We numerically study by lattice Boltzmann simulations the rheological properties of an active emulsion made of a suspension of an active polar gel embedded in an isotropic passive background. We find that the hexatic equilibrium configuration of polar droplets is highly sensitive to both active injection and external forcing and may either lead to asymmetric unidirectional states which break top-bottom symmetry or symmetric ones. In this latter case, for large enough activity, the system develops a shear-thickening regime at low shear rates. Importantly, for larger external forcing a regime with stable negative effective viscosity is found. Moreover, at intermediate activity a region of multistability is encountered and we show that a maximum entropy production principle holds in selecting the most favorable state.
We study the rheology of monodisperse and bidisperse emulsions with various droplet sizes (1 $μ$m -- 2 $μ$m diameter). Above a critical volume fraction $ϕ_c$, these systems exhibit solid-like behavior and a yield stress can be detected. Previous experiments suggest that for small thermal particles, rheology will see a glass transition at $ϕ_c = ϕ_g =0.58$; for large athermal systems, rheology will see a jamming transition at $ϕ_c = ϕ_J =0.64$. However, simulations point out that at the crossover of thermal and athermal regimes, the glass and jamming transitions may both be observed in the same…
Adhesion differentials control the rheology of biomimetic emulsions
Sorbonne Université（索邦大学）；CNRS（法国国家科学研究中心）；Laboratoire Jean Perrin（让·佩兰实验室）；Inserm（法国国家健康与医学研究院）；Institut de Biologie Paris-Seine（巴黎塞纳生物学研究所）；Aix Marseille Univ（艾克斯-马赛大学）；Université de Toulon（土伦大学）；CPT (UMR 7332)（复杂系统物理中心（UMR 7332））；Turing Center for Living Systems（图灵生命系统中心）
The aggregation of protein-stabilised emulsions leads to the formation of emulsion gels. These soft solids are classically envisioned as droplet-filled matrices. Here however, it is assumed that protein-coated sub-micron droplets contribute to the network formation in a similar way to proteins. Emulsion gels are thus envisioned as composite networks made of proteins and droplets. Emulsion gels with a wide range of composition are prepared and their viscoelasticity and frequency dependence are measured. Their rheological behaviours are then analysed and compared with the properties of pure gels…
The effect of a spatially uniform magnetic field on the shear rheology of a dilute emulsion of monodispersed ferrofluid droplets, immersed in a non-magnetizable immiscible fluid, is investigated using direct numerical simulations. The direction of the applied magnetic field is normal to the shear flow direction. The droplets extra stress tensor arising from the presence of interfacial forces of magnetic nature is modeled on the basis of the seminal work of G. K. Batchelor, J. Fluid Mech., 41.3 (1970) under the assumptions of a linearly magnetizable ferrofluid phase and negligible inertia. The …
Protein-stabilised emulsion gels can be studied in the theoretical framework of colloidal gels, because both protein assemblies and droplets may be considered as soft colloids. These particles differ in their nature, size and softness, and these differences may have an influence on the rheological properties of the gels they form. Pure gels made of milk proteins (sodium caseinate), or of sub-micron protein-stabilised droplets, were prepared by slow acidification of suspensions at various concentrations. Their microstructure was characterised, their viscoelasticity, both in the linear and non-l…
