Dynamic pattern formation in a vesicle-generating microfluidic device

Spatiotemporal pattern formation occurs in a variety of nonequilibrium physical and chemical systems. Here we show that a microfluidic device designed to produce reverse micelles can generate complex, ordered patterns as it is continuously operated far from thermodynamic equilibrium. Flow in a microfluidic system is usually simple-viscous effects dominate and the low Reynolds number leads to laminar flow. Self-assembly of the vesicles into patterns depends on channel geometry and relative fluid pressures, enabling the production of motifs ranging from monodisperse droplets to helices and ribbons.     

Estimating surfactant surface coverage and decomposing its effect on drop deformation

A novel method is introduced to estimate surface coverage and the equation of state of insoluble surfactant on droplets, involving measurement of interfacial tension on a single parent drop and progressively subdivided generations of daughter drops. This has enabled quantitative decomposition of the dilution, tip-stretching, and Marangoni effects of surfactants on drop deformation. For a small viscosity ratio of 0.09, the Marangoni effect dominates, increasing first and then decreasing with surface coverage, the dilution effect is significant at high, and tip-stretching only at low surface coverage. For a viscosity ratio of 2.3, the dilution effect dominates, and neither Marangoni nor tip-stretching effects play an important role.     

Numerical simulation on dynamic behaviors of bubbles flowing through bifurcate T-junction in microfluidic device

Based on the volume of fluid(VOF) method, a numerical model of bubbles splitting in a microfluidic device with T-junction is developed and solved numerically. Various flow patterns are distinguished and the effects of bubble length,capillary number, and diameter ratio between the mother channel and branch are discussed. The break-up mechanism is explored in particular. The results indicate that the behaviors of the bubbles can be classified into two categories: break-up and non-break. Under the condition of slug flowing, the branches are obstructed by the bubbles that the pressure difference drives the bubbles into break-up state, while the bubbles that retain non-break state flow into an arbitrary branch under bubbling flow condition. The break-up of the short bubbles only occurs when the viscous force from the continuous phase overcomes the interfacial tension. The behavior of the bubbles transits from non-break to break-up with the increase of capillary number. In addition, the increasing of the diameter ratio is beneficial to the symmetrical break-up of the bubbles.     

Arnold tongues in a microfluidic drop emitter

The Letter reports an experimental study of microfluidic droplets produced in T junctions and subjected to a local periodic forcing. Synchronized and quasiperiodic regimes--organized into Arnold tongues and devil staircases--are reported for the first time for a system dedicated to drop emission. The nature of the dynamical regime controls the droplet characteristics. These phenomena are mostly controlled by the characteristics of the forcing and the flow conditions.     

含活性剂液膜去润湿演化的稳定性特征

针对含非溶性活性剂的液膜在固体基底上的去润湿过程,基于润滑理论建立了基态和扰动态下液膜厚度和表面活性剂浓度的演化模型,应用非模态理论分析了演化过程的稳定性特征,探讨了分子间力对液膜去润湿过程的影响. 研究表明,微扰动波的引入（k=1）有利于液膜去润湿过程的稳定进行,扰动能量逐渐衰减,然而,该效果随着扰动波数的增加而显著改变,k ≥ 2时,液膜演化的稳定性反而恶化,扰动能量被逐步放大,演化呈现出非稳定特征. 增大初始液膜厚度可以有效改善液膜流动的稳定性. 范德华力放大了液膜表面的微扰动,使得液膜演化的稳定性下降;相反,Born斥力和静电斥力具有增强去润湿稳定性的作用. 关键词： 活性剂 去润湿 分离压 稳定性     

Instability of Marangoni flow in the presence of an insoluble surfactant. Experiments

The stability and structure of solutocapillary Marangoni flow initiated by a localized concentration source in the presence of an adsorbed layer of insoluble surfactant is investigated experimentally. It has been established that the main axisymmetric flow becomes unstable with respect to azimuthally periodic disturbances which leads to the appearance of the surface flow with a multi-vortex structure. The structure of the secondary flow is investigated depending on the intensity of the main flow and on the surface density of the surfactant. It has been shown that the azimuthal wave number increases with the growth of the Marangoni number and decreases with the surface density of the surfactant. A threshold value of the surface density of the surfactant, at which the Marangoni flow does not occur, has been defined.     

