Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

流体力学杂志“Journal of Fluid Mechanics”由剑桥大学教授George Batchelor在1956年5月创办，在国际流体力学界享有很高的学术声望，被公认为是流体力学最著名的学术刊物之一，2005年的影响因子为2．061，雄居同类期刊之首．在它创刊50周年之际，2006年5月JFM出版了第554卷的纪念特刊，其中刊登了现任主编（美国西北大学S．H．Davis教授和英国剑桥大学T．J．Pedley教授）合写的述评：“Editorial：JFM at50”，以JFM为背景，从独特的视角对近50年来流体力学的发展进行了简明的回顾和展望，并归纳了一系列非常有启发性的有趣统计数字．2006年7月21日在剑桥大学应用数学和理论物理研究所（DAMTP）举行了创刊50周年的庆祝会．下午2点，JFM的新老编辑和来宾会聚一堂，Pedley教授致开幕词，其后是5个精彩的报告：The mysterious rattleback and its fluid counterpart（Keith Moffatt），Developments in shear instabilities（Patrick Huerre），Falling clouds（Elisabeth Guazzelli），Ecotectural fluid mechanics（Paul Linden），The success of JFM（Herbert Huppert），最后由Davis教授致闭幕词．

Journal of Fluid Mechanics创刊50周年

The mechanical behaviour of cells is largely controlled by a structure that is fundamentally out of thermodynamic equilibrium: a network of crosslinked filaments subjected to the action of energy-transducing molecular motors. The study of this kind of active system was absent from conventional physics and there was a need for both new theories and new experiments. The field that has emerged in recent years to fill this gap is underpinned by a theory that takes into account the transduction of chemical energy on the molecular scale. This formalism has advanced our understanding of living systems, but it has also had an impact on research in physics per se. Here, we describe this developing field, its relevance to biology, the novelty it conveys to other areas of physics and some of the challenges in store for the future of active gel physics. Equilibrium physics is ill-equipped to explain all of life’s subtleties, largely because living systems are out of equilibrium. Attempts to overcome this problem have given rise to a lively field of research—and some surprising biological findings.

Active gel physics

Epithelial tissues (epithelia) remove excess cells through extrusion, preventing the accumulation of unnecessary or pathological cells. The extrusion process can be triggered by apoptotic signalling, oncogenic transformation and overcrowding of cells. Despite the important linkage of cell extrusion to developmental, homeostatic and pathological processes such as cancer metastasis, its underlying mechanism and connections to the intrinsic mechanics of the epithelium are largely unexplored. We approach this problem by modelling the epithelium as an active nematic liquid crystal (that has a long range directional order), and comparing numerical simulations to strain rate and stress measurements within monolayers of MDCK (Madin Darby canine kidney) cells. Here we show that apoptotic cell extrusion is provoked by singularities in cell alignments in the form of comet-shaped topological defects. We find a universal correlation between extrusion sites and positions of nematic defects in the cell orientation field in different epithelium types. The results confirm the active nematic nature of epithelia, and demonstrate that defect-induced isotropic stresses are the primary precursors of mechanotransductive responses in cells, including YAP (Yes-associated protein) transcription factor activity, caspase-3-mediated cell death, and extrusions. Importantly, the defect-driven extrusion mechanism depends on intercellular junctions, because the weakening of cell-cell interactions in an α-catenin knockdown monolayer reduces the defect size and increases both the number of defects and extrusion rates, as is also predicted by our model. We further demonstrate the ability to control extrusion hotspots by geometrically inducing defects through microcontact printing of patterned monolayers. On the basis of these results, we propose a mechanism for apoptotic cell extrusion: spontaneously formed topological defects in epithelia govern cell fate. This will be important in predicting extrusion hotspots and dynamics in vivo, with potential applications to tissue regeneration and the suppression of metastasis. Moreover, we anticipate that the analogy between the epithelium and active nematic liquid crystals will trigger further investigations of the link between cellular processes and the material properties of epithelia.

/pdf/topological-defects-in-epithelia-govern-cell-death-and-3c5ek0vxn9.pdf

Topological defects in epithelia govern cell death and extrusion.

Engineering synthetic materials that mimic the remarkable complexity of living organisms is a fundamental challenge in science and technology. We studied the spatiotemporal patterns that emerge when an active nematic film of microtubules and molecular motors is encapsulated within a shape-changing lipid vesicle. Unlike in equilibrium systems, where defects are largely static structures, in active nematics defects move spontaneously and can be described as self-propelled particles. The combination of activity, topological constraints, and vesicle deformability produces a myriad of dynamical states. We highlight two dynamical modes: a tunable periodic state that oscillates between two defect configurations, and shape-changing vesicles with streaming filopodia-like protrusions. These results demonstrate how biomimetic materials can be obtained when topological constraints are used to control the non-equilibrium dynamics of active matter.

