Soft Matter最新文献

We utilize the results of surface-enhanced Raman spectroscopy (SERS)-based interdiffusion experiments on meso-structured substrates to independently validate direct observations of plasmonic enhancements on these elements. The plasmonic enhancement function (PEF) is crucial for accurately determining interdiffusion coefficients using this newly proposed SERS-based methodology. The substrates feature a microscale inverted pyramid geometry, coated with nanoscale sputtered gold. Interdiffusion experiments involve the sequential deposition of polymer bilayers, with deuterated polystyrene (dPS) at the bottom and polystyrene (PS) on top, followed by annealing while periodically acquiring Raman spectra. The temporal evolution of the PS Raman signal reflects not only the interdiffusion process but also plasmonic effects, as the Raman scattering primarily arises from the substrate's plasmonic hotspots. High-resolution finite element (FE) diffusion simulations, combined with experimental SERS data, are used to infer the PEF of the substrate. The derived PEF is consistent with two hotspots located at the apex and vertices of the pyramidal cavity, extending along the edges and spreading into the molecular layer in direct contact with the substrate. This finding is tested against experiments conducted at various diffusion rates, showing excellent agreement. It corroborates recent observations by Steuwe et al. regarding the localization of hotspots on this specific substrate but contradicts other studies that attribute hotspots solely to the micron-scale geometry. This analysis establishes a solid foundation for reliably determining diffusion coefficients using this SERS-based methodology.

The structure of ionic liquids (ILs) has an influence on their physiochemical properties, determining their performance as self-assembly media. In this study, we focus on the anion effect of aprotic ionic liquids (AILs). The aggregation behaviours of the cationic surfactant 1-hexadecyl-3-methylimidazolium bromide (C₁₆mimBr) have been investigated in the imidazolium AILs with the 1-ethyl-3-methyl imidazolium cation and different anions, including nitrate, ethylsulfate, bis(trifluoromethylsulfonyl) imide and tetrafluoroborate. Surface adsorption parameters of C₁₆mimBr were determined using surface tension measurements, and the critical micellization concentration values in AILs vary for their different cohesive energy. The micellar and lamellar lyotropic liquid crystal phases emerge with the increase of C₁₆mimBr concentrations. The structure and properties of aggregates were determined using small angle X-ray scattering, polarized optical microscopy, rheology and differential scanning calorimetry. The anion effects of AILs on the phase behaviours and structure and properties of aggregates were analysed and discussed. The lamellar lyotropic liquid crystals have shown good conductivity, as confirmed by electrochemical impedance spectroscopy characterization. Our results enhance the understanding of the structure effect of ILs as self-assembly media and contribute to the design of tailorable solvents.

When drops are placed on a sufficiently soft surface, the drop surface tension drives an out of plane deformation around the contact line (i.e., a wetting ridge). For soft elastomeric surfaces that are swollen with a liquid, capillarity from a drop can induce a phase separation in the wetting ridge. Using confocal microscopy, we study the dynamics of phase separation at the wetting ridge of glycerol drops on silicone elastomers, which are swollen with silicone oils of varying viscosity (i.e., molecular weight). We show that the viscosity of the swelling oil plays a large role in the oil separation size and separation rate. For networks swollen to near their maximum swelling (i.e., saturated), lower viscosity oil separates more and separates faster at early times compared to larger viscosity oil. During late-stage wetting, the growth rate of the separation is a function of viscosity and swelling ratio, which can be described by a simple diffusive model and a defined wetting ridge geometry. In this late-stage wetting, the higher viscosity oil evidently grows faster, likely because it is further from reaching equilibrium. Interestingly, the separated oil phase region grows with a nearly constant, geometrically similar shape. Understanding how phase separation occurs on swollen substrates should provide information on how to control drop spreading, sliding, adhesion, or friction on such surfaces.

