Qingfei Fu

Classical structural-kinetics models of thixotropy usually assume that the material structure can fully recover after the imposed shear is removed. This assumption is inadequate for colloidal suspensions in which sufficiently strong shear permanently lowers the recoverable structural level. Here, we formulate a partially reversible thixotropic constitutive model by separating two structural variables: the structural state, λ, and the maximum recoverable structural level, λbreak. The variable λ describes the structural level realized under the current shear condition, whereas λbreak records the shear-history-dependent reduction of the maximum recoverable structure. The resulting model reduces to the classical Moore equation when λbreak=1 but predicts a transition to partially reversible behavior when the imposed shear rate exceeds a critical value γ̇c. For a constant shear rate above the critical value γ̇c, the model gives an analytical solution for the evolution of λbreak and a closed-form expression for the maximum recoverable structural level, λbreak,max. Rheological measurements on 4.5 wt. % fumed-silica suspensions, including three-interval thixotropy tests, repeated high-shear cycles, high-shear duration tests, and critical-shear-rate tests, are used to evaluate the model. The experiments show irreversible viscosity loss after high-shear loading, identify a damage threshold near 10 s−1, and show that the experimentally extracted recovery limit is consistent with the analytical prediction. The proposed framework therefore provides a compact constitutive description of thixotropic materials whose structural recovery is limited by irreversible shear-history effects.

Reverse dialysis based on hydrogel membranes holds promise for the development of soft electrochemical batteries. However, the energy conversion efficiency is often constrained by excessive swelling, which dilates nanochannels and degrades permselectivity. Here, we present a bicontinuous‐network hydrogel (BCNH) engineered by precisely modulating the hydrophilic and hydrophobic phases to achieve high‐performance osmotic energy conversion. Benefiting from the BCN structure, the hydrophilic ion‐conducting pathways are spatially confined by a percolating hydrophobic matrix, which dramatically suppresses swelling. This structural restraint preserves high permselectivity and enhances mechanical robustness. The resulting BCNH membrane, which simultaneously offers high permselectivity and rapid ionic flux, delivers an outstanding osmotic power density of 21.07 W·m ⁻² , significantly outperforming conventional homogeneous hydrogels. By integrating cation‐selective and anion‐selective BCNHs into flexible full cells, we achieve a high‐power density of 10.07 W·m ⁻² , enabling the scalable fabrication of highly flexible battery arrays. Combining integrated high selectivity, ultralow swelling, and robust mechanical properties, the BCNH platform establishes a transformative material foundation for advanced flexible electronics and sustainable energy conversion technologies.

We investigate the rheology of dense suspensions under simple shear and extensional flows using lubrication dynamics-discrete element method simulations. For all deformation modes considered, the relative viscosity is described reasonably well by the Krieger–Dougherty form, suggesting that the divergence is not strongly dependent on flow type, whereas the fitted parameters, namely, the jamming point and exponent, are sensitive to the interparticle friction. To relate shear and extension within a common rate–pressure scaling, we consider a generalized viscous number and examine the effective friction coefficient across deformation modes using a flow-type-dependent normalization. Building on prior stress-decomposition approaches, we employ a minimal modification to the contact contribution by introducing an exponent which improves agreement with simulations for weakly frictional systems. As the interparticle friction increases, this exponent approaches unity, recovering the original formulation. In addition, using the modified hydrodynamic scaling in the low viscous number regime gives an improved prediction relative to the classical square-root dependence. Overall, the proposed framework provides friction-aware constitutive relations that are compatible with both shear and extensional deformations over the full ranges of interparticle friction and viscous number explored here.

Surfactants are widely used to tailor fluid rheology, and interfacial viscoelasticity has received increasing attention in recent years. Previous studies on the effect of interfacial viscosity on liquid-thread breakup have often treated interfacial viscosity as a combined effect or assumed that the interfacial dilatational viscosity is equal to the interfacial shear viscosity. However, the one-dimensional model derived in the present work shows that this assumption masks the independent contribution of dilatational viscosity. Moreover, how interfacial dilatational and shear viscosities differ in their mechanisms and in the magnitude of their effects remains unclear. To address this issue, we develop a one-dimensional model for a surfactant–laden liquid thread, with particular emphasis on separating the effects of the two interfacial viscosities, and investigate the dynamics using linear stability analysis and numerical simulations. The results show that, in the dispersion relation corresponding to the linear stage, the coefficient associated with interfacial shear viscosity is nine times that associated with dilatational viscosity. In suppressing satellite droplets, the overall effect of interfacial shear viscosity is comparable to that of a 12-times larger dilatational viscosity. Moreover, the mechanisms by which the two viscosities suppress satellite-droplet formation are distinct: dilatational viscosity reduces the satellite volume by flattening the satellite droplet, whereas shear viscosity tends to keep the pinch-off location near the midplane, thereby inhibiting satellite-droplet formation. These results show that interfacial dilatational and shear viscosities are not interchangeable in liquid-thread breakup, even within the same interfacial rheological framework.