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在考虑到捕获电子效应的情况下,对求解二维Fokker-Planck方程的编程进行了反弹平均的修改,使用了交替方向隐式法来求解方程。分析和计算了在不同扩散系数和不同共振区间的情况下,捕获电子效应对驱动电流的影响。结果显示：随着逆纵横比的增加驱动电流密度有明显的下降,在磁轴附近捕获电子效应对电流驱动影响很小;提高波功率并不能很好的改善捕获电子效应对电流驱动的影响;右移共振区间提高共振电子的速度,也不能很好的改善捕获电子效应对电流驱动的影响。所得结果与理论分析基本一致。     

Pulsed-field separation of particles in a microfluidic device

We demonstrate the proof-of-principle of a new separation concept for micrometer-sized particles in a structured microfluidic device. Under the action of externally applied, periodic voltage-pulses two different species of like-charged polystyrene beads are observed to simultaneously migrate into opposite directions. Based on a theoretical model of the particle motion in the microdevice that shows good agreement with the experimental measurements, the underlying separation mechanism is identified and explained. Potential biophysical applications, such as cell sorting, are briefly addressed.     

Buoyancy-driven breakup of an isolated drop with surfactant

At moderate values of the surface Peclet number and the dimensionless gas constant, a surfactant-covered, isolated drop that is initially spherical can undergo large deformations and eventually break in inertialess gravitational motion. A long, thin filament forms, extending from the trailing edge of the drop, with a breakup mechanism that is similar to tip streaming. In the range of parameters studied, the emitted drop has an equivalent spherical radius between 0.01 to 0.03 of the original drop radius and an interfacial tension from 0.2 to 0.6 of the clean-interface value.     

捕获电子效应对电流驱动影响的数值研究

在考虑到捕获电子效应的情况下,对求解二维Fokker-Planck方程的编程进行了反弹平均的修改,使用了交替方向隐式法来求解方程。分析和计算了在不同扩散系数和不同共振区间的情况下,捕获电子效应对驱动电流的影响。结果显示：随着逆纵横比的增加驱动电流密度有明显的下降,在磁轴附近捕获电子效应对电流驱动影响很小;提高波功率并不能很好的改善捕获电子效应对电流驱动的影响;右移共振区间提高共振电子的速度,也不能很好的改善捕获电子效应对电流驱动的影响。所得结果与理论分析基本一致。     

Numerical and experimental investigation of burner device for anode gas reburning

A complex numerical and experimental investigation of burner devices for a reburning of the anode gas of aluminum production was carried out. The chosen mathematical model of the processes of aerodynamics, heat exchange, and gas burning was identified and adapted on the basis of data of real-scale measurements. Basing on computational experiments the optimization of the design of elements of the system for anode gas detoxication and transport was carried out. A new system was proposed and tested for reburning the anode gases of electrolysis production, which consists of an improved burner device and a dust-sedimenting chamber with a possibility of regulating the amount of gas suction.     

微小水滴撞击深水液池空腔运动的数值模拟及机理研究

为了探究微米级微小水滴撞击深水液池运动中空腔的成长过程与机理,采用自适应网格技术和流体体积方法对撞击速度为2.5–6.5 m/s的微小水滴撞击深水液池的运动进行数值模拟研究,考察不同撞击速度下水滴撞击深水液池后的水体混掺、毛细波传播、空腔变形规律以及气泡截留过程,并深入探究空腔运动的动力学机制.研究结果表明,不同撞击速度下,在忽略毛细波作用、空腔深度h∈（D,h_max）的前提下,空腔深度随时间的成长仍满足t∝h^5/2的关系;液滴撞击产生的空腔形状有U形和半球形两种,前者一般向V形转变,后者空腔底部会变为圆柱形,产生细长射流,并有可能发生气泡截留现象;在撞击速度较低时,低压区首先在空腔侧壁与底部交界处产生,随后在靠近液面以及空腔底部靠近中心区域各产生一个较大的涡环;在撞击速度较高,产生细长射流时,涡环的生成被抑制,低压区首先在波浪底部与侧壁上交界处产生,随后空腔底部变为圆柱状,空腔侧壁首先坍塌形成气泡截留.     

