This study investigates the neural and behavioral mechanisms of tactile perceptual discrimination using fMRI and a set of wrinkled surface stimuli with varying textures. Fifteen female participants were tasked with distinguishing between different surfaces by touch alone. Behavioral results demonstrated variable discriminability across conditions, reflecting the tactile sensitivity of human fingertips. Neural analysis showed varied brain activations tied to the task’s difficulty. In the easiest least fine-grained discrimination condition, widespread activations were observed across sensory and integration regions. As task difficulty increased, stronger parietal and frontal lobe involvement reflected higher cognitive demands. In the hardest most fine-grained discrimination condition, activation concentrated in the right frontal lobe, indicating reliance on executive functions. These results highlight the brain’s intricate role in processing sensory information during tactile discrimination tasks of varying difficulty. As task difficulty increases, the brain adapts by engaging additional neural resources to meet higher cognitive demands. This research advances our understanding of the psychophysical and neural bases of tactile discrimination acuity, with practical implications for designing materials that enhance tactile feedback.

Long, straight chain saturated fatty acids form homogeneous, featureless monolayers on a supramolecular length scale at the water–air interface. In contrast, a naturally occurring saturated branched fatty acid, 18-methyl eicosanoic acid (18-MEA) has been observed to form three-dimensional domains of size 20–80 nm, using a combination of Langmuir trough, Atomic Force Microscopy (AFM) images of the deposited monolayers, and Neutron reflectometry (NR) and X-Ray reflectometry (XRR). It is hypothesized that these domains result from the curvature of the water surface induced by the steric constraints of the methyl branch. Accordingly, in this work, we investigate in situ the structure of such films using Grazing Incidence Small Angle X-ray Scattering and Diffraction (GISAXS and GIXD). The branched fatty acids indeed form curved nanodomains as revealed by their two-dimensional scattering pattern whereas straight chain fatty acids form the expected featureless film, with no GISAXS scattering peaks. Mixed monolayers consisting of 18-MEA and eicosanoic acid (EA) display a phase transition in the structure from hexagonally packed at high 18-MEA ratio to structures with one-dimensional translational ordering (aligned stripes) for 50:50 mol% and lower ratios. Moreover, the GIXD patterns of monolayers containing 18-MEA display a peak with curved distribution of intensity, indicating a continuous distribution of collective molecular orientations, consistent with the local curvature of the water surface. Finally, we report on an unusual double peak phenomenon in the GISAXS data that is interpreted as being due to a hexagonal packing of elliptical domains – i.e. with two characteristic dimensions. Synchrotron X-Ray scattering experiments have thus unambiguously confirmed the self-assembly, out of plane, “cobbling” of the water interface by these branched structures. 

Branched fatty acids, such as those found on the surface of hair and wool, have recently been shown to form novel 3D self-assembly curvature structures at the air–water interface—nanocaps. On the hair surface, the branched fatty acid 18-methyleicosanoic acid (18-MEA) is expressed together with shorter, unbranched, straight chain fatty acids to form a protective palisade layer. The biological function of the chain length differences was hitherto unknown. Using a combination of atomic force microscopy and Langmuir isotherms, a safe, versatile route for tuneable nanopatterning of solid surfaces is demonstrated, via fatty acid interfacial nanocap deposition from biomimetic mixtures. Further, it is shown that chain length dependence of the interaction with the branched chain is exquisitely sensitive, leading to profoundly different morphologies in the self-assembly structures. The vastly enhanced properties of the mixed films compared to the individual components alone reveals the biological origin of the hair surface composition. 

