Towards 4D Printing of Very Soft Heterogeneous Magnetoactive Layers for Morphing Surface Applications via Liquid Additive Manufacturing.

This work explores the use of liquid additive manufacturing (LAM) to print heterogeneous magnetoactive layers. A general method is proposed where, by studying the printing of pure silicone lines, the successful printing of closed shapes, open shapes, and a combination thereof, can be achieved while accounting for the continuous deposition that is specific to LAM. The results of this characterization are subsequently exploited for the printing of a heterogeneous layer composed of four magnetoactive discs embedded in a pure silicone square. Such a layer, when affixed to a softer silicone substrate, yields a system that produces truly three-dimensional surface patterns upon application of a magnetic field. Hence, this work demonstrates that LAM is a promising approach for the rapid 4D printing of morphing surfaces exhibiting 3D surface patterns that can be actuated remotely and reversibly via a magnetic field. Such heterogenous layers have a wide range of applications, ranging from haptics to camouflage to differential cell growth.

Mechanofluorochromic Difluoroboron ß-Diketonates Based Polymer Composites: Toward Multi-Stimuli Responsive Mechanical Stress Probes.

Developing mechano-responsive fluorescent polymers that exhibit distinct responses to distinct mechanical stresses requires a careful design of the fluorophore in order to tune its interactions with the polymer. A series of mechanofluorochromic (MFC) polymer composites are prepared by dispersing difluoroboron diketonates complexes with various alkyl side-chain lengths (DFB-alkyl) in linear low-density polyethylene. Observation of the resulting polymer composites under a microscope reveals different aggregate sizes of the three DFB-alkyls, thus confirming the functionalization by alkyl side chains as a powerful approach to control the aggregation process in a polymer. Besides, the three polymer composite samples are shown to be sensitive to both stretching and scratching, thereby consisting in the first reported example of MFC polymer responding to these two distinct mechanical stimuli. To establish a structure-property relationship, the strategy consisted in applying controlled tensile or friction forces while simultaneously monitoring fluorescence changes. Interestingly, the intensity of the MFC response to both stretching and scratching depends on the alkyl chain length and thus on the aggregation properties of the fluorophore. According to a time-resolved fluorescence study, the emission is found to originate from different species following the type of applied stress (tensile or friction force).

Aggregate-driven reconfigurations of carbon nanotubes in thin networks under strain: in-situ characterization.

This work focuses on the in-situ characterization of multi-walled carbon nanotube (CNT) motions in thin random networks under strain. Many fine-grain models have been devised to account for CNT motions in carbon nanotube networks (CNN). However, the validation of these models relies on mesoscopic or macroscopic data with very little experimental validation of the physical mechanisms actually arising at the CNT scale. In the present paper, we use in-situ scanning electron microscopy imaging and high-resolution digital image correlation to uncover prominent mechanisms of CNT motions in CNNs under strain. Results show that thin and sparse CNNs feature stronger strain heterogeneities than thicker and denser ones. It is attributed to the complex motions of individual CNTs connected to aggregates within thin and sparse CNNs. While the aggregates exhibit a collective homogeneous deformation, individual CNTs connecting them are observed to fold, unwind or buckle, seemingly to accommodate the motion of these aggregates. In addition, looser aggregates feature internal reconfigurations via cell closing, similar to foam materials. Overall, this suggests that models describing thin and sparse CNN deformation should integrate multiphase behaviour (with various densities of aggregates in addition to individual CNTs), heterogeneity across surface, as well as imperfect substrate adhesion.

Controlled, Low-Temperature Nanogap Propagation in Graphene Using Femtosecond Laser Patterning.

Graphene nanogap systems are promising research tools for molecular electronics, memories, and nanodevices. Here, a way to control the propagation of nanogaps in monolayer graphene during electroburning is demonstrated. A tightly focused femtosecond laser beam is used to induce defects in graphene according to selected patterns. It is shown that, contrary to the pristine graphene devices where nanogap position and shape are uncontrolled, the nanogaps in prepatterned devices propagate along the defect line created by the femtosecond laser. Using passive voltage contrast combined with atomic force microscopy, the reproducibility of the process with a 92% success rate over 26 devices is confirmed. Coupling in situ infrared thermography and finite element analysis yields a real-time estimation of the device temperature during electrical loading. The controlled nanogap formation occurs well below 50 °C when the defect density is high enough. In the perspective of graphene-based circuit fabrication, the availability of a cold electroburning process is critical to preserve the full circuit from thermal damage.

Two-field surface pattern control via marginally stable magnetorheological elastomers.

The stability and post-bifurcation of a non-linear magnetoelastic film/substrate block is experimentally exploited to obtain active control of surface roughness. The non-intuitive interplay between magnetic field and elastic deformation owes to material and geometry selection, namely a ferromagnetic particle composite film bonded on a compliant passive foundation. Cooperation of the two otherwise independent loading mechanisms-mechanical pre-compression and magnetic field-allows one to bring the structure near a marginally stable state and then destabilize it with either magnetic or mechanical fields. We demonstrate for the first time that the critical magnetic field is a decreasing function of pre-compression and vice versa. The experimental results are then probed successfully with full-field finite element simulations at large strains and magnetic fields. The magnetoelastic coupling allows for reversible on/off control of surface wrinkling under adjustable critical magnetic and mechanical fields, thus this study constitutes a first step towards realistic active haptic and morphing devices.