Numerical investigation of the deformation mechanism of a bubble or a drop rising or falling in another fluid

A numerical method for simulating the motion and deformation of an axisymmetric bubble or drop rising or falling in another infinite and initially stationary fluid is developed based on the volume of fluid （VOF） method in the frame of two incompressible and immiscible viscous fluids under the action of gravity, taking into consideration of surface tension effects. A comparison of the numerical results by this method with those by other works indicates the validity of the method. In the frame of inviseid and incompressible fluids without taking into consideration of surface tension effects, the mechanisms of the generation of the liquid jet and the transition from spherical shape to toroidal shape during the bubble or drop deformation, the increase of the ring diameter of the toroidal bubble or drop and the decrease of its cross-section area during its motion, and the effects of the density ratio of the two fluids on the deformation of the bubble or drop are analysed both theoretically and numerically.     

基于微泡共振的快速微流体声学混合方法研究

微流体在生物医学、化学工程等领域应用广泛,并具有重大意义.在预处理中,液体混合也是关键且最为必要的前序.为了提高微流控腔道内液体混合的效率,本文提出基于单微泡振动的声学混合器,通过微泡共振,产生声微流,声微流形成的剪切力将在流体中产生微扰动,实现液体的混合.设计了底面直径为40μm的微孔结构,由于液体表面张力作用形成微泡,在共振频率为165 kHz的压电换能器激励下,气泡发生共振产生声微流.通过对压电换能器输入不同能量,获取混合液体的最优参数,可在37.5 ms内实现混合效果,混合均匀度达到92.7%.本文设计的单微泡振动混合器结构简单、混合效率高、混合时间短、输入能量低,可为生物化学等方面的研究提供强有力的技术支撑.     

Numerical investigation of the air injection effect on the cavitating flow in Francis hydro turbine

At full and over load operating points, some Francis turbines experience strong self-excited pressure and power oscillations. These oscillations are occuring due to the hydrodynamic instability of the cavitating fluid flow. In many cases, the amplitude of such pulsations may be reduced substantially during the turbine operation by the air injection/ admission below the runner. Such an effect is investigated numerically in the present work. To this end, the hybrid one-three-dimensional model of the flow of the mixture “liquid?vapor” in the duct of a hydroelectric power station, which was proposed previously by the present authors, is augmented by the second gaseous component — the noncondensable air. The boundary conditions and the numerical method for solving the equations of the model are described. To check the accuracy of computing the interface “liquid?gas”, the numerical method was applied at first for solving the dam break problem. The algorithm was then used for modeling the flow in a hydraulic turbine with air injection below the runner. It is shown that with increasing flow rate of the injected air, the amplitude of pressure pulsations decreases. The mechanism of the flow structure alteration in the draft tube cone has been elucidated, which leads to flow stabilization at air injection.     

On dynamic excitation of Marangoni instability in a liquid layer with insoluble surfactant on the deformable surface

The European Physical Journal Special Topics - This paper is a continuation of our previous work presented at the IMA-6, see [1]. We continue to analyze the parametric excitation of Marangoni...     

Numerical and experimental investigation of the effect of filtering on chaotic symbolic dynamics

Motivated by the practical consideration of the measurement of chaotic signals in experiments or the transmission of these signals through a physical medium, we investigate the effect of filtering on chaotic symbolic dynamics. We focus on the linear, time-invariant filters that are used frequently in many applications, and on the two quantities characterizing chaotic symbolic dynamics: topological entropy and bit-error rate. Theoretical consideration suggests that the topological entropy is invariant under filtering. Since computation of this entropy requires that the generating partition for defining the symbolic dynamics be known, in practical situations the computed entropy may change as a filtering parameter is changed. We find, through numerical computations and experiments with a chaotic electronic circuit, that with reasonable care the computed or measured entropy values can be preserved for a wide range of the filtering parameter.