Impact response of a thin shallow doubly curved linear viscoelastic shell rectangular in plan

In this paper, we consider the problem on a transverse impact of a viscoelastic sphere upon a viscoelastic shallow doubly curved shell with rectangular platform, the viscoelastic features of which are defined via the fractional derivative standard linear solid models; in so doing, only Young’s time-dependent operators are preassigned, while the bulk moduli are considered to be constant values, since the bulk relaxation for the majority of materials is far less than the shear relaxation. Shallow panel’s displacement subjected to the concentrated contact force is found by the method of expansion in terms of eigen functions, and the sphere’s displacement under the action of the contact force, which is the sum of the shell’s displacement at the place of contact and local bearing of impactor and target’s materials, is defined from the equation of motion of the material point with the mass equal to sphere’s mass. Within the contact domain, the contact force is defined by the modified Hertzian contact law with the time-dependent rigidity function. For decoding the viscoelastic operators involving the problem under consideration, the algebra of Rabotnov’s fractional operators is employed. A nonlinear integro-differential equation is obtained either in terms of the contact force or in the local bearing of the target and impactor materials. Using the duration of contact as a small parameter, approximate analytical solutions have been found, which allow one to define the key characteristics of impact process.

Shear relaxation governs fusion dynamics of biomolecular condensates

Phase-separated biomolecular condensates must respond agilely to biochemical and environmental cues in performing their wide-ranging cellular functions, but our understanding of condensate dynamics is lagging. Ample evidence now indicates biomolecular condensates as viscoelastic fluids, where shear stress relaxes at a finite rate, not instantaneously as in viscous liquids. Yet the fusion dynamics of condensate droplets has only been modeled based on viscous liquids, with fusion time given by the viscocapillary ratio (viscosity over interfacial tension). Here we used optically trapped polystyrene beads to measure the viscous and elastic moduli and the interfacial tensions of four types of droplets. Our results challenge the viscocapillary model, and reveal that the relaxation of shear stress governs fusion dynamics. These findings likely have implications for other dynamic processes such as multiphase organization, assembly and disassembly, and aging.

Thermo-Viscoelastic Response of Protein-Based Hydrogels

Because of the bioactivity and biocompatibility of protein-based gels and the reversible nature of bonds between associating coiled coils, these materials demonstrate a wide spectrum of potential applications in targeted drug delivery, tissue engineering, and regenerative medicine. The kinetics of rearrangement (association and dissociation) of the physical bonds between chains has been traditionally studied in shear relaxation tests and small-amplitude oscillatory tests. A characteristic feature of recombinant protein gels is that chains in the polymer network are connected by temporary bonds between the coiled coil complexes and permanent cross-links between functional groups of amino acids. A simple model is developed for the linear viscoelastic behavior of protein-based gels. Its advantage is that, on the one hand, the model only involves five material parameters with transparent physical meaning and, on the other, it correctly reproduces experimental data in shear relaxation and oscillatory tests. The model is applied to study the effects of temperature, the concentration of proteins, and their structure on the viscoelastic response of hydrogels.

Shear Relaxation Governs Dynamic Processes of Biomolecular Condensates

Phase-separated biomolecular condensates must respond agilely to biochemical and environmental cues in performing their wide-ranging cellular functions, but our understanding of condensate dynamics is lagging. Ample evidence now indicates biomolecular condensates as viscoelastic fluids, where shear stress relaxes at a finite rate, not instantaneously as in viscous liquids. Yet the fusion dynamics of condensate droplets has only been modeled based on viscous liquids, with fusion time given by the viscocapillary ratio (viscosity over interfacial tension). Here we used optically trapped polystyrene beads to measure the viscous and elastic moduli and the interfacial tensions of four types of droplets. Our results challenge the viscocapillary model, and reveal that the relaxation of shear stress governs fusion and other dynamic processes of condensates.

Shape Recovery of Deformed Biomolecular Droplets: Dependence on Condensate Viscoelasticity

ABSTRACTPhase-separated biomolecular condensates often appear as micron-sized droplets. Due to interfacial tension, the droplets usually have a spherical shape and, upon deformation, tend to recover their original shape. Likewise, interfacial tension drives the fusion of two droplets into a single spherical droplet. In all previous studies on shape dynamics, biomolecular condensates have been modeled as purely viscous. However, recent work has shown that biomolecular condensates are viscoelastic, with shear relaxation occurring not instantaneously as would in purely viscous fluids. Here we present an exact analytical solution for the shape recovery dynamics of biomolecular droplets, which exhibits rich time dependence due to viscoelasticity. For condensates modeled as purely viscous, shape recovery is an exponential function of time, with the time constant given by the “viscocapillary” ratio, i.e., viscosity over interfacial tension. For viscoelastic droplets, shape recovery becomes multi-exponential, with shear relaxation yielding additional time constants. The longest of these time constants can be dictated by shear relaxation and independent of interfacial tension, thereby challenging the currently prevailing viscocapillarity-centric view derived from purely viscous fluids. These results highlight the importance of viscoelasticity in condensate shape dynamics and expand our understanding of how material properties affect condensate dynamics in general, including aging. The analytical solution presented here can also be used for validating numerical solutions of fluid-dynamics problems and for fitting experimental and molecular simulation data.

Viscoelasticity of biomolecular condensates conforms to the Jeffreys model

ABSTRACTBiomolecular condensates, largely by virtue of their material properties, are revolutionizing biology, and yet physical understanding of these properties is lagging. Here I show that the viscoelasticity of condensates can be captured by a simple model, comprising a component where shear relaxation is an exponential function of time and a component that is purely viscous (corresponding to instantaneous shear relaxation). Modulation of intermolecular interactions, e.g., by adding salt, can disparately affect the two components, such that the exponentially-relaxing component may dominate at low salt whereas the purely viscous component may dominate at high salt. Condensates have a tendency to fuse, with the dynamics accelerated by surface tension and impeded by viscosity. For fast-fusion condensates, shear relaxation may become rate-limiting. These insights help narrow the gap in understanding between the biology and physics of biomolecular condensates.

Polymer-Magnetic Composite Particles of Fe3O4/Poly(o-anisidine) and Their Suspension Characteristics under Applied Magnetic Fields

Fe3O4/poly(o-anisidine) (POA) magnetic composite nanoparticles with their core-shell structure were synthesized by chemical oxidation polymerization technique and adopted as a magneto-responsive magnetorheological (MR) material. The chemical structure and morphology of core-shell nanoparticles were identified by FT-IR, SEM, TEM, and elemental analyzer. Pycnometer and vibrating sample magnetometer showed that the magnetic saturation and density of the Fe3O4/POA particles were reduced by the POA shell coatings. The rheological properties of the MR suspension dispersed in a silicone oil at various magnetic field strengths were investigated using a rotating rheometer under a magnetic field. The resulting MR suspension showed a typical Newtonian fluid behavior in the absence of external stimuli. When an external magnetic field was applied, it formed a strong chain structure, acting like a solid with a yield stress. Further solid-like behaviors were observed from storage shear relaxation and viscoelastic tests. Finally, the Fe3O4/POA nanoparticles showed better dispersion stability than pure Fe3O4 nanoparticles with 50% improvement.