Recent advances in the production of controllable multiple emulsions using microfabricated devices

This review focuses on recent developments in the fabrication of multiple emulsions in micro-scale systems such as membranes, microchannel array, and microfluidic emulsification devices. Membrane and microchannel emulsification offer great potential to manufacture multiple emulsions with uniform drop sizes and high encapsulation efficiency of encapsulated active materials. Meanwhile, microfluidic devices enable an unprecedented level of control over the number, size, and type of internal droplets at each hierarchical level but suffer from low production scale. Microfluidic methods can be used to generate high-order multiple emulsions (triple, quadruple, and quintuple), non-spherical (discoidal and rod-like) drops, and asymmetric drops such as Janus and ternary drops with two or three distinct surface regions. Multiple emulsion droplets generated in microfabricated devices can be used as templates for vesicles like polymersomes, liposomes, and colloidosomes with multiple inner compartments for simultaneous encapsulation and release of incompatible active materials or reactants.     

Controllable preparation of particles with microfluidics

This paper reviews recent development and achievements in controllable preparation of nanoparticles, micron spherical and non-spherical particles, using microfluidics. A variety of synthesis strategies are presented and compared, including single-phase and multiphase microflows. The main structures of microfluidic devices and the fundamental principles of microflows for particle preparation are summarized and identified. The controllability of particle size, size distribution, crystal structure, morphology, physical and chemical properties, is examined in terms of the special features of microfluidic reactors. An outlook on opinions and predictions concerning the future development of powder technology with microfluidics is specially provided.     

MULTI-SCALE STRUCTURES IN EMULSION AND MICROSPHERE COMPLEX SYSTEMS

Multi-scale structures involved in emulsion and microsphere complex systems are presented and discussed. The stability and spatio-temporal structures of emulsions, as well as nano-structures formed on the surface of microspheres after polymerization, are affected by the molecular emulsifier/stabilizer structures and the adsorbed emul-sifier/stabilizer nano-structures on the oil/water interface. The broad size distribution and variation of surface features of droplets are responsible for variations of the adsorbed emulsifier/stabilizer structures and the stability of the emulsions. On the other hand, preparation of a uniformly sized emulsion and employment of a combined emulsifier/stabilizer system can preserve the stability of the emulsions and microspheres. The above phenomena should be modeled by a multi-scale method, In order to maintain the stability of individual emulsion systems and realize the desired nano-structures of microspheres by choosing adequate emulsifier/stabilizer and experimental parameters.     

Experimental techniques for mechanical characterization of hydrogels at the microscale

Hydrogel actuators in microfluidic devices must endure the forces of aqueous flow, the constraint of device walls, and the restoring force of elastic membranes. In order to assess the capabilities of hydrogels, three experimental techniques for determining the uniaxial tensile properties and functional swelling properties of microscale hydrogel structures have been developed. Tensile tests were conducted to determine Young's modulus and Poisson's ratio at varying degrees of swelling equilibrium. Force response tests were performed to determine the force exerted by cylindrical hydrogel structures on compression platens held at fixed displacement. Particle image velocimetry, a method originally developed to measure velocity fields in fluid flows, was adapted to investigate the deformation rates at various times within hydrogel structures during volumetric swelling. The techniques and sample fabrication methods outlined are applicable to a variety of hydrogel chemistries.     

Pickering emulsions stabilized by biocompatible particles: A review of preparation,bioapplication, and perspective

The phenomenon of adsorption of solid particles at fluid interfaces to stabilize emulsions or foams have been known for more than a century. Today, particle-stabilized emulsions, often referred to as Pickering emulsions, are receiving growing attention as they are encountered in oil recovery and have long been used in personal care products and food industry. Over the past 10 years the focus of the Pickering emulsion has also increasingly shifted to biomedical applications with thanks to novel syntheses of a wide range of biocompatible particle stabilizers. Here, a brief overview of the development of biocompatible particles is given for Pickering emulsion stabilization, including alginate, poly(lactic-co-glycolic acid) (PLGA), and protein-based particles. The materials prepared by templating from emulsion stabilized with biocompatible particles include colloidal capsules and hierarchically porous materials. It is hoped that the understanding gained from the recent intense activity in the field will enable more researchers to modify existing materials and design new formulations, which would be beneficial for exploring more biological applications.     

