In the classic theory of solid adhesion, surface energy drives deformation to increase contact area whereas bulk elasticity opposes it. Recently, solid surface stress has been shown also to play an important role in opposing deformation of soft materials. This suggests that the contact line in soft adhesion should mimic that of a liquid droplet, with a contact angle determined by surface tensions. Consistent with this hypothesis, we observe a contact angle of a soft silicone substrate on rigid silica spheres that depends on the surface functionalization but not the sphere size. However, to satisfy this wetting condition without a divergent elastic stress, the gel phase separates from its solvent near the contact line. This creates a four-phase contact zone with two additional contact lines hidden below the surface of the substrate. Whereas the geometries of these contact lines are independent of the size of the sphere, the volume of the phase-separated region is not, but rather depends on the indentation volume. These results indicate that theories of adhesion of soft gels need to account for both the compressibility of the gel network and a nonzero surface stress between the gel and its solvent.wetting | adhesion | soft matter | surface tension | phase separation S olid surfaces stick together to minimize their total surface energy. However, if the surfaces are not flat, they must conform to one another to make adhesive contact. Whether or not this contact can be made, and how effectively it can be made, are crucial questions in the study and development of solid adhesive materials (1, 2). These questions have wide-ranging technological consequence. With applications ranging from construction to medicine, and large-scale manufacturing to everyday sticky stuff, adhesive materials are ubiquitous in daily life. However, much remains unknown about the mechanics of solid adhesion, especially when the solids are very compliant (3-5). This limits our understanding and development of anything that relies on the mechanics of soft contact, including pressure-sensitive adhesives (6, 7), rubber friction (8), materials for soft robotics (9-12), and the mechanical characterization of soft materials, including living cells (13)(14)(15)(16)(17).Adhesion is favorable whenever the adhesion energy, W = γ 1 + γ 2 − γ 12 , is positive, where γ 1 and γ 2 are the surface energies of the free surfaces and γ 12 is the interfacial energy in contact. When W > 0, the solids are driven to deform spontaneously to increase their area of contact, but at the cost of incurring elastic strain. The foundational and widely applied Johnson-Kendall-Roberts (JKR) theory of contact mechanics (18, 19) was the first to describe this competition between adhesion and elasticity. However, it was recently shown that the JKR theory does not accurately describe adhesive contact with soft materials because it does not account for an additional penalty against deformation due to solid surface stress, ! (4). Unlike a fluid, the surface stress of a solid is not a...

During mating season, males of synchronous firefly species flash in unison within swarms of thousands of individuals. These strongly correlated collective displays have inspired numerous mathematical models to explain how global synchronous patterns emerge from local interactions. Yet, experimental data to validate these models remain sparse. To address this gap, we develop a method for three-dimensional tracking of firefly flashes, using a stereoscopic set-up of 360-degree cameras. We apply this method to record flashing displays of the North American synchronous species Photinus carolinus in its natural habitat as well as within controlled environments, and obtain the three-dimensional reconstruction of flash occurrences in the swarm. Our results show that even a small number of interacting males synchronize their flashes; however, periodic flash bursts only occur in groups larger than 15 males. Moreover, flash occurrences are correlated over several metres, indicating long-range interactions. While this suggests emergent collective behaviour and cooperation, we identify distinct individual trajectories that hint at additional competitive mechanisms. These reveal possible behavioural differentiation with early flashers being more mobile and flashing longer than late followers. Our experimental technique is inexpensive and easily implemented. It is extensible to tracking light communication in various firefly species and flight trajectories in other insect swarms.

Fireflies flashing in unison is a mesmerizing manifestation of animal collective behavior and an archetype of biological synchrony. To elucidate synchronization mechanisms and inform theoretical models, we recorded the collective display of thousands of Photinus carolinus fireflies in natural swarms, and provide the first spatiotemporal description of the onset of synchronization. At low firefly density, flashes appear uncorrelated. At high density, the swarm produces synchronous flashes within periodic bursts. Using three-dimensional reconstruction, we demonstrate that flash bursts nucleate and propagate across the swarm in a relay-like process. Our results suggest that fireflies interact locally through a dynamic network of visual connections defined by visual occlusion from terrain and vegetation. This model illuminates the importance of the environment in shaping self-organization and collective behavior.

