Fibril-forming tropocollagens (TCs) play an essential role in tissue biomechanics. They are ubiquitous in mammals and other animal tissues, where they provide passive mechanical functions. While molecular dynamics simulations have targeted the mechanics of individual TCs, experimental data on their tensile mechanical properties remain scarce. As a consequence, the link between the unique triple-helix structure of the collagen molecule and macro-mechanical properties of collagenous tissues is not well understood. To close this gap, we have investigated isolated TCs grafted on the tip of atomic force microscopy (AFM) probes as well as adsorbed TC films using a surface force apparatus (SFA). AFM force spectroscopy showed that an individual TC can be stretched without failing to a contour length of up to 900 nm─nearly three times its native length─over thousands of stretching cycles. The molecule was retracted from a strongly adhering mica surface by pulling on one of the α-chains, forcing the triple-helix to unravel. During this process, the α-chains slipped progressively, irreversibly, and almost entirely past each other before being caught by strong physical interactions between overlapping chain ends. SFA measurements showed that strong electrostatic interactions bind TC to mica and prevent TC aggregation, supporting the AFM results. These findings indicate that a controlled slippage mechanism underpins the exceptional toughness of TCs, collagen fibrils, and collagen-rich tissues such as tendons and skin.

We report on the dielectrowetting of sessile droplets of two common liquid crystals, 4-cyano-4 '-pentylbiphenyl (5CB) and 4-cyano-4 '-n-octylbiphenyl (8CB), deposited on interdigitated electrodes that were treated to induce homeotropic anchoring. We found a pronounced hysteretic response of the contact angle to the applied voltage caused by the pinning and depinning of the droplet contact line. Depinning occurred as the voltage exceeded a threshold value that increased from the nematic to the isotropic phase, whereas the smectic phase showed an intermediate value. Above the threshold, the contact angle decreased linearly and rapidly as a function of the voltage square, as expected from the dielectrowetting equation originally formulated for dielectric and isotropic liquids, with a slope larger in the anisotropic liquid crystal phases than in the isotropic phase. Observation between crossed polarizers showed that the molecular director realigned along the applied field in the anisotropic phase near the surface between the electrodes, thereby increasing the effective dielectric constant and strengthening the dielectrophoretic force compared to the isotropic phase. Director realignment involved the nucleation of topological defects in the nematic phase and was inhibited by large energy barriers in the smectic phase, which weakened the dielectrowetting response.

Collagen, the most abundant protein in mammals, plays a key role in tissue formation and mechanics due to its triple-helix structure. We used atomic force microscopy to study individual type-I and type-III human collagen molecules adsorbed on smooth mica surfaces from low-salt, near-neutral aqueous solutions. Statistical analysis of their two-dimensional contours revealed nonuniform curvature in both collagen types, which persisted after surface drying and molecular dehydration, owing to robust collagen-mica adsorption. In addition, the angle between tangent vectors at the ends of molecular segments followed a non-Gaussian probability distribution, indicative of nonequilibrium quenching of fluctuations upon adsorption to mica. These results suggest that collagen either possesses an intrinsic three-dimensional curvature in solution or acquires a two-dimensional curvature upon adsorption. The first scenario has implications for the self-assembly and elasticity of collagen fibrils, whereas the second has implications in biomaterial design and tissue-engineering strategies.

Epsilon-near-zero (ENZ) metamaterials represent a powerful toolkit for selectively transmitting and localizing light through cavity resonances, enabling the study of mesoscopic phenomena and facilitating the design of photonic devices. In this experimental study, we demonstrate the feasibility of engineering and actively controlling cavity modes, as well as tuning their mutual coupling, in an ENZ multilayer structure. Specifically, by employing a high-birefringence liquid crystal film as a tunable nanocavity, the polarization-dependent coupling of resonant modes with narrow spectral width and spatial extent was achieved. Surface forces aparatus (SFA) allowed us to continuously and precisely control the thickness of the liquid crystal film contained between the nanocavities and thus vary the detuning between the cavity modes. Hence, we were able to manipulate nanocavities anti-crossing behaviors. The suggested methodology unlocks the full potential of tunable optical coupling in epsilon-near-zero metamaterials and provides a versatile approach to the creation of tunable photonic devices, including bio-photonic sensors, and/or tunable planar metamaterials for on-chip spectrometers.

Surface force measurements provide deep insights into the structure and response of liquid crystals (LCs) near surfaces and in nanoscale confinement geometries. Thin LC films in the nematic, smectic-A, and cholesteric phases generate equilibrium repulsive forces that are a direct expression of the order, elasticity, and defect-mediated plasticity of the phase. On the other hand, surface interactions modify the molecular order over nanoscale distances and can produce new structure and response in nanoscale LC films. In this review, we discuss equilibrium surface force measurements obtained in thin LC films using the surface forces apparatus (SFA) and the atomic force microscope (AFM). The results illustrate general theoretical concepts related to surfaces and confinement, such as phase frustration, heterophases, capillary condensation, and defect nucleation, many of which are yet to be explored in ferroelectric, spontaneously modulated, and chiral LC phases. These concepts are relevant to technological applications as nanoscale LC films can effectively modulate colloidal interactions in response to external stimuli in composite materials and are increasingly used in nanoscale photonic devices as polarisation-controlling elements.