Alginate-gelatin emulsion droplets for encapsulation of vitamin A by 3D printed microfluidics

Microfluidics is characterized by the manipulation of fluids in submillimeter channels and has great application potential in encapsulation. To further extend the application of microfluidics in food industries, a 3D printed microfluidic device is used to encapsulate vitamin A and improve its stability. Two natural macromolecules, sodium alginate and gelatin, are added to water as the continuous phase to generate monodisperse emulsion. Under different flow rate ratios, the diameter of droplets decreases with the increase of continuous flow rate. However, at the same flow rate ratio, varying the dispersed and continuous flow rates does not significantly change the diameter and size distribution of emulsion collected. The prepared O/W (oil/water) single emulsion can form microgel particles and avoid degradation of vitamin A by simulated gastric acid; the encapsulated vitamin A will not be released until particles reach simulated intestinal tract. In the simulated digestion in vitro, no vitamin A is released for 2 h in the acidic environment; under an alkaline or neutral environment such as those in intestinal fluids, vitamin A can be released from the microgel particles within 2.5 h. Using the presented approach, emulsions encapsulating vitamin A have been prepared and can potentially be applied to encapsulate other oil-soluble substances in the food industry.     

High-throughput generation of uniform droplets from parallel microchannel droplet generators and the preparation of polystyrene microsphere material

To prepare uniform polystyrene particles with ten microns of diameter, a parallel scaling-up strategy for the capillary-assembled stepwise microchannel was developed, which created uniform droplets with high-throughput and formed a large amount of emulsion templates for the polymerization of styrene and cross-linker. The microchannel droplet generator was robust for the flow rate deviation of the continuous phase in the jetting flow, and droplet generation frequency up to 2.8 × 10⁴ Hz was achieved with only four parallel droplet generators, which were much more efficient than the parallelly scaled microfluidic devices working in dripping flow. 32–52 μm average diameter droplets with 4.5%–8.4% diameter variation coefficients were successfully prepared from the microchannel device fabricated by low-cost 3D-print method, and the droplets were subsequently turned to solid particles via a two-step polymerization in the platform. The polystyrene particles were further reduced to 16.9–23.5 μm with 5.0%–8.6% diameter variation coefficients due to the accompanying emulsion polymerization, and the working capacity of the platform reached hundred milligrams of particles per hour.     

Effect of phase change on the rheology and stability of paraffin wax-in-water Pickering emulsions

“Pickering” emulsions are widely found in nature and industry including food, pharmaceuticals, and oil industries. Often, Pickering emulsion studied have a Newtonian dispersed phase. However, the dispersed phase can be non-Newtonian such as one that can be subjected to a phase change under certain experimental conditions. This work examines how changing the physical state of dispersed phase alters the shear stability and bulk viscoelasticity of o/w emulsions. Model silica-stabilized, paraffin wax-in-water emulsions are synthesized. The wax, with a melting temperature of about 55 ^°C is subjected to a phase change by changing the temperature between 15 and 80 ^°C. At lower temperatures (< 55 ^°C), the droplet deformability and particle mobility at the interface are significantly restricted while at higher temperatures (> 55 ^°C), the wax melts and expands, making the emulsion droplets deformable and the particles more relaxed. These directly affect bulk emulsion rheology. Flow curves and oscillatory shear experiments indicate that emulsion droplets are flocculated and the emulsions behave as elastic solids. These properties are directly influenced by temperature, which alters the state of aggregation and network-structure of the emulsion droplets. The effect of emulsion concentration is also analyzed. Three different concentrations are tried—15, 30, and 45 vol% (as measured at 25 ^°C when the wax is solid). At a given temperature, the rheological properties seem to scale with concentration. Further, we show that the emulsions are sensitive to destabilization (gelation) under flow with the sensitivity directly varying with temperature and magnitude of shear fields (steady shear) applied.     

Third-order polynomial model for analyzing stickup state laminated structure in flexible electronics

Laminated hard-soft integrated structures play a significant role in the fabrication and development of flexible electronics devices. Flexible electronics have advantageous characteristics such as soft and light-weight, can be folded, twisted, flipped inside-out, or be pasted onto other surfaces of arbitrary shapes. In this paper, an analytical model is presented to study the mechanics of laminated hard-soft structures in flexible electronics under a stickup state. Third-order polynomials are used to describe the displacement field, and the principle of virtual work is adopted to derive the governing equations and boundary conditions. The normal strain and the shear stress along the thickness direction in the bi-material region are obtained analytically, which agree well with the results from finite element analysis. The analytical model can be used to analyze stickup state laminated structures, and can serve as a valuable reference for the failure prediction and optimal design of flexible electronics in the future.     

