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  1. Abstract

    The skin of the cephalopod is a 3D display, where the papillae muscles control the protrusion of each voxel by several millimeters out of the skin plane, create hierarchical textures, and collectively change the overall skin pattern in a fraction of a second. A material system capable of mimicking this response using electromechanical actuation of twisted spiral artificial muscles (TSAMs) is presented in this study. TSAMs leverage the mechanics of their twisted geometry to extend out of plane by 8 mm, corresponding to 2000% strain using a voltage of only 0.02 V mm−1. They are made of polymer fibers wrapped with a helical metal wire. These actuators are assembled on a stretchable skin with the required flexible electrical connections to form an array of digital texture voxels (DTVs). The DTV array produces arbitrary 3D surface patterns on‐demand, and provides opportunities to control hydrodynamic drag, camouflage, and haptic displays.

     
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  2. Soft robotics offer unusual bioinspired solutions to challenging engineering problems. Colorful display and morphing appendages are vital signaling modalities used by natural creatures to camouflage, attract mates, or deter predators. Engineering these display capabilities using traditional light emitting devices is energy expensive and bulky and requires rigid substrates. Here, we use capillary-controlled robotic flapping fins to create switchable visual contrast and produce state-persistent, multipixel displays that are 1000- and 10-fold more energy efficient than light emitting devices and electronic paper, respectively. We reveal the bimorphic ability of these fins, whereby they switch between straight or bent stable equilibria. By controlling the droplets temperature across the fins, the multifunctional cells simultaneously exhibit infrared signals decoupled from the optical signals for multispectral display. The ultralow power, scalability, and mechanical compliance make them suitable for curvilinear and soft machines. 
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    Free, publicly-accessible full text available June 30, 2024
  3. The diverse chemical and physical reactions encountered during cooking connect us to science every day. Here, we theoretically and experimentally investigate the swelling and softening of pasta due to liquid imbibition as well as the elastic deformation and adhesion of pasta due to capillary force. As water diffuses into the pasta during cooking, it softens gradually from the outside inward as starch swells and relaxes. The softening follows three sequential regimes: regime I, the hard-glassy region, shows a slow decrease in modulus with cooking time; regime II, the glassy to rubbery transition region, or leathery region, is characterized by a very fast, several orders of magnitude drop in elastic modulus and regime III, the rubbery region, has an asymptotic modulus about four orders of magnitude lower than the raw pasta. We present experiments and theory to capture these regimes and relate them to the heterogeneous microstructure changes associated with swelling. Interestingly, we observe a modulus drop of two orders of magnitude within the range of “al dente” cooking duration, and we find the modulus to be extremely sensitive to the amount of salt added to the boiling water. While most chefs can gauge the pasta by tasting its texture, our proposed experiment, which only requires a measurement with a ruler, can precisely provide an optimal cooking time finely tuned for various kinds of pasta shapes.

     
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  4. Drying of fine hair and fibers induces dramatic capillary-driven deformation, with important implications on natural phenomena and industrial processes. We recently observed peculiar self-assembly of hair bundles into various distinct patterns depending on the interplay between the bundle length and the liquid drain rate. Here, we propose a mechanism for this pattern selection, and derive and validate theoretical scaling laws for the polymorphic self-assembly of polygonal hair bundles. Experiments are performed by submerging the bundles into a liquid bath, then draining down the liquid. Depending on the interplay between the drain rates and the length of the fibers, we observe the bundles morphing into stars (having concave sides), polygons (having straight edges and rounded corners), or circles. The mechanism of self-assembly at the high drain regime is governed by two sequential stages. In the first stage of the high drain rate regime, the liquid covers the outside of the bundles, and drainage from inside the bundle does not play a role in the self-assembly due to the high viscous stress. The local pressure at the corners of the wet bundles compresses the fibers inward blunting the corners, and the internal lubrication facilitates fiber rearrangement. In the second stage, the liquid is slowly draining from within the fiber spacing, and the negative capillary pressure at the perimeter causes the fibers to tightly pack. In the slow drainage regime, the first stage is absent, and the fibers slowly aggregate without initial dynamic rearrangement. Understanding the mechanism of dynamic elastocapillarity offers insights for studying the complicated physics of wet granular drying. 
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  6. Heat engines have long been used for transportation, but electric motors, which deliver clean mechanical actuation and come in a wide range of sizes, have enabled the spread of automated motion used, for example, in pumps, compressors, fans, and escalators. Nonetheless, natural muscles, their biological counterparts, are far more ubiquitous. Human bodies have more than 600 muscles that drive functions such as heartbeat, facial expressions, and locomotion. There are many opportunities for expanding the use of actuators that mimic muscles by directly using electric, thermal, or chemical energy to generate motion and enable more pervasive automation. In this issue, on pages 150, 155, and 145, Mu et al. (1), Yuan et al. (2), and Kanik et al. (3), respectively, describe new types of fiber-shaped artificial muscles that exploit advantages derived from the mechanics of twisted and coiled geometries. 
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  7. Heat engines have long been used for transportation, but electric motors, which deliver clean mechanical actuation and come in a wide range of sizes, have enabled the spread of automated motion used, for example, in pumps, compressors, fans, and escalators. Nonetheless, natural muscles, their biological counterparts, are far more ubiquitous. Human bodies have more than 600 muscles that drive functions such as heartbeat, facial expressions, and locomotion. There are many opportunities for expanding the use of actuators that mimic muscles by directly using electric, thermal, or chemical energy to generate motion and enable more pervasive automation. In this issue, on pages 150, 155, and 145, Mu et al. (1), Yuan et al. (2), and Kanik et al. (3), respectively, describe new types of fiber-shaped artificial muscles that exploit advantages derived from the mechanics of twisted and coiled geometries. 
    more » « less