Soft Matter最新文献

Microorganisms such as bacteria and spermatozoa often inhabit confined viscoelastic environments. These organisms exhibit self-organization/collective dynamics in such complex surroundings. Here, we report a simulation study of active particle suspensions in viscoelastic fluids under confinement—representing an experimental scenario where motile organisms suspended in aqueous viscoelastic fluid are surrounded by an oil medium. We employed dissipative particle dynamics, a particle-based mesoscopic approach, to model the system with minimalistic ingredients and qualitatively reproduced some of the experimental observations by Liu et al. Nature, 2021, 590 (7844), 80–84. The collective dynamics within the suspended drop, mediated by the viscoelastic nature of the medium, manifest into two steady state configurations, namely a unidirectional vortex or an oscillatory vortex. We present a phase diagram for the drop's steady state configuration as a function of system parameters, such as strength and packing fraction of active agents, polymer concentration etc.

Ribbons are a subset of polymerized networks that occupy an intermediate space between polymers and surfaces. We perform extensive numerical simulations to understand how to interpolate the statistical properties of ribbons across the two limits by studying their behavior as a function of their widths and bending rigidities, taking into consideration both ideal and self-avoiding ribbons. We map out a two-dimensional phase diagram of the morphology of ideal ribbons, and uncover the onset of a crumpling transition for ribbons of sufficiently large width. We also discuss the onset width above which a ribbon behaves effectively as a surface. Finally, we suggest scaling laws and functional forms that properly link and interpolate the shape of self-avoiding polymers to that of self-avoiding surfaces.

This study investigated the effects of inorganic electrolytes (NaCl, NaBr, and Na₂SO₄) on the interfacial behavior, rheological properties, and foam performance of a ternary surfactant system composed of sodium dodecyl sulfate (SDS), dodecyl dimethyl betaine (BS-12), and a silicone surfactant (RH-288) at a mass ratio of 10 : 1 : 1. The mixed surfactant system exhibited enhanced surface activity with a critical micelle concentration close to that of RH-288 alone. Upon addition of electrolytes, a sphere-to-worm-like micellar transition was induced, significantly altering the solution's rheological behavior. Unlike NaBr and Na₂SO₄, NaCl promoted the formation of elongated and densely entangled worm-like micelles with long relaxation times, resulting in pronounced viscoelasticity. Although foamability slightly decreased with salt addition, foam stability markedly improved, as evidenced by reduced drainage, slower bubble coarsening, and enhanced thermal resistance up to 80 °C. Cryogenic transmission electron microscopy (cryo-TEM) confirmed the presence of worm-like micellar networks in the NaCl-containing system. The results demonstrated that anion-specific effects, interpreted by the Hofmeister series, played a critical role in modulating micellar dynamics and foam stability. Furthermore, the dynamic relaxation characteristics of the micellar network should be regarded as a key factor alongside bulk viscosity. The SDS/BS-12/RH-288 system with 0.4 mol L⁻¹ NaCl shows great potential as a high-performance, environmentally friendly, fluorine-free foam extinguishing agent. This study can provide a suitable approach to develop fluorine-free foam extinguishing agents for forest and grassland firefighting.

In this work, we discuss the development of an active colloidal system with controllable interactions with an artificial lipid bilayer membrane as a model for investigating the interplay of membrane mechanics and the transport of particles during adhesion and wrapping. We use polystyrene microspheres coated with a hemispherical platinum cap as model swimmers whose active motion is initiated by the addition of hydrogen peroxide (H₂O₂). Two classes of particle–membrane interactions and particle swimming direction are assessed. For the former, carboxylated particles are used to passively interact with the membrane through electrostatic interactions, while streptavidin coated particles are used to form a strong bond with biotinylated lipid membranes. For the latter, these active Janus particles are designed to be “pushers”, which swim toward their metal face into the bilayer, or “pullers”, which swim away from the membrane, by changing the concentration of CTAB, a cationic surfactant, in the aqueous phase. We find that a negative gravitaxis effect causes the steady movement of unbound pullers up and away from the membrane with increasing H₂O₂. When the particles are bound, a threshold H₂O₂ concentration is needed before overcoming the strength of the biotin–neutravidin bond and releasing the particles from the interface. In the case of the pusher system, as the H₂O₂ concentration increases the particles become increasingly wrapped in the membrane, as evidenced by their altered translational and rotational dynamics. We apply active Brownian models to characterize the nature of the particle–membrane interactions and also particle pair interactions. These results lay the groundwork to combine active colloidal systems with model lipid membranes to understand active transport in cellular contexts.