Novel shear modulus equations for concentrated emulsions of two immiscible elastic liquids with interfacial tension

Starting from the shear modulus equation for a dilute emulsion system of two immiscible liquids with interfacial tension, four new equations have been developed for the shear modulus of concentrated emulsions using a differential scheme. The continuous phase and the dispersed droplets are treated as elastic liquids in the derivation. Out of the four models developed in the paper, two models predict the relative shear modulus (ratio of emulsion modulus to continuous phase modulus) to be a function of three variables: elastocapillary number, modulus ratio (dispersed phase modulus to continuous phase modulus) and volume fraction of dispersed phase. The remaining two models include an additional parameter, i.e. the maximum packing volume fraction of droplets. The proposed models are evaluated using three sets of experimental data on high frequency shear modulus of concentrated polymer-thickened oil-in-water emulsions.     

A study of emulsion expansion by a boundary integral method

A new implementation of the conventional three-dimensional boundary-integral formulation for deformable drops in a viscous medium at low Reynolds numbers is presented. We describe a scheme to generate concentrated emulsions and a numerical procedure for computing interactions among deformable drops. Direct numerical simulations are used to describe the expansion of an ordered emulsion at dispersed-phase volume fraction up to 0.98. We also study the viscoelastic response of a monodisperse emulsion which have been concentrated up to 0.68. The results have demonstrated the feasibility of simulating high-volume fraction emulsions, and the method can readily be extended to explore three-dimensional foam problems.     

Motion control of the flexible manipulator via controllable local degrees of freedom

In order to improve motion accuracy of the flexible manipulator, an idea of using its topological characteristics to suppress vibration is suggested. The concept of controllable local degree of freedom is proposed and introduced to the topological structure of the flexible manipulator. It is shown that the arbitrary motion provided by the controllable local degrees of freedom plays an important role in eliminating undesired effects of the flexibility. On this basis, a method for reducing motion error of the flexible manipulator is put forward. By planning the motion of controllable local degrees of freedom, the appropriate control force can be constructed to increase the damping force and eliminate the exciting force of the flexible manipulator, thereby improving the end-effector accuracy. The results, demonstrated by the numerical simulations, are highly promising and suggest that controllable local degrees of freedom can be a useful tool in combating the undesired vibration deformation of the flexible manipulator.     

柔性多体系统的计算策略

对柔性多体系统计算建模的研究现状和近期进展进行了总结. 重点讨论了柔性多体 动力学的以下内容: 柔性构件的建模, 约束建模, 求解技术, 控制策略, 耦合问 题, 设计和实验的研究. 对柔性多体系统建模的浮动坐标系,转动坐标系和惯性系 等3种坐标系的特点进行了对比. 指出了未来的研究方向, 包括柔性多体系统的新 的应用,如微观力学系统和超微观力学系统等; 提高这些模型的计算精度和效率的 技巧和策略; 以及可以用于改善柔性多体系统的工具. 本综述文章引用了877篇参 考文献.     

航天柔性结构振动控制的若干新进展

围绕航天柔性结构的振动控制，从结构及材料的数学模型、材料及器件、基本理论与方法和一体化振动控制几个方面对一些研究的最新进展进行了介绍．主动控制和被动控制的一体化技术研究是当今航天柔性结构振动控制研究的重点，两种控制方法的结合不仅优点互补，而且提高了控制系统的性能．控制用材料和器件的研究在工程应用的推动下，也取得了较快的发展，并促进了振动控制技术的实用化     

Cinemicrophotographic study of boiling of water-in-oil emulsions

Cinemicrophotography is applied to the boiling of emulsions of water dispersed in oils whose boiling points are higher than that of water. The layer of emulsion is made thin enough to make possible a microscopic observation by transmitted light through the layer. Two alternative processes of bubble formation are found: a periodical bubble formation at specified sites on the heated solid surface contacting the emulsion and a casual bubble formation some distance away from the surface. The basic mechanisms of those processes are discussed.     

