No friction required: Self-locking in entangled assemblies of curved filaments
Filaments are incredible material building blocks that, when entangled together, form robust structures with remarkable adaptive mechanics, from stiff cables to stretchable knits. Basic questions about the mechanism that holds together such structures, especially assemblies like rope, without attractive physical or chemical bonds between constituents are among the oldest considered in materials science. Contradicting a long-standing picture that structural integrity of rope-like assemblies rely on a combination of external loading and inter-filament friction to prevent spontaneous detangling (e.g. fraying of a cut rope), we demonstrate that assemblies of intrinsically-curved filaments can achieve structural integrity purely from the competition of elastic forces that resist shape-change and (traction-free) repulsive forces that prevent overlap. We analyze a friction-free model of assembly of intrinsically-helical filaments, and show that depending on filament number and preferred filament shape, overcrowding in the assemblies generate elastic pre-stresses that give rise to either self-locking entanglement or spontaneous detanglement through propagation of a soliton-like detanglement front through assemblies. We test a generalized fracture criterion for spontaneous detangling and confirm the role of shape-elastic costs to self-locking via experiment and simulation of frictional filaments, demonstrating a range of bundled structures which cannot be made self-locking for any value of friction. The mechanism of ``purely elastic cohesion" illuminated here sheds light on self-locking in a broader class of entangled structures, and further, offers design principles for new classes of active filament materials capable of stimuli-responsive assembly and disassembly.