We examine the diffusiophoresis of core–shell structured soft particles, focusing on a structural model relevant to many biological and environmental systems. This model features a rigid hydrophobic inner core enclosed by a shell that is penetrable to both ions and fluid. A key novelty of this approach is assuming the slipping plane resides within the polyelectrolyte layer (PEL), rather than being fixed precisely at the core–shell interface, reflecting more complex internal hydrodynamics. This study assumes the shell layer possesses a dielectric permittivity lower than that of the bulk electrolytic solution. Such a situation is frequently encountered and relevant in the analysis of both biological and environmental colloids. As a result, the ion partitioning effect is operational across the PEL, which is however directly related to the penetration of mobile electrolyte ions across the PEL and controls its net volumetric charge. Thus, we model the diffusiophoresis of soft particles by integrating several crucial physical mechanisms: a hydrophobic and charged inner core, a slipping plane located within the surface PEL, the volume charge of the PEL, and the ion partitioning effect. The analysis is conducted within the flat-plate regime and the Debye–Hückel electrostatic framework. Based on these assumptions, we have derived a general expression for the diffusiophoretic velocity of the undertaken soft particle, which is applicable when the particle is exposed to a concentration gradient of valence-symmetric electrolytes with equal or unequal ionic diffusivities. We further illustrate the results to indicate the impact of the pertinent parameters.

We demonstrate that the Whispering Gallery Mode (WGM) lasing spectroscopy is a versatile high resolution tool to study the structure of interfaces of liquid crystalline (LC) droplets immersed in an immiscible fluid, such as water. The eigenfrequencies of WGMs in spherical microcavities are very sensitive to the refractive index profile in the nanometer thin interfacial region. This makes it possible to detect interfacial phenomena and temperature change in LC droplets with interferometric accuracy. We use 10–30 µm diameter droplets of a nematic liquid crystal labeled with a fluorescent dye and floating in water as an optical microcavity that sustains the WGMs. At the isotropic–nematic transition we observe wetting of the droplet's interface by a nanometer-thin layer of paranematic LC. Just below this transition, we observe red-shift and strong fluctuations of WGM spectra just before spherical droplet elongates into a fiber. The experiments are modeled with Finite-Difference Time-domain (FDTD) analysis of WGMs in nematic droplet and we find very good qualitative agreement.

The complexation between proteins and polyelectrolytes is fundamental to materials science and biology, yet the driving forces under non-ideal electrostatic conditions remain debated. Here, we systematically investigated the interaction between casein (CAS) and chitosan (CHI) at pH 5.5 and 3.0 using a combined experimental and theoretical approach. At pH 5.5, the oppositely charged macromolecules formed compact complexes through conventional electrostatic attraction (≈190–340 nm). A more intriguing behavior emerges at pH 3.0, where both CAS and CHI carry net positive charges yet still assemble into stable aggregates (≈340–370 nm). Spectroscopic analyses revealed that even under these conditions, CHI induced pronounced conformational and microenvironmental changes in CAS, including quenching of tryptophan fluorescence and secondary-structure remodeling, evidencing complex formation. To elucidate this counterintuitive phenomenon, we combined constant-pH Monte Carlo simulations with a semi-quantitative Kirkwood–Schumaker (KS) analysis. Our model quantified the mean charge of the representative αS1-casein as 〈Z〉 = +17.7 at pH 3.0, confirming strong electrostatic repulsion. However, we showed that the attraction is driven by the protein's significant charge regulation capacity (C = 3.45), resulting in a slightly shorter-range mesoscopic force that overcomes the repulsion. Within the KS framework, we also evaluated the ion–dipole (patch) contribution and demonstrated that, across the investigated pH range, it remains consistently smaller than the charge regulation term. The decisive role of this peculiar mechanism was confirmed experimentally: the complex dissociated upon the addition of salt, consistent with the screening of electrostatic interactions. Although both charge regulation (1/R²) and ion–dipole (1/R⁴) contributions are attenuated by the same exponential Debye screening, the longer-range nature of charge regulation makes it the dominant effect in the investigated pH range, thereby ruling out ion–dipole interactions as the primary driving force. This work provided a quantitative and mechanistic confirmation that charge regulation was the dominant driving force for protein–polyelectrolyte association on the “wrong side” of the isoelectric point, offering fundamental insights for the rational design of biomolecular complexes.

