Polymorphic display and texture integrated systems controlled by capillarity.

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.

Plant cell-like tip-growing polymer precipitate with structurally embedded multistimuli sensing ability.

Soft systems that respond to external stimuli, such as heat, magnetic field, and light, find applications in a range of fields including soft robotics, energy harvesting, and biomedicine. However, most of the existing systems exhibit nondirectional, nastic movement as they can neither grow nor sense the direction of stimuli. In this regard, artificial systems are outperformed by organisms capable of directional growth in response to the sense of stimuli or tropic growth. Inspired by tropic growth schemes of plant cells and fungal hyphae, here we report an artificial multistimuli-responsive tropic tip-growing system based on nonsolvent-induced phase separation of polymer solution, where polymer precipitates as its solvent dissolves into surrounding nonsolvent. We provide a theoretical framework to predict the size and velocity of growing precipitates and demonstrate its capability of sensing the directions of gravity, mechanical contact, and light and adjusting its growing direction in response. Exploiting the embedded physical intelligence of sensing and responding to external stimuli, our soft material system achieves multiple tasks including printing 3D structures in a confined space, bypassing mechanical obstacles, and shielded transport of liquids within water.

Dynamic pattern selection in polymorphic elastocapillarity.

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.

Coalescence of oil drops and films on micropillared substrates enabled by enhanced water drainage through pillar gaps.

Collecting or removing oil drops dispersed in water is essential in a range of industrial processes including oil recovery and oil spill cleanup. However, it is by no means easy to induce merging of oil drops with another body of oil due to slow drainage of the intervening water film. Here, we report immediate coalescence of oil drops with oil films that lie on micropillar arrays as immersed in various aqueous solutions. In addition to experimental demonstrations, we theoretically construct a regime map to predict whether the drop will bounce off or coalesce with the oil film, which is determined by the oil film thickness, geometry of the pillar array and liquid properties. Good agreement between theory and experiment indicates that micropillar arrays provide additional drainage paths of the intervening film, which helps drop-film coalescence. Our results suggest potential implications of utilizing oil-laden microporous structures to achieve efficient demulsification of oil drops in contaminated water.

Hydrodynamic Elastocapillary Morphing of Hair Bundles.

We report polymorphic self-assembly of hair arranged in hollow bundles driven by capillarity, hydrodynamics, and elasticity. Bundles emerging from a liquid bath shrink but remain hollow at slow drainage due to the negative pressure of the menisci trapped between the hairs. The timescale allows the collective stiffening of the fibers to resist closure. At fast drainage, the bundles fully close before the liquid can drain through the hair. A liquid column trapped in the hole closes the bundle while the lubricated hairs still behave softly. Scaling laws predict this reversible hair polymorphism.

Poro-elasto-capillary wicking of cellulose sponges.

We mundanely observe cellulose (kitchen) sponges swell while absorbing water. Fluid flows in deformable porous media, such as soils and hydrogels, are classically described on the basis of the theories of Darcy and poroelasticity, where the expansion of media arises due to increased pore pressure. However, the situation is qualitatively different in cellulosic porous materials like sponges because the pore expansion is driven by wetting of the surrounding cellulose walls rather than by increase of the internal pore pressure. We address a seemingly so simple but hitherto unanswered question of how fast water wicks into the swelling sponge. Our experiments uncover a power law of the wicking height versus time distinct from that for nonswelling materials. The observation using environmental scanning electron microscopy reveals the coalescence of microscale wall pores with wetting, which allows us to build a mathematical model for pore size evolution and the consequent wicking dynamics. Our study sheds light on the physics of water absorption in hygroscopically responsive multiscale porous materials, which have far more implications than everyday activities (for example, cleaning, writing, and painting) carried out with cellulosic materials (paper and sponge), including absorbent hygiene products, biomedical cell cultures, building safety, and cooking.

Hygrobot: A self-locomotive ratcheted actuator powered by environmental humidity.

Microrobots that are light and agile yet require no artificial power input can be widely used in medical, military, and industrial applications. As an actuation system to drive such robots, here we report a biologically inspired bilayer structure that harnesses the environmental humidity energy, with ratchets to rectify the motion. We named this actuator-ratchet system the hygrobot. The actuator uses a hygroscopically responsive film consisting of aligned nanofibers produced by directional electrospinning, which quickly swells and shrinks in lengthwise direction in response to the change of humidity. The ratchets based on asymmetric friction coefficients rectify oscillatory bending motion in a directional locomotion. We mathematically analyzed the mechanical response of the hygrobot, which allowed not only prediction of its performance but also the optimal design to maximize the locomotion speed given geometric and environmental constraints. The hygrobot sterilized a trail across an agar plate without any artificial energy supply.