Composite microcapsules have been aggressively pursued as designed chemical entities for biomedical and other applications. Common preparations rely on multi-step, time consuming processes. Here, we present a single-step approach to fabricate such microcapsules with shells composed of nanoparticle-polyelectrolyte and protein-polyelectrolyte complexes, and demonstrate control of the mechanical and release properties of these constructs. Interfacial polyelectrolyte-nanoparticle and polyelectrolyte-protein complexation across a water-oil droplet interface results in the formation of capsules with shell thicknesses of a few μm. Silica shell microcapsules exhibited a significant plastic response at small deformations, whereas lysozyme incorporated shells displayed a more elastic response. We exploit the plasticity of nanoparticle incorporated shells to produce microcapsules with high aspect ratio protrusions by micropipette aspiration.

Translocation from one cavity to another through a narrow constriction (i.e., a "hole") represents the fundamental elementary process underlying hindered mass transport of nanoparticles and macromolecules within many natural and synthetic porous materials, including intracellular environments. This process is complex and may be influenced by long-range (e.g., electrostatic) particle−wall interactions, transient adsorption/desorption, surface diffusion, and hydrodynamic effects. Here, we used a three-dimensional (3D) tracking method to explicitly visualize the process of nanoparticle diffusion within periodic porous nanostructures, where electrostatic interactions were mediated via ionic strength. The effects of electrostatic interactions on nanoparticle transport were surprisingly large. For example, an increase in the Debye length of only a few nanometers (in a material with a hole diameter of ∼100 nm) increased the mean cavity escape time 3-fold. A combination of computational and experimental analyses indicated that this hindered cavity escape was due to an electrostatic energy barrier in the region of the hole, which was quantitatively explained using DLVO theory. These findings explicitly demonstrate that the cavity escape process was barrier-limited and dominated by electrostatic effects.

We used single-molecule tracking to probe the local rheology of interfacial water.Fluorescent rhodamine molecules were tracked on silica surfaces as a function of ambient relative humidity, which controlled the thickness of condensed water nanofilms. At low humidity, the molecules exhibited confined diffusion in the vicinity of isolated adsorption sites characterized by a broad distribution of binding stiffness constants; subsequent chemical or physical surface passivation selectively eliminated stiffer binding sites. At increased humidity, molecularly thin water films condensed, permitting near-surface transport of rhodamine molecules. Motion was subdiffusive, with an anomalous exponent increasing with the nanofilm thickness. Molecular trajectories were temporally anticorrelated, ergodic, but also featured transient binding and intermittent diffusion.Statistical modeling demonstrated that this complex motion in water nanofilms had the characteristics of fractional Brownian motion combined with a continuous time random walk. This was consistent with diffusion within viscoelastic nanofilms, suggesting persistent molecular structuring in the vicinity of the silica surface. 1 arXiv:2001.06072v1 [cond-mat.soft] 16 Jan 2020 Keywords silica, water nanofilm, single-molecule tracking, anomalous diffusion, confined diffusion, molecular structuringThe interface between water and silica (or related silicate minerals) is central to a wide range of geological and industrial processes, 1,2 yet many of its properties remain elusive, notably in the nanoscale limit where molecules are highly confined and constrained. The different forms of Si-O groups at the surface are believed to interact strongly with water molecules through hydrogen bonding, leading to molecular ordering that extends several Å from the surface. 3Water structuring, in turn, may impact transport properties at the interface, by influencing the hydrodynamic slip or no-slip boundary conditions, 4 and changing local rheology. The viscosity of water nanofilms remains hotly debated. While studies of water confined between two mica (an aluminosilicate mineral) surfaces separated by 5 nm and less found no substantial increase in viscosity, 5,6 experiments using scanning-probe techniques on silica and other hydrophilic surfaces have reported viscosity enhancement by as much as 10 6 . 4,7 Hydrophilic surfaces exposed to water vapor become spontaneously coated by a thin film of water at the Å-to-nm scale, with the exact thickness depending on substrate chemistry, temperature, and vapor pressure. Water physisorption is therefore ubiquitous in the Earth's subsurface, surface, and atmosphere, including on aerosol particles and ice (glacier and snow), 8 with significant consequences for environmental processes, for example in heterogeneous chemical reactions. 9 Thus, there is significant interest in understanding molecular transport in water nanofilms, despite the uncertainty with respect to the structure of these thin films. 10 Recent studies have elucidated the mechanisms o...

Many biological systems fold thin sheets of lipid membrane into complex three-dimensional structures. This microscopic origami is often mediated by the adsorption and self-assembly of proteins on a membrane. As a model system to study adsorption-mediated interactions, we study the collective behavior of micrometric particles adhered to a lipid vesicle. We estimate the colloidal interactions using a maximum likelihood analysis of particle trajectories. When the particles are highly wrapped by a tense membrane, we observe strong long-range attractions with a typical binding energy of 150k_{B}T and significant forces extending a few microns.

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