strains, of the order 10(-3) for polystyrene. Not only is this critical strain found to be small in comparison to bulk measurement, we show that it is influenced by thin film confinement--leading to an increase in the critical strain for plastic failure as film thickness approaches zero.

Looking beyond fibrillar features to scale gecko-like adhesion.

Hand-sized gecko-inspired adhesives with reversible force capacities as high as 2950 N (29.5 N cm(-2) ) are designed without the use of fibrillar features through a simple scaling theory. The scaling theory describes both natural and synthetic gecko-inspired adhesives, over 14 orders of magnitude in adhesive force capacity, from nanoscopic to macroscopic length scales.

Ordering of a lamella-forming fluid near an interface.

By using wedged thin films, we have measured the effect of interfaces on the ordering of an anisotropic fluid in real space. Symmetric diblock copolymers can form an ordered lamellar fluid, and the preference of the substrate for one of the blocks can induce order well into the disordered bulk phase. The induced order decays away from the substrate with a length scale that diverges at the bulk ordering transition. Ordering and disordering kinetics are found to differ: all layers relax identically upon disordering, whereas the formation of lamellae is found to vary with the distance from the substrate and can be understood from the time-dependent Ginzburg-Landau theory.

Droplet shape of an anisotropic liquid.

We investigate how a droplet of a complex liquid is modified by its internal nanoscale structure. As the liquid passes from an isotropic disordered state to an anisotropic layered morphology, the droplet shape switches from a smooth spherical cap to a terraced hyperbolic profile, which can be modeled as a stack of thin concentric circular disks with a repulsion between adjacent disk edges. Our ability to resolve the detailed shape of these defect-free droplets offers a unique opportunity to explore the underlying physics.