We present a dissipative particle dynamics (DPD) model for surfactants at oil-water interfaces, parametrised directly against experimental data. The model is applied to pure and mixed systems of ionic and nonionic surfactants, including sodium dodecyl sulfate (SDS), dodecyldimethylamine oxide (DDAO), and polyoxyethylene alkyl ethers (CiEj). Our simulations reveal that exceeding the maximum interfacial packing density can lead to the spontaneous production of micelles from the interfacial surfactant layer into the bulk. This behaviour, together with our parametrisation scheme, enables quantitative prediction of interfacial tension as a function of surface concentration in excellent agreement with experiment. In addition, the model provides access to other interfacial properties such as maximum surface coverage and interfacial monolayer thickness. Moreover, we show that the model can be used to investigate synergistic effects in mixed surfactant systems, where combinations of surfactants yield lower interfacial tensions than either component alone. In particular, simulations of DDAO and SDS systems demonstrate a lower interfacial tension than the corresponding pure surfactants. Utilising these results, we propose that these synergistic effects result from a balance between the intrinsic ability of a surfactant molecule to lower the IFT and its achievable maximum packing at the interface.

Droplets far from equilibrium experience different compositions and local environments compared with bulk oil and water phases at equilibrium. Understanding the pathways involved in emulsion progression towards equilibrium is valuable for designing complex fluids for many purposes including coatings, food, chemical separations, active matter, and enhanced oil recovery. Here we report how microscale oil droplets, which partition nonionic surfactants and also solubilize, can follow an unexpected pathway wherein a spherical droplet transitions through an interfacial instability and dissociates. This process depends on the oil hydrophobicity, the concentration and ethylene oxide number of the surfactant, the initial droplet diameter, and the presence of neighboring droplets. We propose a mechanism based on local phase inversion that explains both the visual appearance of the droplet dissociation behavior as well as the trends in its counterintuitive dependence on specific conditions like oil and surfactant chemical structure and surfactant concentration.

Rubber-ice friction is governed by the coupled contributions of viscoelastic adhesion and interfacial heating. This work provides unique in situ/in operando insight into the friction mechanisms from the macroscopic to the molecular scale using combined experimental measurements and analytical modelling approaches. Simultaneous force and real contact area imaging were used to quantify the real shear stress of SBR-silica elastomers spanning a small-strain, low-frequency shear-modulus range of 1-8 MPa (a low glass transition temperature of ∼-60 °C) during pure sliding over 50 µm s-1-1 m s-1 and at environmental temperatures down to -30 °C. The normalized real contact area exhibited a non-monotonic velocity dependence; together with a bell-shaped shear stress-velocity curve, this revealed three friction regimes: at low velocity, adhesion and viscoelastic dissipation dominated thermal effects; near the peak, frictional heat generation increased; and at high velocity, thermally driven mechanisms, including advection-dominated heat removal, became more significant, with the possible occurrence of localized interfacial melting. No bulk melting was observed. The novelty of this work relies on bridging the scales in ice-rubber friction mechanisms. At the macroscopic scale, a simple thermal analysis of the rubber-ice sliding interface yielded a dimensionless average contact temperature that collapsed across all compounds and test conditions considered. At the molecular scale, Chernyak-Leonov kinetics was used to describe elastomer chain attachment-detachment on ice and its temperature dependence. Successfully confronting our experimental results with this multiscale model led to a predictive interfacial shear stress model that, when supplied with the measured real contact area, reproduced both the magnitude and shape of the friction response.

We present an experimental method to fabricate centimetric thin elastic capsules with a highly uniform thickness and negligible bending stiffness using silicone elastomers. In our experiments, the capsule thickness is tunable at fabrication, while internal pressure and hoop (circumferential) stress are adjustable via hydrostatic inflation once the capsules are filled and immersed in water. Capsule mechanics are probed through hydro-elastic waves generated by weak mechanical perturbations at the capsule interface. By analyzing the surface wave dynamics in the Fourier domain, we extract the in-plane stress and demonstrate that the hydro-elastic waves are exclusively governed by hoop stress. This provides a controllable macroscopic analogue of liquid drops characterised by an effective surface tension, allowing the capsules to be modeled as large-scale "elasto-drops" with an inflation and thickness tunable effective surface tension. In this limit, bending stiffness is negligible over the experimentally relevant wavelengths, so that the shell dynamics are governed primarily by in-plane tension. Our work demonstrates that elasto-drops serve as a robust model system for parametric studies of large-scale analogues of liquid drops with experimentally adjustable surface tension.