Previously, we and others have used cantilever-based techniques to measure droplet friction on various surfaces, but typically at low speeds U < 1 mm s⁻¹; at higher speeds, friction measurements become inaccurate because of ringing artefacts. Here, we are able to eliminate the ringing noise using a critically damped cantilever. We measured droplet friction on a superhydrophobic surface over a wide range of speeds U = 10⁻⁵–10⁻¹ m s⁻¹ and identified two regimes corresponding to two different physical origins of droplet friction. At low speeds U < 1 cm s⁻¹, the droplet is in contact with the top-most solid (Cassie–Baxter), and friction is dominated by contact-line pinning with F_fric force that is independent of U. In contrast, at high speeds U > 1 cm s⁻¹, the droplet lifts off the surface, and friction is dominated by viscous dissipation in the air layer with F_fric ∝ U^2/3 consistent with Landau–Levich–Derjaguin predictions. The same scaling applies for superhydrophobic and underwater superoleophobic surfaces despite their very different surface topographies and chemistries, i.e., the friction scaling law derived here is universal.

Polymeric, soft, and biological materials exhibit viscoelasticity, which is a time dependent mechanical response to deformation. Material viscoelasticity emerges from the movement of a material's constituent molecules at the nano- and microscale in response to applied deformation. Therefore, viscoelastic properties depend on the speed at which a material is deformed. Recent technological advances, especially in atomic force microscopy (AFM), have provided tools to measure and map material viscoelasticity with nanoscale resolution. However, to obtain additional information about the viscoelastic behavior of a material from such measurements, theoretical grounding during data analysis is required. For example, commercially available bimodal AFM imaging maps two different viscoelastic properties of a sample, the storage modulus, E′, and loss tangent, tan δ, with each property being measured by a different resonance frequency of the AFM cantilever. While such techniques provide high resolution maps of E′ and tan δ, the different measurement frequencies make it difficult to calculate key viscoelastic properties of the sample such as: the model of viscoelasticity that describes the sample, the loss modulus, E′′, at either frequency, elasticity E, viscosity η, and characteristic response times τ. To overcome this difficulty, we present a new data analysis procedure derived from linear viscoelasticity theory. This procedure is applied and validated by performing amplitude modulation–frequency modulation (AM–FM) AFM, a commercially available bimodal imaging technique, on a styrene–butadiene rubber (SBR) with known mechanical behavior. The new analysis procedure correctly identified the type of viscoelasticity exhibited by the SBR and accurately calculated SBR E, η, and τ, providing a useful means of enhancing the amount of information gained about a sample's nanoscale viscoelastic properties from bimodal AFM measurements. Additionally, being derived from fundamental models of linear viscoelasticity, the procedure can be employed for any technique where different viscoelastic properties are measured at different and discrete frequencies with applied deformations in the linear viscoelastic regime of a sample.

Incorporating associative interactions as the energy dissipation units has been recognized as an effective strategy to develop tough hydrogels. For hydrogen-bond associations, however, it is highly challenging to stabilize them under aqueous conditions. Although affording cooperativity can enhance and stabilize the hydrogen bonds, it usually requires stepwise polymerization to form these cooperative associations between different polymers and networks. Here, we report a series of tough supramolecular hydrogels with robust hydrogen-bond associations between grafted polymers that are synthesized by polymerization of a macromonomer of poly(N,N-dimethylacrylamide) (PDMAA) and a small monomer of methacrylic acid. The grafted chains of PDMAA form cooperative hydrogen bonds with the main chain of poly(methacrylic acid) (PMAAc), forming supramolecular hydrogels with high toughness and good stability. The tough and stiff hydrogels are in a glassy state, exhibit forced elastic deformation at room temperature, and remain stable over a wide pH range. In contrast, hydrogels prepared by the copolymerization of DMAA and MAAc are swollen and weak in water due to the lack of successive hydrogen donor/acceptor units and the absence of cooperative hydrogen bonds. In addition, these tough hydrogels exhibit good recyclability and shape memory properties, owing to the supramolecular nature of the network and the temperature-dependent mechanical properties. The influence of polymer structure on the associative interactions and macroscopic properties of the hydrogels should be informative for the design of tough soft materials with versatile applications.

