Bending Instability of Rod-Shaped Bacteria

Non-Classical Euler Buckling and Brazier Instability in Cylindrical Liquid Droplets

Crystalline monolayers prevalent in nature and technology possess elusive elastic properties with important implications in fundamental physics, biology, and nanotechnology. Leveraging the recently discovered shape transitions of oil-in-water emulsion droplets, upon which these droplets adopt cylindrical shapes and elongate, we investigate the elastic characteristics of the crystalline monolayers covering their interfaces. To unravel the conditions governing Euler buckling and Brazier kink formation in these cylindrical tubular interfacial crystals, we strain the elongating cylindrical droplets within confining microfluidic wells. Our experiments unveil a nonclassical relation between the Young's modulus and the bending modulus of these crystals. Intriguingly, this relation varies with the radius of the cylindrical crystal, presenting a nonclassical mechanism for tuning of elasticity in nanotechnology applications.

Reversible kink instability drives ultrafast jumping in nematodes and soft robots

Entomopathogenic nematodes (EPNs) exhibit a bending-elastic instability, or kink, before becoming airborne, a feature hypothesized but not proven to enhance jumping performance. Here, we provide the evidence that this kink is crucial for improving launch performance. We demonstrate that EPNs actively modulate their aspect ratio, forming a liquid-latched closed loop over a slow timescale O (1 s), then rapidly open it O (10 µs), achieving heights of 20 body lengths (BL) and generating ∼ 10 4 W/Kg of power. Using jumping nematodes, a bio-inspired Soft Jumping Model (SoftJM), and computational simulations, we explore the mechanisms and implications of this kink. EPNs control their takeoff direction by adjusting their head position and center of mass, a mechanism verified through phase maps of jump directions in simulations and SoftJM experiments. Our findings reveal that the reversible kink instability at the point of highest curvature on the ventral side enhances energy storage using the nematode's limited muscular force. We investigated the impact of aspect ratio on kink instability and jumping performance using SoftJM, and quantified EPN cuticle stiffness with AFM, comparing it with C. elegans . This led to a stiffness-modified SoftJM design with a carbon fiber backbone, achieving jumps of ∼25 BL. Our study reveals how harnessing kink instabilities, a typical failure mode, enables bidirectional jumps in soft robots on complex substrates like sand, offering a novel approach for designing limbless robots for controlled jumping, locomotion, and even planetary exploration.

Soft Deployable Structures via Core-Shell Inflatables

Deployable structures capable of significant geometric reconfigurations are ubiquitous in nature. While engineering contraptions typically comprise articulated rigid elements, soft structures that experience material growth for deployment mostly remain the handiwork of biology, e.g., when winged insects deploy their wings during metamorphosis. Here we perform experiments and develop formal models to rationalize the previously unexplored physics of soft deployable structures using core-shell inflatables. We first derive a Maxwell construction to model the expansion of a hyperelastic cylindrical core constrained by a rigid shell. Based on these results, we identify a strategy to obtain synchronized deployment in soft networks. We then show that a single actuated element behaves as an elastic beam with a pressure-dependent bending stiffness which allows us to model complex deployed networks and demonstrate the ability to reconfigure their final shape. Finally, we generalize our results to obtain three-dimensional elastic gridshells, demonstrating our approach's applicability to assemble complex structures using core-shell inflatables as building blocks. Our results leverage material and geometric nonlinearities to create a low-energy pathway to growth and reconfiguration for soft deployable structures.