A major challenge hampering the industrial and biological exploitation of lipases is the self-limiting effect of the lipolytic reaction. Defying sustained attention throughout history, the influence of lipase catalysed lipolysis on the internal structure of thin triglyceride films has proved elusive. An in-depth understanding of the lipolysis process at the triglycerides/aqueous interface will assist in creating innovative methods to enhance yields. This study furthers our understanding of the influence of solution pD (pH equivalent for aqueous solutions prepared with D2O) on the effect of lipolysis on the structure of thin triolein films. All experiments were performed at pD 7.0 or pD 8.5, either side of the apparent pKa of the primary product of the hydrolysis (oleic acid/oleate). Spectroscopic ellipsometry measurements were employed to kinetically track the changes in thickness of the triolein film after the introduction of 2 ppm Thermomyces lanuginosus lipase (TLL). Neutron reflectometry experiments revealed the internal structure of the thin films before and after TLL digestion, while fast kinetic measurements capturing changes to the reflectivity profile throughout lipolysis. Both techniques revealed significant variations in the physical properties and enzymatic conversion of the triolein films between pD 7.0 and pD 8.5.

New di-chain anionic surfactants containing silicon (Si) atoms in the hydrophobic chain-tips (trimethylsilyl (TMS) hedgehog surfactants) are able to reduce air-water (A-W) surface tension γcmc to as low as ≈22 mN m-1 (A. Czajka, C. Hill, J. Peach, J. C. Pegg, I. Grillo, F. Guittard, S. E. Rogers, M. Sagisaka and J. Eastoe, Phys. Chem. Chem. Phys., 2017, 19, 23869). However, the extent to which these surfactants stabilize alkane oil-water (O-W) interfaces is unexplored. Here, it is shown that such TMS surfactants are able to stabilize water-in-oil microemulsions (W/O-µEs). The O-W interfacial tensions γo/w in these µEs are ultra-low, in the range of 10-2 to 10-4 mN m-1, and µE-stability can be optimized by varying surfactant and solvent chemical structures. For example, with aliphatic n-alkanes and cycloalkanes, the surfactant AOT-SiC alone stabilizes W/O-µEs over a wide temperature window, but not with the aromatic solvent toluene. Likewise, AOT-SiB forms W/O-µEs, but preferably in aromatic solvents, such as toluene. Contrast-variation small-angle neutron scattering (SANS) measurements indicate that the water droplets in these W/O-µEs are stabilized by surfactant-monolayers. In all of these systems, the droplet morphologies and shapes are correlated with the proximity to (from) the µE-phase stability boundaries. The results show that Si-containing TMS surfactants are effective at O-W interfaces, promoting the ultra-low interfacial tensions necessary for stabilization of µEs. These TMS surfactants offer credible alternatives to environmentally damaging and health-hazardous fluorinated surfactants (FSURFs).

The viscoelastic behaviors of aqueous solutions of hydroxypropylmethyl cellulose (HpMC) samples with narrow molar mass distributions were investigated over a wide concentration (c) range. The degree of substitution by methyl groups and the molar substitution number by hydroxypropyl groups of the samples were 1.8 and 0.15. The weight average molar masses (Mw) were 210, 310 and 440 kg mol-1, and the molar mass distribution indices were less than 1.3 for each sample. The zero-shear viscosity (η0), the average relaxation time (τw), and the steady state compliance (Je) determined in the c range for HpMC molecules to fully entangle were determined using the molecular number density (ν = cNA/Mw, where NA is the Avogadro constant) and the rod particle length (L) of HpMC molecules. The results were reasonably understood on the basis of entangled rod particle suspension rheology, in which η0 and τw are described with L3ν and Je-1 is described with ν. Consequently, all the obtained viscoelastic data clearly revealed that the HpMC molecules behave as rod-like particles even in a fully entangled concentration range.

Growing evidence indicates that the motility of multicellular systems exhibits active nematic characteristics. However, the impact of cell-to-cell variability, particularly the relationship between a cell's dynamic phenotype and its contribution to nematic order, remains poorly understood. Here, we examine the motility of monolayers of micropatterned breast cancer cells and observe the emergence of robust nematic order that evolves spatiotemporally, despite the absence of coherent tissue flow. We identify a distinct subpopulation of cells, termed "patrollers", which display strongly polarized migration and appear to reinforce local nematic alignment. To elucidate the underlying mechanisms, we develop a mean-field theoretical model that captures the essential contributions of this subpopulation and yields predictions consistent with our experimental observations. Our results indicate that nematic order within multicellular systems may be driven not by uniform behavior across the entire population, but rather by the dominant influence of a specialized subset of cells that orchestrate collective alignment.

Mechanical oscillations play fundamental roles in cellular processes such as motility, signalling, and structural regulation; however, the mechanisms by which artificial cytoskeletal networks can be engineered to reproduce such autonomous oscillatory behaviours remain poorly understood. In this study, we demonstrate that a chemically polyethylene glycol-crosslinked filamentous actin hydrogel exhibits autonomous, long-lasting, and synchronised mechanical oscillations during self-organised polymerisation. These oscillations arise from chemo-mechanical responses coupled with the treadmilling polymerisation-depolymerisation equilibrium of filamentous actin. We propose that the rigid and highly hierarchical structure of the chemically crosslinked network plays an important role in the emergence of such autonomous mechanical oscillations. Our results reveal how hierarchical crosslinking and chemo-mechanical coupling drive sustained oscillations in active polymer networks, providing new insight into the fundamental mechanisms underlying autonomous dynamics in soft materials.