Liquid crystal films play a key role in advancing next-generation optical and photonic devices that require a precise in-plane modulation of optical anisotropy. This study employs multiphoton direct laser writing, a high-resolution three-dimensional (3D) printing method, to fabricate pseudoperiodic patterns of lines and grooves on glass surfaces for the in-plane alignment of liquid crystal films. Single layers of lines with submicron thickness and line spacing were fabricated in less than half an hour and forced the in-plane alignment of a liquid crystal film with a thickness of about 10 mu m. We validate the method on patterns with singular topologies designed to induce the nucleation of disclination defects with a predetermined spatial arrangement, orientation, and topological strength. Compared to other surface patterning methods, high-resolution 3D printing provides the unique advantage of direct surface fabrication, enabling the creation of nonflat geometries such as terraces and lenses and expanding the design and functionalities of liquid crystal devices. We anticipate that this method will be used to create thin-film devices such as polarization gratings, beam steerers, and q-plates for manipulating polarized and structured light.

Polymer adhesion is underpinned by a network of chain molecules forming both adhesive bonds with the surfaces and cohesive bonds among themselves. Understanding how this network propagates mechanical stress and dissipates energy under tension is a challenging but essential task for improving polymer adhesives. To this end, we present a series of coarse-grained molecular dynamics simulations of polymer films joining two parallel flat surfaces. The polymer molecules are modeled as chains of beads that can covalently bind to the surfaces and to each other via a Morse potential, allowing bond breakage and chain scission. The mechanical response of the polymer film is studied under both small oscillatory strains and large deformations, leading to complete breakup. Starting from a melt of ungrafted polymer chains and introducing one feature at a time, we find that polymer cross-linking enhances adhesion more than covalent surface grafting. Cross-linking and grafting may also act in synergy, provided the grafting density is homogeneous. Moreover, surface heterogeneity at the nanometer scale affects the viscoelastic response at small strains. The simple model used in our simulations provides a valuable platform to translate molecular-level features of the polymer network, such as chain connectivity and bond strength, into measurable quantities, such as adhesion forces and energy dissipation.

Polaritons can be generated by tuning the optical transitions of a light emitter to the resonances of a photonic cavity. We show that a dye-doped cavity generates resonant tunneling polaritons with Epsilon-Near-Zero (ENZ) effective permittivity. We studied the polariton spectral dispersion in dye-doped metal-dielectric-metal (MDM) cavities as a function of the in-plane (k||) and out-of-plane (k⊥) components of the incident wavevector. The dependence on k|| was investigated through ellipsometry, revealing the ENZ modes. The k⊥ dependence was measured by varying the cavity thickness under normal incidence using a Surface Force Apparatus (SFA). Both methods revealed a large Rabi splitting well exceeding 100 meV. The SFA-based investigation highlighted the collective nature of strong coupling by producing a splitting proportional to the square root of the involved photons. This study demonstrates the possibility of generating ENZ polaritons and introduces the SFA as a powerful tool for the characterization of strong light-matter interactions.

Smectic liquid crystals exhibit a remarkable ability to self-organize into a variety of textures, patterns, and arrays extending over large surface areas with microscale periodicity. These structures provide a useful platform for the bottom-up fabrication of microdevices and functional materials, such as microlens arrays and nanoparticle templates. Periodic structures appear in response to a conflict of director alignment created within the smectic film by external fields such as hybrid (planar-homeotropic) anchoring conditions at the film interfaces or an electric field applied normal to the surface anchoring direction. In this article, we review the formation, structure, and applications of one-dimensional (1d) patterns obtained in response to a director bend, typically induced by unidirectional hybrid anchoring conditions in thin films or under applied electric fields. Compared to lattices of focal conics, 1d patterns harbor an unusual array of topological defects, including disclination and dislocation lines, grain boundaries, and curvature walls, running along periodically spaced and bend-free cylindrical stacks of smectic layers. These defects, most of which have been described by Maurice Kleman, appear to drive the structural evolution of 1d patterns as a function of the film thickness. Under unidirectional hybrid anchoring, the defects are oriented perpendicular to the planar anchoring direction and provide highly anisotropic templates for building ordered networks of close-packed nanoparticles. Moreover, the array formation at the transition from the nematic to the smectic phase is intimately connected to the smectic phase frustration by bend distortions, producing an 'intermediate' smectic phase with partial penetration of the bend distortion through undercooled nematic domains.

