Soft Matter最新文献

Pickering emulsions (PEs) stabilized by nanoparticles have shown significant potential in diverse applications. However, the complex influence of particles on PE makes it difficult to predict how particle hydrophobicity and surface charge affect viscoelastic behavior. In this work, an in situ hydrophobization method was employed to modify silica nanoparticles (SNPs), yielding a series of SNPs with controlled hydrophobicity and surface charge for the preparation of PEs. The findings demonstrated a significant relationship between the surface characteristics of SNPs and the viscoelasticity of PEs. Under conditions of strong electrostatic repulsion, increased hydrophobicity reduces the viscoelasticity of PEs. In contrast, when the surface charge was weak, the maximum viscoelasticity was observed at a hydrophobicity of 90°. These results clarify the mechanisms underlying PE viscoelasticity and provide valuable guidance for the rational design of tunable PEs. The established property–performance relationships offer valuable guidance for tailoring PE rheology through precise particle surface engineering.

We report an approach to obtain effective pair potentials which describe the structure of two-dimensional systems of active Brownian particles. The pair potential is found by an inverse method, which matches the radial distribution function found from two different schemes. The inverse method, previously demonstrated via simulated equilibrium configurations of passive particles, has now been applied to a suspension of active particles. Interestingly, although active particles are inherently not in equilibrium, we still obtain effective interaction potentials which accurately describe the structure of the active system. Treating these effective potentials as if they were those of equilibrium systems, furthermore allows us to measure effective chemical potentials and pressures. Both the passive interactions and active motion of the active Brownian particles contribute to their effective interaction potentials.

This work develops a technique to create and quantify tethered molecular concentration gradients in a hydrogel using a flow chamber. This device is designed to enable isotropic scaffold swelling, nutrient diffusion and real-time microrheological measurements. A hydrogel is first photopolymerized in the flow chamber ensuring that the mechanical properties of the hydrogel across samples are the same prior to molecular concentration gradient creation. Then molecules are passively diffused into the scaffold and a second photopolymerization tethers the concentration gradient into the material. This technique creates in vitro mimics of aspects of biological environments, such as the environment around a hydrogel implanted in the body for cell delivery. We use a well-defined synthetic scaffold with a poly(ethylene glycol) (PEG)-norbornene backbone cross-linked with a matrix metalloproteinase (MMP)-degradable peptide, a standard material for cell encapsulation. The method to tether molecular concentration gradients is validated using a fluorescent PEG-thiol (FITC-PEG-SH), an ideal polymer. We first create a calibration curve by measuring the fluorescence intensity of hydrogels with known uniform concentrations of the tethered fluorescent molecule. The calibration curve is used to calculate spatial concentration from measured fluorescence intensity in hydrogels with polymer or protein concentration gradients. FITC-PEG-SH is diffused through our hydrogel in a flow chamber for 6, 24 and 48 hours. We make consistent gradients and quantify the concentration of the fluorescent molecule every 25 µm along the material. Next, we make tethered concentration gradients of tumor necrosis factor-α (TNF-α), a pro-inflammatory cytokine found in the wound environment, after 24 hours of diffusion. These gradients are consistent when normalized by the concentration at the edge of the hydrogel, which varies due to pore clogging. From both molecular concentration gradients, we calculate an effective diffusion coefficient that is the same order of magnitude as the value calculated using the multiscale diffusion model. Significant advances made with this technique include limited confinement of the material, which enables isotropic swelling and facile nutrient diffusion, the ability to image through the device and the same hydrogel rheological properties across samples. This technique can be used in future work to characterize cell-laden hydrogels which present the same physical cues to cells and tethered concentration gradients of chemical cues using microrheology.

