Thiosquaramide-Based Supramolecular Polymers: Aromaticity Gain in a Switched Mode of Self-Assembly.

Despite a growing understanding of factors that drive monomer self-assembly to form supramolecular polymers, the effects of aromaticity gain have been largely ignored. Herein, we document the aromaticity gain in two different self-assembly modes of squaramide-based bolaamphiphiles. Importantly, O → S substitution in squaramide synthons resulted in supramolecular polymers with increased fiber flexibility and lower degrees of polymerization. Computations and spectroscopic experiments suggest that the oxo- and thiosquaramide bolaamphiphiles self-assemble into "head-to-tail" versus "stacked" arrangements, respectively. Computed energetic and magnetic criteria of aromaticity reveal that both modes of self-assembly increase the aromatic character of the squaramide synthons, giving rise to stronger intermolecular interactions in the resultant supramolecular polymer structures. These examples suggest that both hydrogen-bonding and stacking interactions can result in increased aromaticity upon self-assembly, highlighting its relevance in monomer design.

Uncovering friction dynamics using hydrogel particles as soft ball bearings.

Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontrivial rate-dependent lubrication behavior. Unlike Newtonian lubrication or dry solid friction, hydrogel particles in suspension transition through four frictional regimes as a function of sliding velocity. We relate the different regimes to the deformation of the particles at different gap sizes, which changes the effective contact area between the sliding surfaces. By systematically varying the particle characteristics and the surface properties of the sliding surfaces, we assign potential mechanisms for each of the different lubricating regimes as a function of velocity: (I) relatively high friction due to particle flattening and direct contact between interacting bodies (II) decrease of friction owing to the presence of rolling particles (III) large inflow of particles in a confined space leading to compressed particles and (IV) the formation of a thick lubricating layer. Using these suspensions with soft, deformable particles as a ball bearing system, we provide new insights into soft material friction with applications in emulsions, powders, pastes or other granular materials.