/pdf/topology-and-dynamics-of-active-nematic-vesicles-1xj7wgz3xk.pdf

Topology and dynamics of active nematic vesicles

Hydrodynamics can play an important role in determining the behaviour of colloidal particles in many soft matter systems. Analytical solutions for fluid dynamics are limited and incorporating the particle dynamics in numerical methods is challenging, since grid points belonging to fluid and solid phases are exchanged during the simulations. As a solution, here, we introduce a quasi-steady method to simulate dynamics of particles within the frame work of lattice Boltzmann method. This method not only carries the advantages of lattice Boltzmann, namely the simple and straight forward algorithm of programming and the simplicity in imposing the boundary conditions, but it also avoids the complications associated with exchange of particle and fluid nodes. Exploiting the smallness of Reynold’s number associated with colloidal hydrodynamics, the proposed algorithm works in an instantaneous frame of reference and particle velocities are then calculated by imposing additional constraints of force and torque acting on the particle. We illustrate the method using the classic examples of settling particles and a system of recent interest-dynamics of active particles, both in the presence of a wall. Therefore, we expect the proposed method to be suitable and useful in variety of soft and active matter systems.

https://link.springer.com/content/pdf/10.1007/s12034-020-2074-z.pdf

Colloidal hydrodynamics using a quasi-steady algorithm in lattice Boltzmann method

It has previously been shown that non-isothermal directional polymer crystallisation driven by local melting (Zone Annealing), has a close analogy with an equivalent isothermal crystallisation protocol. This surprising analogy is due to the low thermal conductivity of polymers-because they are poor thermal conductors, crystallisation occurs over a relatively narrow spatial domain while the thermal gradient spans a much wider scale. This separation of scales, which occurs in the limit of small sink velocity, allows replacing the crystallinity profile with a step and the temperature at the step acts as an effective isothermal crystallisation temperature. In this paper, we study directional polymer crystallisation under faster moving sinks using both numerical simulations and analytical theory. While, only partial crystallisation occurs, regardless, a steady state exists. At large velocity, the sink quickly moves ahead of a region that is still crystallizing; since polymers are poor thermal conductors, the latent heat dissipation to the sink becomes inefficient, eventually resulting in the temperature increasing back to the melting point thereby resulting in incomplete crystallization. This transition occurs when the two length scales measuring the sink-interface distance and the width of the crystallizing interface become comparable. For steady state and in the limit of large sink velocity, regular perturbation solutions of the differential equations governing heat transport and crystallization in the region between the heat sink and the solid-melt interface are in good agreement with numerical results.

Directional polymer crystallisation with a fast-moving sink.

We derive a closed analytical form of planar rotational equilibria that exist in the three-dimensional motion of two hydrodynamically coupled nonidentical microswimmers, each modeled as a force dipole with intrinsic self-propulsion. Using the method of images for zero Reynolds number flows near interfaces, we demonstrate that our results remain equally applicable at a stress-free liquid–gas interface as in the bulk of a fluid. For a pair of two pullers and a pair of two pushers the linear stability of the equilibria is analyzed with respect to two- and three-dimensional perturbations. A universal stability diagram of the orbits with respect to two-dimensional perturbations is constructed and it is shown that two nonidentical pushers or two nonidentical pullers moving at a stress-free interface may form a stable rotational equilibrium. For two nonidentical pullers we find stable quasi-periodic localized states, associated with the motion on a two-dimensional torus in the phase space. Stable tori are born from the circular periodic orbits as the result of a torus bifurcation. All stable equilibria in two dimensions are shown to be monotonically unstable with respect to three-dimensional perturbations.

Planar Rotational Equilibria of Two Nonidentical Microswimmers

Compound particles are a class of composite systems in which solid particles encapsulated in a fluid droplet are suspended in another fluid. They are encountered in various natural and biological processes, for e.g., nucleated cells, hydrogels, microcapsules etc. In this work, we analyze the flow in and around a concentric compound particle and investigate the deformation and reorientation dynamics of the confining drop and its stability against breakup in imposed linear flows. We obtain analytical expressions for the flow fields upto O(Ca) (capillary number) and the deformed shape of the confining drop upto O(Ca^2) using a domain perturbation technique. Further, we develop an O(Ca) constitutive equation for the volume-averaged stress for a dilute dispersion of compound particles. Compared to linear theory, O(Ca^2) calculations are found to be important as they make qualitatively different predictions in some linear flows. We find that the strong hydrodynamic interaction between the encapsulated particle and the confining interface results in an increased deformation of the confining drop compared to that of a simple drop, and enhances the rheological quantities such as the effective shear viscosity, extensional viscosity, and normal stress differences in a dilute dispersion. These measures pertaining to particles, drops, and particles coated with a fluid film are also derived as limiting cases of compound particles. Moreover, linear viscoelastic behavior of a dilute dispersion of compound particles is characterized in terms of complex modulus by subjecting the dispersion to a small amplitude oscillatory shear flow.

Sumesh P. Thampi

Papers

Colloidal hydrodynamics using a quasi-steady algorithm in lattice Boltzmann method

Directional polymer crystallisation with a fast-moving sink.

Planar Rotational Equilibria of Two Nonidentical Microswimmers

Dilute dispersion of compound particles: deformation dynamics and rheology