Layered Metal-Dielectric-Metal nanocavities sustain resonances with vanishing permittivity. The wavelength of these Epsilon-Near-Zero (ENZ) modes can be tuned straightforwardly by the thickness of the dielectric layer. Multiple ENZ resonances can be engineered by including higher frequency harmonics, and by stacks of strongly interacting cavities. Integrating such cavity systems with light emitting nanomaterials enables light emission enhancement, directional control, and strong light-matter interaction.

Protein-based underwater adhesives of marine organisms exhibit extraordinary binding strength in high salinity based on utilizing a variety of molecular interaction mechanisms. These include acid-base interactions, bidentate bindings or complex hydrogen bonding interactions, and electrochemical manipulation of interfacial bonding. In this Perspective, we briefly review recent progress in the field, and we discuss how interfacial electrochemistry can vary interfacial forces by concerted tuning of surface charging, hydration forces, and tuning of the interfacial ion concentration. We further discuss open questions, controversial findings, and new paths into understanding and utilizing redox-proteins and derived polymers for enhancing underwater adhesion in a complex salt environment.

Polymer adhesion is ubiquitous in both the natural world and human technology. It is also a complex multiscale phenomenon, such that the solution of adhesion problems requires a convergence of chemistry, physics, and engineering. In this Perspective, we provide an overview of some of the fundamental concepts that have emerged in the field of polymer adhesion, discuss recent work, and identify challenges in three specific areas: (a) theories and simulations, with an emphasis on problems involving chain scission; (b) experimental methods for measuring forces and characterizing interfaces at the molecular scale; and (c) strategies inspired by living organisms to generate underwater adhesion.

Common fluids cannot sustain static mechanical stresses at the macroscopic scale because they lack molecular order. Conversely, crystalline solids exhibit long-range order and mechanical strength at the macroscopic scale. Combining the properties of fluids and solids, liquid crystal films respond to mechanical confinement by both flowing and generating static forces. The elastic response, however, is very weak for film thicknesses exceeding 10 nm. In this study, the mechanical strength of a fluid film was enhanced by introducing topological defects in a cholesteric liquid crystal, producing unique viscoelastic and optomechanical properties. The cholesteric was confined under strong planar anchoring conditions between two curved surfaces with sphere-sphere contact geometry similar to that of large colloidal particles, creating concentric dislocation loops. During surface retraction, the loops shrank and periodically disappeared at the surface contact point, where the cholesteric helix underwent discontinuous twist transitions, producing weak oscillatory surface forces. On the other hand, new loop nucleation was frustrated by a topological barrier during fluid compression, creating a metastable state. This generated exceptionally large forces with a range exceeding 100 nm as well as extended blueshifts of the photonic bandgap. The metastable cholesteric helix eventually collapsed under a high compressive load, triggering a stick-slip-like cascade of defect nucleation and twist reconstruction events. These findings were explained using a simple theoretical model and suggest a general approach to enhance the mechanical strength of one-dimensional periodic materials, particularly cholesteric colloid mixtures.

Optical fields in metal-dielectric multilayers display typical features of quantum systems, such as energy level quantization and avoided crossing, underpinned by an isomorphism between the Helmholtz and Schrödinger wave equations. This article builds on the fundamental concepts and methods of quantum theory to facilitate the understanding and design of multicavity resonators. It also introduces the surface forces apparatus (SFA) as a powerful tool for rapid, continuous, and extensive characterization of mode dispersion and hybridization. Instead of fabricating many different resonators, two equal metal-dielectric-metal microcavities were created on glass lenses and displaced relative to each other in a transparent silicone oil using the SFA. The fluid thickness was controlled in real time with nanometer accuracy from more than 50 μm to less than 20 nm, reaching mechanical contact between the outer cavities in a few minutes. The fluid gap acted as a third microcavity providing optical coupling and producing a complex pattern of resonance splitting as a function of the variable thickness. An optical wave in this symmetric three-cavity resonator emulated a quantum particle with nonzero mass in a potential comprising three square wells. Interference between the wells produced a 3-fold splitting of degenerate energy levels due to hybridization. The experimental results could be explained using the standard methods and formalism of quantum mechanics, including symmetry operators and the variational method. Notably, the interaction between square wells produced bonding, antibonding, and nonbonding states that are analogous to hybridized molecular orbitals and are relevant to the design of “epsilon-near-zero” devices with vanishing dielectric permittivity.