High-pressure foaming properties of carbon dioxide-saturated emulsions

The aeration of emulsions with tailored properties and structure is of widespread importance in processing of foods and cosmetics. This report addresses the micro-cellular foam formation of carbon dioxide-saturated oil-in-water emulsions triggered by the application of a controlled pressure drop. The experimental setup combines a stirred pressure vessel with a pressure cell-equipped rheometer and pneumatic expansion valves. This allows to systematically study the process of gas dissolution, bubble nucleation, and growth under defined pressure, temperature, and flow conditions. Investigations on the impact of relevant process parameters show that dissolved gas fraction, emulsion viscosity, and shear rate have a major influence on foam formation. Dissolution of carbon dioxide leads to a viscosity reduction of the emulsion which and is described by a viscosity reduction factor. The point of bubble nucleation is derived from rheological patterns during depressurization. Experiments show that lower emulsion viscosity and higher shear rate favor bubble nucleation upon pressure release. Rheological results are supported by video analysis as the setup allows capturing nucleation, growth, and destabilization of bubbles as a function of pressure, supersaturation, and time. The results of this work yield the understanding of the high-pressure foaming mechanism from a rheological perspective and foster the design of such processes.     

流体动压下乳化液迁移特性及润滑机理研究

乳化液广泛应用于机械加工装备,其在动压条件下迁移特性及润滑机理亟待明晰.采用光干涉法测量了试制和商用两种乳化液在不同浓度梯度下的流体动压成膜厚度,使用光致荧光法研究了接触区外围乳液池三维分布及其迁移行为,结合各浓度下乳化液黏度、红外吸收光谱特征峰和粒径分布等表征结果,对低副接触下乳化液润滑机制进行了研究.结果表明:在稀释过程中乳化液会发生油包水向水包油的流型转变,转变前后乳化液均有较好成膜能力;可观察到出口气穴和入口弯月面所需的卷吸速度随着乳化液浓度降低而增大;粒径较小乳化液滴可直接进入接触区,粒径大的液滴在进入接触区前发生破裂,油相经离水展着进入接触区起主要润滑作用.     

A theoretical model of reversible adhesion in shape memory surface relief structures and its application in transfer printing

Transfer printing is an important and versatile tool for deterministic assembly and integration of micro/nanomaterials on unusual substrates, with promising applications in fabrication of stretchable and flexible electronics. The shape memory polymers (SMP) with triangular surface relief structures are introduced to achieve large, reversible adhesion, thereby with potential applications in temperature-controlled transfer printing. An analytic model is established, and it identifies two mechanisms to increase the adhesion: (1) transition of contact mode from the triangular to trapezoidal configurations, and (2) explicit enhancement in the contact area. The surface relief structures are optimized to achieve reversible adhesion and transfer printing. The theoretical model and results presented can be exploited as design guidelines for future applications of SMP in reversible adhesion and stretchable electronics.     

Pickering emulsions stabilized by colloidal surfactants: Role of solid particles

Pickering emulsions are emulsions stabilized by colloidal surfactants, i.e. solid particles. Compared with traditional molecular surfactant-stabilized emulsions, Pickering emulsions show many advantages, such as high resistance to coalescence, long-term stability, good biocompatibility and tunable properties. In recent years, Pickering emulsions are widely applied in scientific researches and industrial applications. In this review, we focus on the influences of particle properties on Pickering emulsions, including particle amphiphilicity, concentration, size and shape, and summarize the strategies developed for the preparation of amphiphilic Janus particles. The applications of Pickering emulsions in food industry, cosmetic industry, material science, drug delivery, biomedical research and vaccine adjuvant will also be covered. Pickering emulsions are a unique system for multi-disciplinary studies and will become more and more important in the future.     

流动聚焦及射流不稳定性

流动聚焦是一种有效的微细射流产生方法,其原理可以描述为从毛细管流出的流体由另一种高速运动的流体驱动,经小孔聚焦后形成稳定的锥–射流结构,射流因不稳定性破碎成单分散的液滴.自从1998年流动聚焦被提出以来,陆续发展了单轴流动聚焦、电流动聚焦、复合流动聚焦和微流控流动聚焦等毛细流动技术.这些技术稳定、易操作、没有苛刻的环境条件的要求,能够制备单分散性较好的微纳米量级的液滴、颗粒和胶囊,在科学研究和实际应用中具有重要价值.流动聚焦涉及了多尺度、多界面和多场耦合的复杂流体力学问题,其中稳定的锥形是形成稳定射流的先决条件,过程参数是影响射流界面扰动发展的关键因素,而射流不稳定性分析是揭示射流破碎的最主要理论工具.该文回顾了近二十年来不同结构流动聚焦的研究进展,概述这些技术涉及的过程控制、流动模式、尺度律和不稳定性分析等关键力学问题,总结射流不稳定性的研究方法和已取得的成果,最后展望流动聚焦的研究方向和应用前景.