A comprehensive understanding of chemical interactions at the surface of hair represents an important area of research within the cosmetic industry and is essential to obtain new products that exhibit both performance and sustainability. This paper aims at contributing to this research by applying a combination of surface techniques (neutron reflectometry, quartz-crystal microbalance and atomic force microscopy) to study adsorption of surface active ingredients onto hair-mimetic surfaces. The surface of hair is not homogeneous due to chemical and physical damage, and this work focuses on partly damaged hair models, in which both hydrophobic and charged moieties are present. Examples of such mixed-surface models are rare in the literature, despite the interest in the topic. The studied actives were an anionic surfactant (sodium dodecyl sulphate, SDS) and a natural polysaccharide (chitosan) of two different molecular weights, to represent soluble polymer-surfactant associations of cosmetic interest in hair-care rinsing applications. The effect of the concentration of SDS, the molecular weight of chitosan, and the order in which SDS and chitosan are introduced are studied, and compared to totally hydrophobic and totally hydrophilic surfaces. Results show that SDS can interact with the hydrophobic portions of the mixed surface, and its adsorption increases if associated with chitosan. Interestingly, differences have been found in the adsorption behaviour of chitosan depending on its chain size. Both types can deposit onto the surface, but when SDS is added, the lower molecular weight chitosan keeps its extended conformation in a ca. 70 Å thick layer, while the higher molecular weight chitosan collapses to form a layer of about 30 Å. This knowledge opens the door to developing hair-care formulations with improved performance and sustainability. 

Three nonhalogenated ionic liquids (ILs) dissolved in 2-ethylhexyl laurate (2-EHL), a biodegradable oil, are investigated in terms of their bulk and electro-interfacial nanoscale structures using small-angle neutron scattering (SANS) and neutron reflectivity (NR). The ILs share the same trihexyl(tetradecyl)phosphonium ([P6,6,6,14]+) cation paired with different anions, bis(mandelato)borate ([BMB]−), bis(oxalato)borate ([BOB]−), and bis(salicylato)borate ([BScB]−). SANS shows a high aspect ratio tubular self-assembly structure characterized by an IL core of alternating cations and anions with a 2-EHL-rich shell or corona in the bulk, the geometry of which depends upon the anion structure and concentration. NR also reveals a solvent-rich interfacial corona layer. Their electro-responsive behavior, pertaining to the structuring and composition of the interfacial layers, is also influenced by the anion identity. [P6,6,6,14][BOB] exhibits distinct electroresponsiveness to applied potentials, suggesting an ion exchange behavior from cation-dominated to anion-rich. Conversely, [P6,6,6,14][BMB] and [P6,6,6,14][BScB] demonstrate minimal electroresponses across all studied potentials, related to their different dissociative and diffusive behavior. A mixed system is dominated by the least soluble IL but exhibits an increase in disorder. This work reveals the subtlety of anion architecture in tuning bulk and electro-interfacial properties, offering valuable molecular insights for deploying nonhalogenated ILs as additives in biodegradable lubricants and supercapacitors. 

Ionic liquids (ILs) have emerged as viable solutions for developing new-age lubricants, both as neat lubricants and lubricant additives. Enabled by the presence of discrete ions, ILs have the possibility to render electrically conductive lubricants, which is a feasible strategy for developing lubricant systems compatible with modern e-drive conditions. However, this requires the characterization of the electrical properties of lubricants, which is a bottleneck for developing electrically conductive greases, given their complex architecture. This work introduces an electrochemical impedance spectroscopy measurement methodology to evaluate grease samples’ electrical properties. Compared to the commonly used conductivity meters, this method, through its multi-frequency alternating current (AC) impedance approach, can effectively distinguish the individual contributions of the bulk and the sample-electrode interface to the measured electrical response. Impedance spectra of grease samples are obtained using an electrochemical cell with parallel plate electrodes, mounted on a temperature-controlled cell stand and coupled with a potentiostat. The grease’s bulk conductivity is extracted by fitting the impedance data to relevant equivalent electrical circuits. The bulk conductivity of lithium complex grease doped with ILs is evaluated and compared to greases with conventional conductivity additives (copper powder and conductive carbon black). The analysis of temperature-dependent conductivity reveals the rather different conductivity mechanisms for different additives. For greases doped with ILs, a comparison against the electrical conductivity of neat ILs reveals that, in addition to the ion dissociation, the interaction of the ions with the different grease components (base oil, thickener) is crucial in defining the grease conductivity. 