Filamentous bacteriophages fd are viral particles, highly monodisperse in size, that have been widely used as a model colloidal system for studying the self-assembly of rod-shaped particles as well as a versatile template in nanoscience. In aqueous suspensions, fd viruses exhibit lyotropic behavior, forming liquid crystalline phases as their concentration increases. Here, we report a solvent-free system displaying thermotropic phase behavior, achieved through covalent coupling of low molecular weight PEG-based polymer surfactant onto the fd virus surface. Upon lyophilization of aqueous suspensions of these polymer-grafted bacteriophages and subsequent thermal annealing, a solvent-free material is obtained, exhibiting both viscoelasticity and, notably, thermotropic liquid crystalline properties. A combination of small-angle X-ray scattering and optical microscopy experiments reveals the formation of an ordered hexagonal mesophase below 30 °C, which undergoes a melting transition into an isotropic liquid at higher temperatures. Our results demonstrate an efficient approach for converting lyotropic into thermotropic phase behavior in the columnar liquid crystalline phase of filamentous fd colloids. This approach paves the way for extending such functionalization to other technologically relevant rod-like systems, such as carbon nanotubes and cellulose nanocrystals, enabling the introduction of thermotropic properties in anhydrous colloidal materials.

Instrumented indentation is a common technique for measuring the elastic properties of thin materials, including elastomers, gels, and biological materials. Traditional indentation analysis yields a reduced modulus, which is a function of Young's modulus and Poisson's ratio, thus requiring one of the parameters to be estimated or independently measured to decouple the properties. It is difficult in some cases to know the true deformation of the surface due to substrate deformations, machine compliance, and thermal drift. To address these issues, a new technique is demonstrated in which 3D displacements are measured at discrete points along the surface of a transparent specimen during indentation tests using microscopy and fluorescent micrometer-scale particles embedded in the specimen. The out-of-plane displacements of the particles are measured using a defocused imaging technique, taking advantage of the change in spherical aberration ring radius with distance from the focal plane. A technique for tracking the motion of the particles and calibrating the system is described, and experimental measurements on a silicone elastomer are presented. Two optimization algorithms were developed to extract Young's modulus and Poisson's ratio from the experimental measurements. The first algorithm uses radial and normal displacements measured along the surface of the specimen. The second algorithm uses a combination of traditional indentation analysis and radial surface displacements. The elastic properties of polydimethylsiloxane (PDMS) were calculated from experimental data using both algorithms. The results from both methods were in agreement with each other, as well as with values of Young's modulus reported in the literature.

We employed intensity fluctuations of evanescent light scattering to probe spatiotemporal correlations in the near-wall confined motion of microspheres on supported lipid bilayers (SLBs). Normalized cross-correlation analysis revealed long-range, time-resolved correlations in particle–wall separation distances, demonstrating that interfacial stress propagation can transmit mechanical signals across membrane interfaces. The motion exhibited broadly corralled diffusion, with both the corral size and diffusion constant confined to the nanoscale. This confinement could be further classified into fast and slow modes, with most diffusion constants residing in the slow regime, indicating that SLBs predominantly retard and localize microsphere dynamics at the interface. Furthermore, a transition in the interbilayer interaction profile—from bimodal to single-peak behavior—introduced a characteristic length scale and a kT-scale energy barrier, underscoring the cooperative interplay between interfacial stress propagation and membrane shape remodeling.

Cells have the ability to sense and respond to various mechanical cues from their immediate surroundings. One of the manifestations of such a process, which is also known as "mechanosensing", is directed cell migration. Various biological processes have been shown to be controlled by extracellular matrix (ECM) stiffness. Substrates with a high stiffness gradient have been used as a platform to investigate cellular motion in response to mechanical cues. However, creating a cell scale stiffness gradient in such a cell adhesion friendly substrate still remains elusive. In this study, we present a simple and versatile method for fabricating substrates with a periodically varying stiffness profile at the cellular scale, featuring customizable high stiffness gradients. Fibroblast cells, when presented with such continuous yet anisotropic variation of stiffness, preferentially position their nuclei in stiffer regions of the substrate and align themselves along the direction of the lowest rigidity gradient. Furthermore, when the rigidity of the substrate is sufficiently high, cells exhibit less sensitivity to stiffness gradients, with their elongation and nuclear positioning becoming independent of stiffness variations. Overall, our experimental results reveal new insights into the process of cellular mechanosensing where the cell-scale gradient drives strong positional and orientational order.

