Improving the sustainability of cosmetic products while maintaining a good performance requires a deeper understanding on the way that new eco-respectful ingredients interact with hair or skin. In the case of shampoos, the surface science is dominated by the diverse changes on the hair fiber due to both chemical and physical damages that particularly affect physicochemical properties such as hydrophobicity. A native, undamaged fiber is covered with a monolayer of lipids, mainly 18-methyleicosanoic acid (18-MEA), while a highly damaged hair surface, having completely lost the protective lipids, is hydrophilic and negatively charged. Intermediate states exist, where there is a partial loss of 18-MEA (“partially damaged hair”). Here, four model surfaces have been produced, to mimic different types of hair surfaces. Their interaction with selected surfactants and polyelectrolytes (natural and synthetic) has been studied by neutron reflectometry (NR). NR can reveal hierarchical adsorption from mixtures thanks to the scattering contrast between deuterated and hydrogenous molecules. Atomic force microscopy (AFM) measurements complement the study by adding information about the in-plane structure of adsorbed species. The presence of the methyl branch of 18-MEA is found to affect the interaction of the surface with adsorbates. For surfactant/polyelectrolyte mixtures, for example, the adsorption of polymer is enhanced. Of particular interest are the results on the partially damaged hair model, as it manifests patches of hydrophobic and hydrophilic moieties; it is possible to separately observe the different adsorption behaviors to the different sites in a single experiment.

Hyaluronic acid (HA) and phospholipids (PLs) are key components of joint lubrication. In osteoarthritis (OA), the molecular weight (MW) of HA is reduced, which has been proposed to weaken the anchoring capacity of PL and impair lubrication. This study reveals a different mechanism by directly linking the MW to the structure of HA-PL (hybrid) assemblies and frictional properties. Using mixed-MW HA and PL to model this difference between healthy and OA synovial composition, we found interfacial lamellar structures form under healthy-like conditions, while hybrid vesicles predominate in OA-like conditions. At physiologically relevant shear rates, lamellar assemblies maintain ultralow friction, whereas vesicles are removed, causing a tenfold friction increase. These findings provide mechanistic insight into how HA-PL structural organization controls lubrication. While this simplified system does not capture the biochemical complexity of synovial fluid, this study advances understanding and offers a framework for designing structure-informed therapeutic strategies and biomimetic lubricants.

We hypothesise that the recent discovery of nanodomains at the air-water interface can be leveraged to nano-functionalize surfaces through casting with incorporated functional species. The interfacial self-assembly of the amphiphilic molecules, 18-methyleicosanoic acid 18-MEA and 4-(tetradecyl)benzene diazonium tetrafluoroborate TDDS, at the air-water interface and cast on silicon wafer has been investigated using Langmuir-Blodgett (LB) techniques and atomic force microscopy. The impact of composition and surface pressure (SP) on the formation of nanodomains and microstructures was examined. TDDS (which can be used to modify the electronic structure of graphene) behaves as a co-surfactant in the 18-MEA film at low concentrations, facilitating the formation of homogeneous nanodomains with functional capacity. At higher TDDS concentrations, there is evidence for phase separation in the domains, and the TDDS furthermore partitions to the aqueous phase at higher pressures. By manipulating the 18-MEA:TDDS ratio and SP, regular nano-patterns can be transitioned into novel 2D structures reminiscent of 3D water-in-oil-in-water (W/O/W) analogues ("cookie systems"), offering a versatile strategy for designing nanoarchitectures with potential applications in graphene patterning.

Lithium salts are by far the most common type of thickener in lubricating greases. The battery industry is placing increasing demands on lithium supply and pricing, however, and a probable toxin classification of precursor material looms. It is thus important to explore a transition to a lithium-independent lubricant industry. In addition to providing the consistency properties of grease, a primary function of the thickener is to act as a passive lubricant supply system, releasing oil into the contact and making functional additives available to the surface as required. In this study, the performance of lithium complex grease was stress-tested by subjecting it to highly loaded rolling-sliding contact conditions that are now commonplace in modern machines. Tribological evaluation and Time-of-Flight Secondary Ion Mass Spectrometry analysis of tribofilms together reveal serious limitations in the properties of lithium thickeners; instead, a lithium-free grease matrix system, based on polypropylene, is shown to display better lubricating performance, rendering it a viable and more effective alternative.

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.

Tactile perception deteriorates with age, resulting in a negative impact on life quality. A clinical assessment of this decline could help to reduce its effects. Such a clinical apparatus for fine texture does not yet exist. Femtosecond laser surface texturing (LST) is capable of manufacturing fine textures on materials that are sufficiently robust for clinical requirements. This paper starts by addressing how LST can be used to manufacture surfaces for tactile tests, i.e. of sufficient dimensions to permit interrogation, and with a minimum quantity of uncontrolled surface features. Vibrotactile interrogation tests on textured surfaces demonstrate that the surface textures have controllable tactile signature and thus underline the suitability of the process for generating fine textures for tactile perception assessment.

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.

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.