Production of biofuels from biological feedstocks, such as soybean oil, is an important piece of the transition to renewable energy sources. Processes have been developed to co-refine these feedstocks with traditional feedstocks, however, the high concentration of polar functional groups in biofeedstocks can cause a wide range of intermediate chemical reactions and interactions. An improved understanding of the interactions of biofeedstocks and their degradation products is needed to continue to expand the usage of biofeedstocks in fuel production. In this study, the equilibrium structures of glycerol monooleate (GMO), a common intermediate product of biofeedstock processing, in white mineral oil at a wide range of compositions, temperatures, and additional byproduct concentrations (water and/or oleic acid) were characterized using small angle X-ray scattering (SAXS). It was determined that GMO can exist as crystalline aggregates in white oil or as reverse micelles depending on the concentration and temperature. The critical micelle temperature increases significantly with increasing GMO concentration but remains relatively stable with increasing water or fatty acid concentration. Fitting of the SAXS data revealed that for many compositions, the GMO formed roughly spherical reverse micelles, however, at high water concentrations (∼1 wt%), the GMO formed elongated reverse micelles. Additionally, when >1 wt% oleic acid was added to the system, bi-continuous structures were stabilized rather than discreet reverse micelles. These results help increase our understanding of the structural behavior of biofeedstock intermediate products at concentrations and temperatures relevant to biofuel production and can enable processers to design systems and products that can either leverage or prevent these interactions for improved processing performance.

Photoresponsive hydrogels are an emerging class of stimuli-responsive materials that exhibit changes in physical or chemical properties in response to light. Previous investigations have leveraged photothermal mechanisms to achieve reversible changes in hydrogel friction, although few have focused on photochemical means. To date, the tribological properties of photoswitchable hydrogels (e.g., friction and lubrication) have remained underexplored. In this work, we incorporated photoresponsive methoxy-spiropyran-methacrylate monomers (methoxy-SP-MA) into a hydrogel network to form a copolymerized system of poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropane sulfonic acid-co-methoxy-spiropyran-methacrylate) (p(NIPAAm-co-AMPS-co-SP)). We demonstrated repeatable photoresponsive changes to swelling, friction, and stiffness over three light cycles. Our findings suggest that volume changes driven by the decreased hydrophilicity of the methoxy-SP-MA upon light irradiation are responsible for differences in the mechanical and tribological properties of our photoresponsive hydrogels. Our results could inform future designs of photoswitchable hydrogels for applications ranging from biomedical applications to soft robotics.

Amyloid fibrils are proteinaceous nanostructures known for their ability to activate the innate immune system, which has been recently exploited for their use as self-adjuvanted antigen delivery systems for vaccines. Among mechanisms of immunostimulation, the activation of the heterodimeric toll-like receptor 2/6 (TLR2/TLR6) by the cross-β-sheet quaternary conformation appears important. Nonetheless, the lack of control over the process of self-assembly and the polydispersity of the resulting supramolecular architectures make it challenging to elucidate the molecular basis of TLR2/TLR6 engagement by amyloid assemblies. In this context, we harnessed the effects of N- and C-terminal modifications of a short 10-mer β-peptide derived from the islet amyloid polypeptide (I₁₀) to investigate the relationships between the morphology and physicochemical properties of amyloid assemblies and their TLR2/TLR6 activity. Chemical substitutions at the N- and C-termini of the I₁₀ peptide, including addition of charged residues at the N-terminus and α-amidation of C-terminus, allowed the controlled formation of a diversity of architectures, including belt-like filaments, rigid nanorods as well as flat and twisted fibrils. These fully cytocompatible peptide nanostructures showed different potencies to activate TLR2/TLR6, which correlated with the charge exposed on the surface. These results further demonstrate the potent modulatory effect of N- and C-terminal electrostatic capping on the self-assembly of short synthetic β-peptides. This study also indicates that self-assembly into cross-β-sheet nanostructures is essential for the activation of the TLR2/TLR6 by amyloidogenic peptides, albeit the structural requirements of the engagement of this promiscuous immune receptor by the nanostructures remain challenging to precisely untangle.