We use coarse-grained molecular dynamics to isolate how azobenzene isomer identity (cis vs. trans) modulates polymer dynamics in a guest–host setting without covalent attachment and without explicit photoisomerization. Segmental relaxation is quantified from the incoherent intermediate scattering function F_s(k,t), with relaxation times τ(T) extracted from the F_s(k,τ) = e⁻¹ criterion, fitted by Vogel–Fulcher–Tammann, and a glass-transition temperature T_g defined by a standard operational threshold. Across compositions, global structure (density and pair correlations) is nearly isomer-invariant. In contrast, within our model, cis systems exhibit systematically shorter τ and lower T_g than trans—differences consistent with a localized dynamic facilitation near chromophores. Voronoi analysis shows that the average monomer free volume around azobenzene is essentially insensitive to isomer identity, whereas cis chromophores occupy larger Voronoi cells at low T. Isoconfigurational ensembles (propensity analysis) reveal that monomers in the first-neighbor shell of cis are more mobile than near trans, and that immobilizing the chromophores suppresses this contrast. Overall, in this fixed-isomer equilibrium setting, our results cannot support a purely homogeneous free-volume softening between isomers (and, by construction, do not test illumination-induced macroscopic stress gradients); instead they point to a local, cooperative, mobility-dependent pathway that provides a geometry-only baseline for the still-debated microscopic origin of light-driven mass transport in azobenzene materials.

Elastic MicroPhase separation (EMPS) provides a simple route to create soft materials with homogeneous microstructures by leveraging the supersaturation of crosslinked polymer networks with liquids. At low supersaturation, network elasticity stabilizes a uniform mixture, but beyond a critical threshold, metastable microphase-separated domains emerge. While previous theories have focused on describing qualitative features about the size and morphology of these domains, they do not make quantitative predictions about EMPS phase diagrams. In this work, we extend Flory–Huggins theory to quantitatively capture EMPS phase diagrams by incorporating strain-stiffening effects. This model requires no fitting parameters and relies solely on independently measured solubility parameters and large-deformation mechanical responses. Our results confirm that strain-stiffening enables metastable microphase separation within the swelling equilibrium state and reveal why the microstructures can range from discrete droplets to bicontinuous networks. This works highlights the critical role of nonlinear elasticity in controlling phase-separated morphologies in polymer gels.

Many functional materials, such as paints and inks used in applications like coating and 3D printing, are concentrated granular suspensions. In such systems, the contact line dynamics and the internal structure of the suspension interact through shear rate dependent viscosity and microstructural rearrangements. The local shear rate increases sharply near moving contact lines, leading to the non-Newtonian rheology of dense suspensions in this region. While hydrodynamic solutions can describe dilute suspensions, their applicability near advancing contact lines in dense suspensions remains unclear. This study quantifies the deviation from the Newtonian solution by systematically varying interparticle interactions through the choice of dispersion medium. We use silica particles suspended in two refractive index-matched fluids: (i) aqueous 2,2′-thiodiethanol (weak interactions) and (ii) aqueous sodium thiocyanate solution (strong interactions). These systems exhibit substantially different rheological responses, shear-thickening and yield-stress behaviour, respectively. Using astigmatism particle tracking velocimetry (APTV), we resolve the three-dimensional trajectories of tracer particles within a drop driven over a substrate, in an arrangement enabling tracking of the internal flows over a long travel distance of the drop. We observe distinct flow behaviours depending on the particle interactions and the resulting suspension rheology. The more the particle interactions play a role, i.e., the more pronounced the non-Newtonian effects, the more strongly the measured flow profiles differ from the Newtonian solution of the hydrodynamic equations. In the case of the shear-thickening suspension, a notable deviation from Newtonian behaviour is observed. Conversely, the yield-stress suspension exhibits plug flow over the substrate, with Newtonian-like behaviour restricted to the yielded region near the substrate.