We investigate a two-dimensional polydisperse suspension of self-propelled semiflexible filaments and reveal a collective wrapping mechanism that is absent in monodisperse systems. At intermediate activity levels, long filaments coil around shorter ones, forming nested spiral structures stabilized by filament length disparity. These assemblies generalize the single-filament spiraling seen in active systems into cooperative, multi-filament configurations. As activity increases, the nested spirals undergo structural transitions: medium-length filaments unwind, longer filaments encapsulate shorter ones, and eventually all spiral structures dissolve. This reorganization is reflected in the dynamics, where van Hove distributions uncover coexisting confined and motile filament populations. Our findings identify filament length as a key control parameter for nonequilibrium self-assembly and establish inter-filament wrapping as a minimal mechanism for hierarchical organization in active matter. This mechanism provides a simple model for the cooperative confinement and structural hierarchy observed in both biological and synthetic active systems.

Soft materials such as colloids, pastes, and polymer liquids are defined rheologically by how they build and relax stress during flow and deformation. Their internal connectivity manifests in a broad spectrum of viscoelastic eigenmodes, with relaxation ranging from fast to slow and contributions that vary from weak to strong. The interplay of these modes determines how the material deforms under shear, compression, or stretching across different processing timescales. Traditional measures of viscoelasticity, such as the Deborah number (De) and the Weissenberg number (Wi), condense this complexity into single scalar values. While useful for certain purposes, these scalar measures mask the fast/slow interplay of relaxation processes that shape the distinct responses of soft matter. To overcome this limitation, we introduce the “spectral classification of processes and eigenmodes” (SCOPE) framework. SCOPE explicitly accounts for the distributed nature of both process times and material relaxation times. It generalizes the classical De and Wi into their functional counterparts—the Deborah function and the Weissenberg function—which connect applied stress and strain to the full spectrum of relaxation times (0 < τ < τ_max), thereby covering the entire range of process timescales and types of deformation. By doing so, SCOPE provides a spectral perspective on viscoelasticity that integrates fast and slow dynamics within a single, unified rheological framework. SCOPE provides criteria that separate viscous from elastic eigenmodes, and modes below or above the onset of nonlinearity. In what follows, we introduce the SCOPE framework in detail and demonstrate its functions for viscoelastic liquids.

The precise arrangement of different chemical moieties in a polymer determines its thermophysical properties. How the sequence of moieties impacts the properties of a polymer is an outstanding problem in polymer science. Herein, we address this problem for the thermoresponsive property of poly(N-isopropylacrylamide-co-acrylamide) in water using all-atom molecular dynamics (MD) simulations for temperatures ranging from 260 K to 360 K. Our simulations classify four distinct classes of thermoresponsive behavior in PNIPAM-co-PAM: (i) sequence exhibiting lower critical solution temperature (LCST) behavior, (ii) sequence exhibiting upper critical solution temperature (UCST) behavior, (iii) sequence displaying both LCST and UCST transitions, and (iv) sequence showing no discernible phase transition within the investigated temperature range. The critical temperature exhibits a strong correlation with the mean block length in periodic sequences displaying LCST-type behavior. This variability in thermoresponsive property is found to be closely linked to the extent of hydrogen bond formation in the system. These findings offer new directions in the design of structurally diverse thermoresponsive copolymers.