Ionic liquids (ILs) are a promising class of electrolytes with a unique combination of properties, such as extremely low vapor pressures and nonflammability. Doping ILs with alkali metal salts creates an electrolyte that is of interest for battery technology. These salt-in-ionic liquids (SiILs) are a class of superconcentrated, strongly correlated, and asymmetric electrolytes. Notably, the transference numbers of the alkali metal cations have been found to be negative. Here, we investigate Na-based SiILs with a surface force apparatus, X-ray scattering, and atomic force microscopy. We find evidence of confinement-induced structural changes, giving rise to long-range interactions. Force curves also reveal an electrolyte structure consistent with our predictions from theory and simulations. The long-range steric interactions in SiILs reflect the high aspect ratio of compressible aggregates at the interfaces rather than the purely electrostatic origin predicted by the classical electrolyte theory. This conclusion is supported by the reported anomalous negative transference numbers, which can be explained within the same aggregation framework. The interfacial nanostructure should impact the formation of the solid electrolyte interphase in SiILs. 

The surface of human hair is normally hydrophobic as it is covered by a lipid layer, mainly composed of 18-methyleicosanoic acid (18-MEA). When the hair is damaged, this layer can be partially or fully removed and more hydrophilic, mainly negatively charged surfaces are formed with a wide variety of physical and chemical characteristics. The cosmetic industry is currently embracing the opportunity of increasing the sustainability of their hair-care products whilst improving product performance. To do this, it is vital to have a deeper understanding of the hair surface and how it interacts with hair-care ingredients. This work contributes to this by harnessing the potential of neutron reflectometry (NR) with scattering contrast variation to describe hierarchical adsorption. Three types of hair-mimetic surfaces have been produced: two “healthy hair” models to probe the role of lipid structure, and one “damaged hair” model, to consider the effect of the surface charge. Adsorption of hair-care ingredients has then been studied. The results for these relatively short lipid models indicate that a methyl branch has little effect on adsorption. The “damaged hair” studies, however, reveal the unexpected apparent adsorption of an anionic surfactant to a negative surface. This preferential adsorption of the otherwise solubilised neutral components demonstrates a facile route to selectively deliver a protective film on a damaged hair fibre, without the need for a cationic species. On a more general note, this study also demonstrates the feasibility of using NR to characterize such complex systems.

3D texturing by self-assembly at the air-water interface has recently been proposed. The hypothesis of this work is that, if this is true, such domain formation should be inferable directly from pressure-area isotherms and be thermodynamically stable. Monolayers of branched fatty acid mixtures with straight chain analogues and their stability are thus studied using a combination of pressure-area isotherms, thermodynamic analysis, in situ Brewster angle microscopy, and atomic force microscopy of both LB-deposited and drop-cast films on silicon wafers. Isotherms reflecting the behavior of monodisperse 3D domains are shown to be independent of compression rate and display long-term stability. Gibbs analysis further confirms the thermodynamic rather than kinetic origin of such novel species by revealing that deviations from ideal mixing can be explained only a priori by differences in the topography of the water surface, thus also indirectly confirming the self-assembly deformation of the water interface. The intrinsic self-assembly curvature and miscibility of the two fatty acids is confirmed by drop-casting, which also provides a rapid, tunable thin-film preparation approach. Finally, the longevity of the nanostructured films is extraordinary, the long-range order of the deposited films increases with equilibration time at the water interface, and the integrity of the nanopatterns remains intact on the scale of years.

Interactions between molecules in the synovial fluid and the cartilage surface may play a vital role in the formation of adsorbed films that contribute to the low friction of cartilage boundary lubrication. Osteoarthritis (OA) is the most common degenerative joint disease. Previous studies have shown that in OA-diseased joints, hyaluronan (HA) not only breaks down resulting in a much lower molecular weight (MW), but also its concentration is reduced ten times. Here, we have investigated the structural changes of lipid-HA complexes as a function of HA concentration and MW to simulate the physiologically relevant conditions that exist in healthy and diseased joints. Small angle neutron scattering and dynamic light scattering were used to determine the structure of HA-lipid vesicles in bulk solution, while a combination of atomic force microscopy and quartz crystal microbalance was applied to study their assembly on a gold surface. We infer a significant influence of both MW and HA concentrations on the structure of HA-lipid complexes in bulk and assembled on a gold surface. Our results suggest that low MW HA cannot form an amorphous layer on the gold surface, which is expected to negatively impact the mechanical integrity and longevity of the boundary layer and could contribute to the increased wear of the cartilage that has been reported in joints diseased with OA. © 2023 Author(s).