Nature has evolved several molecular strategies to ensure adhesion in aqueous environments, where artificial adhesives typically fail. One recently‐unveiled molecular design for wet‐resistant adhesion is the cohesive cross‐β structure characteristic of amyloids, complementing the well‐established surface‐binding strategy of mussel adhesive proteins based on 3,4‐l‐dihydroxyphenylalanine (Dopa). Structural proteins that self‐assemble into cross β‐sheet networks are the suckerins discovered in the sucker ring teeth of squids. Here, light is shed on the wet adhesion of cross‐β motifs by producing recombinant suckerin‐12, naturally lacking Dopa, and investigating its wet adhesion properties. Surprisingly, the adhesion forces measured on mica reach 70 mN m−1, exceeding those measured for all mussel adhesive proteins to date. The pressure‐sensitive adhesion of artificial suckerins is largely governed by their cross‐β motif, as evidenced using control experiments with disrupted cross‐β domains that result in complete loss of adhesion. Dopa is also incorporated in suckerin‐12 using a residue‐specific incorporation strategy that replaces tyrosine with Dopa during expression in Escherichia coli. Although the replacement does not increase the long‐term adhesion, it contributes to the initial rapid contact and enhances the adsorption onto model oxide substrates. The findings suggest that suckerins with supramolecular cross‐β motifs are promising biopolymers for wet‐resistant adhesion.

Spontaneous breaking of symmetry in liquid crystal (LC) films often reveals itself as a microscopic pattern of molecular alignment. In a smectic-A LC, the emergence of positional order at the transition from the nematic phase leads to periodic textures that can be used as optical microarrays, templates for soft lithography, and ordering matrices for the organization and manipulation of functional nanoparticles. While both 1d and 2d patterns have been obtained as a function of the LC film thickness and applied fields, the connection has not been made between pattern formation and the peculiar critical behavior of LCs at the nematic-smectic transition, still eluding a comprehensive theoretical explanation. In this article, we demonstrate that an intense bend distortion applied to the LC molecular director while cooling from the nematic phase produces a frustrated smectic phase with depressed transition temperature, and the characteristic 1d periodic texture previously observed in thin films and under applied electric fields. In light of De Gennes' analogy with the normal-superconductor transition of a metal, we identify the 1d texture as the equivalent of the intermediate state in type I superconductors. The bend distortion is analog to the magnetic field in metals and penetrates in the frustrated phase as an array of undercooled nematic domains, periodically intermixed with bend-free smectic-A domains. Our findings provide fundamental evidence for theories of the nematic-smectic transition, highlighting the deep connection between phase frustration and pattern formation, and perspectives on the design of functional smectic microarrays.

Using a surface force apparatus, a cholesteric liquid crystal was confined between two crossed cylindrical surfaces that induced strong planar anchoring and normal alignment of the chiral helix. The film thickness and total twist angle of the chiral molecular structure were simultaneously measured using multiple-beam optical interference. As the film thickness was increased and the chiral structure deformed, the twist angle remained almost unchanged until discontinuous changes occurred at critical distances that were equally spaced by one cholesteric half-pitch length. Structural deformations generated oscillatory elastic forces with periodically spaced maxima corresponding to twist transitions. These findings were reproduced using an equilibrium model of cholesteric confinement and force generation. The analysis indicates that the strength of the azimuthal surface anchoring on mica is high, exceeding 0.2 mJ m-2.

Using a surface forces apparatus and an atomic force microscope, we characterized the adhesive properties of adsorbed layers of two recombinant variants of Perna viridis foot protein 5 (PVFP-5), the main surface-binding protein in the adhesive plaque of the Asian green mussel. In one variant, all tyrosine residues were modified into 3,4-dihydroxy-L-phenylalanine (DOPA) during expression using a residue-specific incorporation strategy. DOPA is a key molecular moiety underlying underwater mussel adhesion. In the other variant, all tyrosine residues were preserved. The layer was adsorbed on a mica substrate and pressed against an uncoated surface. While DOPA produced a stronger adhesion than tyrosine in contact with the nanoscopic Si3N4 probe of the atomic force microscope, the two variants produced comparable adhesion on the curved macroscopic mica surfaces of the surface forces apparatus. These findings show that the presence of DOPA is not a sufficient condition to generate strong underwater adhesion. Surface chemistry and contact geometry affect the strength and abundance of protein-surface bonds created during adsorption and surface contact. Importantly, the adsorbed protein layer has a random and dynamic polymer-network structure that should be optimized to transmit the tensile stress generated during surface separation to DOPA surface bonds rather than other weaker bonds.

The response of a cholesteric liquid crystal film to mechanical confinement is theoretically investigated considering planar anchoring conditions on the limiting surfaces and pure twist distortion of the liquid crystal director. We evaluate the total twist angle and normal force acting on the surfaces as a function of the film thickness. Assuming the Rapini-Papoular functional form for the surface anchoring energy, we show that the surface twist angle undergoes discontinuous jumps or continuous transitions as a function of the film thickness and anchoring strength. The transitions take place at well-defined film thicknesses, related to the intrinsic periodicity of the cholesteric liquid crystal, and produce oscillations in the normal force and position of the optical band-gap as the film thickness is varied.