We study the rheology of a two-fluid emulsion in semi-concentrated conditions; the solute is Newtonian while the solvent an inelastic power law fluid. The problem at hand is tackled by means of direct numerical simulations using the volume of fluid method. The analysis is performed for different volume fractions and viscosity ratios under the assumption of negligible inertia and zero buoyancy force. Several carrier fluids are considered encompassing both the shear-thinning and thickening behaviors. We show that the effective viscosity of the system increases for shear-thickening fluids and dec…
By varying the oil volume fraction, the microscopic droplet size and the macroscopic rheology of emulsions are investigated in a Taylor-Couette (TC) turbulent shear flow. Although here oil and water in the emulsions have almost the same physical properties (density and viscosity), unexpectedly, we find that oil-in-water (O/W) and water-in-oil (W/O) emulsions have very distinct hydrodynamic behaviors, i.e., the system is clearly asymmetric. By looking at the micro-scales, the average droplet diameter hardly changes with the oil volume fraction neither for O/W nor for W/O. However, for W/O it is…
Interfacial rheology is important for understanding properties such as Pickering emulsion or foam stability. Currently, the response is measured using a probe directly attached to the interface. This can both disturb the interface and is coupled to flow in the bulk phase, limiting its sensitivity. We have developed a contactless interfacial method to perform interfacial shear rheology on liquid/liquid interfaces with no tool attached directly to the interface. This is achieved by shearing one of the liquid phases and measuring the interfacial response via confocal microscopy. Using this method…
Exotic rheology of materials with active rearrangements
University of Bristol（布里斯托大学）；Max Planck Institute for Dynamics and Self-Organization（马克斯·普朗克动力学与自组织研究所）；University of Exeter（埃克塞特大学）；Isaac Newton Institute for Mathematical Sciences（牛顿数学科学研究所）；Leiden University（莱顿大学）
Bi-axial shear of confined colloidal suspensions: the structure and rheology of the vorticity-aligned string phase
利用一种新型的双轴共焦流变仪，我们研究了约束条件下剪切胶体悬浮液的结构和流变性。与以前的工作一致[X. Cheng \textit{et al., Proc. Natl. Acad. Sci. U. S. A.}，2011，109，63]，我们在单轴剪切下在中等浓度的胶体悬浮液中观察到涡旋对齐的弦相。使用双轴剪切协议，我们直接操纵的字符串结构的方向和形态。同时，我们使用双轴力测量装置测量了沿流动和涡度方向的悬浮液流变学。我们的研究结果表明，尽管高度各向异性的微观结构，悬浮液粘度保持各向同性和恒定的剪切速率探索。这些结果表明，流体动力学的贡献占主导地位的悬浮液响应。此外，他们强调双轴共焦流变仪的能力，阐明复杂流体的微观结构和流变学之间的关系。
On the rheology of red blood cell suspensions with different amounts of dextran: separating the effect of aggregation and increase in viscosity of the suspending phase
On the rheology of pulmonary surfactant: effects of concentration and consequences for the surfactant replacement therapy
肺表面活性物质的作用是降低肺的表面张力，促进呼吸。表面活性剂替代疗法（SRT）旨在通过将替代品滴注到气道中，这是一种已被证明对早产儿有效和挽救生命的技术。使这种疗法适用于成人需要根据患者体重调整给药剂量，并增加脂质浓度，同时保持其表面和流动特性相似。在这里，我们利用磁线为基础的微流变学技术来测量在各种实验条件下的外源性肺表面活性剂固尔苏的粘度。发现固尔苏粘度随脂质浓度呈指数增加，遵循胶体的Krieger-Doughnard定律。Krieger-Doughnard行为还预测了液体到凝胶转变时粘度的发散。对于固尔苏，发现过渡浓度接近其配制浓度（117 g L-1 vs 80 g L-1）。这一结果表明，对于SRT的表面活性剂的流变性能需要进行监测，并保持在一定的范围内。这里发现的结果可以帮助生产适合成人的呼吸窘迫综合征悬浮液。本工作还表明磁线微流变学技术作为一种精确的工具，探索生物软物质动力学的潜力。
In this work we present a simple qualitative model to describe shear rheological behavior of the twist-bend nematic liquid crystals. We find that at relatively low shear rate the effective viscosity decreases with the shear rate manifesting so-called shear-thinning phenomenon. At intermediate shear rate the stress is almost independent of the shear rate (a sort of plateau), and at large shear rate Newtonian rheology takes place. Within our theory we estimate the critical values of the shear rate in terms of coarse grained shear viscosity coefficients of the effective smectics equivalent to the…