Flows in many biological and engineering applications are viscoelastic and subject to pulsatile disturbances. In this work, we analyze a pulsatile viscoelastic channel flow with the Oldroyd-B model. Using a Fourier framework, we obtain closed-form expressions for the velocity, polymer shear stress, pressure gradient, and wall shear stress for retained modes of a general periodic forcing. The polymer normal stress is evaluated through a spectral convolution that captures coupling between frequency modes. The frequency response is organized by a complex transfer function that relates the imposed flow rate waveform to the required pressure-gradient response. At low frequency, Oldroyd-B fluids require a smaller pressure-gradient pulsation than Newtonian fluids to deliver the same flow rate modulation, which defines a dynamic flow enhancement regime dominated by elasticity. A low frequency expansion yields an explicit critical elasticity for the onset of enhancement and its dependence on polymer concentration through the solvent viscosity ratio, β. As the forcing frequency and fluid elasticity increase, oscillations of the polymer shear stress weaken, and the dynamics approach a Newtonian limit. In this regime, the Oldroyd-B and Newtonian momentum equations become asymptotically equivalent under an effective Womersley number based on the solvent viscosity. Numerical evaluations confirm that this mapping accurately predicts the velocity distribution, critical pulsation amplitude, and reversal regions near the wall. These findings suggest a simplified way to account for pulsatile viscoelastic flows at high frequency, with potential implications that can guide the design and optimization of related biological and industrial applications.

The bursting of surface bubbles can produce jet droplets and thus mediates the mass transfer across liquid–gas interfaces, which is ubiquitous over a wide range of natural and industrial processes. Though the bubble bursting jetting behavior in pure liquids has been well documented, how the widely existing particulates in liquids affect jet dynamics remains unclear. Here, we experimentally investigate the jet dynamics induced by bubble bursting at the suspension surfaces of submicrometer and micrometer particles with a mass concentration up to 200 g/L. We find that the submicrometer particles in liquids make the jet droplets faster and smaller, which are attributed to the influence of the particles on the viscosity and surface tension of the liquid phase. In contrast, in micrometer particle suspensions, the jet droplets become slower and larger and finally are completely suppressed as the particle concentration increases. By analyzing the influence of particles on the cavity collapse time, we deduce that the micrometer particles modify the jet dynamics by introducing extra surface effects of Marangoni stress or surface elasticity, as micrometer particles are easier to be collected to the bubble surface than submicrometer particles. These results promote our understanding of bubble bursting jetting in complex liquid systems and imply the multiscale hydrodynamic interaction between the particle of different scales and bursting bubbles, which deserves further studies.

Cavitation bubbles at solid–liquid interfaces are central to microfluidic actuation, biomedical manipulation, cleaning, and erosion. So far, the effect of surface curvature on cavitation bubbles remains insufficiently understood. Here, we investigate photothermal cavitation bubbles generated on a planar gold-nanoparticle substrate and on microspheres of varying diameters using high-speed imaging and numerical simulations. Bubbles on both surfaces exhibit similar early expansion, producing strong transient pressure that dominates the peak force and impulse. Planar walls promote stable hemispherical attachment with pronounced necking collapse, whereas spherical surfaces induce curvature-controlled transitions from partial attachment to full encapsulation. These curvature-dependent effects arise primarily during collapse, when curved interfaces reshape pressure gradients, modify reentrant inflow, and alter collapse morphology. These behaviors scale with the ratio of the maximum bubble width to microsphere diameter, which governs near-wall flow asymmetry, shifts collapse-induced pressure focusing, and determines both the transient force and accumulated impulse transmitted to the surface. Furthermore, a mathematical model was developed to establish quantitative relationships between the accumulated impulse, bubble size, and interface curvature for both planar and spherical surfaces, and its validity was confirmed through numerical simulations. Overall, these results establish a unified framework for curvature- and size-dependent cavitation dynamics.