振动剪切流场的等效稳态剪切速率

根据瞬态网络结构理论,建立了振动剪切流场中聚合物熔体缠结密度的动态速率微分方程。计算结果表明,当振动剪切达到动态平衡时,缠结密度在平衡位置处周期性地波动,其时均值等于稳态剪切速率下的缠结密度。因此,在时均意义上,振动剪切流的时均网络结构状态可等效为稳态剪切流的缠结平衡状态。通过计算发现:当应变振幅小于1.7时,等效稳态剪切速率可取振动剪切速率的周期平均值,由此产生的均方根误差随着应变振幅增加而增大,但小于0.017;而当应变振幅大于1.7时,则取峰-峰均值作为等效剪切速率,其均方根误差随着应变振幅增加而减小。     

A boundary-Integral Equation for Two-Dimensional Oscillatory Stokes Flow Past an Arbitrary Body

The fundamental singular velocity and pressure fields generated by the presence of an isolated line force acting at a point in a two-dimensional unbounded viscous incompressible medium executing oscillatory motions are used to formulate an integral equation which governs the flow past an arbitrarily shaped body. The Fredholm integral equation of the first kind is then solved by means of a boundary-element method, for the translational oscillatory flow past circular, elliptic and orthogonally intersecting cylinders. The asymptotic behaviour of the force on the cylinder for large values of the frequency parameter is obtained.     

Pulsatile Flow in Microfluidic Systems

This review describes the current knowledge and applications of pulsatile flow in microfluidic systems. Elements of fluid dynamics at low Reynolds number are first described in the context of pulsatile flow. Then the practical applications in microfluidic processes are presented: the methods to generate a pulsatile flow, the generation of emulsion droplets through harmonic flow rate perturbation, the applications in mixing and particle separation, and the benefits of pulsatile flow for clog mitigation. The second part of the review is devoted to pulsatile flow in biological applications. Pulsatile flows can be used for mimicking physiological systems, to alter or enhance cell cultures, and for bioassay automation. Pulsatile flows offer unique advantages over a steady flow, especially in microfluidic systems, but also require some new physical insights and more rigorous investigation to fully benefit future applications.     

Preparation of Monodisperse Biodegradable Polymer Microparticles Using a Microfluidic Flow‐Focusing Device for Controlled Drug Delivery

Degradable microparticles have broad utility as vehicles for drug delivery and form the basis of several therapies approved by the US Food and Drug Administration. Conventional emulsion‐based methods of manufacturing produce particles with a wide range of diameters (and thus kinetics of release) in each batch. This paper describes the fabrication of monodisperse, drug‐loaded microparticles from biodegradable polymers using the microfluidic flow‐focusing (FF) devices and the drug‐delivery properties of those particles. Particles are engineered with defined sizes, ranging from 10 µm to 50 µm. These particles are nearly monodisperse (polydispersity index = 3.9%). A model amphiphilic drug (bupivacaine) is incorporated within the biodegradable matrix of the particles. Kinetic analysis shows that the release of the drug from these monodisperse particles is slower than that from conventional methods of the same average size but a broader distribution of sizes and, most importantly, exhibit a significantly lower initial burst than that observed with conventional particles. The difference in the initial kinetics of drug release is attributed to the uniform distribution of the drug inside the particles generated using the microfluidic methods. These results demonstrate the utility of microfluidic FF for the generation of homogenous systems of particles for the delivery of drugs.     

Retraction: Guiding Cell Migration Using One‐Way Micropattern Arrays

Adv. Mater. 2007 , 19, 1084–1090 The above article, published online on March 20, 2007 in Wiley Online Library ( https://onlinelibrary.wiley.com ) has been retracted by agreement between the authors, the journal Editor‐in‐Chief, and the publisher. The retraction has been agreed following an internal review by the University of Cincinnati wherein it was found that there were problems with Figure 2 and 3 and also with the microscope metadata for Figures 2, 3A, 3C, 3D, 3E, and 4. G. Kumar, C.‐C. Ho, C. C. Co, Adv. Mater. 2007 , 19 , 1084–1090; DOI: 10.1002/adma.200601629     

A New Approach to In‐Situ “Micromanufacturing”: Microfluidic Fabrication of Magnetic and Fluorescent Chains Using Chitosan Microparticles as Building Blocks

An in situ microfluidic assembly approach is described that can both produce microsized building blocks and assemble them into complex multiparticle configurations in the same microfluidic device. The building blocks are microparticles of the biopolymer chitosan, which is intentionally selected because its chemistry allows for simultaneous intraparticle and interparticle linking. Monodisperse chitosan‐bearing droplets are created by shearing off a chitosan solution at a microfluidic T‐junction with a stream of hexadecane containing a nonionic detergent. These droplets are then interfacially crosslinked into stable microparticles by a downstream flow of glutaraldehyde (GA). The functional properties of these robust microparticles can be easily varied by introducing various payloads, such as magnetic nanoparticles and/or fluorescent dyes, into the chitosan solution. The on‐chip connection of such individual particles into well‐defined microchains is demonstrated using GA again as the chemical “glue” and microchannel confinement as the spatial template. Chain flexibility can be tuned by adjusting the crosslinking conditions: both rigid chains and semiflexible chains are created. Additionally, the arrangement of particles within a chain can also be controlled, for example, to generate chains with alternating fluorescent and nonfluorescent microparticles. Such microassembled chains could find applications as microfluidic mixers, delivery vehicles, microscale sensors, or miniature biomimetic robots.     

Pore‐Scale Flow Characterization of Polymer Solutions in Microfluidic Porous Media

Polymer solutions are frequently used in enhanced oil recovery and groundwater remediation to improve the recovery of trapped nonaqueous fluids. However, applications are limited by an incomplete understanding of the flow in porous media. The tortuous pore structure imposes both shear and extension, which elongates polymers; moreover, the flow is often at large Weissenberg numbers, Wi, at which polymer elasticity in turn strongly alters the flow. This dynamic elongation can even produce flow instabilities with strong spatial and temporal fluctuations despite the low Reynolds number, Re. Unfortunately, macroscopic approaches are limited in their ability to characterize the pore‐scale flow. Thus, understanding how polymer conformations, flow dynamics, and pore geometry together determine these nontrivial flow patterns and impact macroscopic transport remains an outstanding challenge. This review describes how microfluidic tools can shed light on the physics underlying the flow of polymer solutions in porous media at high Wi and low Re. Specifically, microfluidic studies elucidate how steady and unsteady flow behavior depends on pore geometry and solution properties, and how polymer‐induced effects impact nonaqueous fluid recovery. This work thus provides new insights for polymer dynamics, non‐Newtonian fluid mechanics, and applications such as enhanced oil recovery and groundwater remediation.     

Computational Analysis of the Oscillatory Mixed Convection Flow along a Horizontal Circular Cylinder in Thermally Stratified Medium

The present work emphasizes the significance of oscillatory mixed convection stratified fluid and heat transfer characteristics at different stations of non-conducting horizontally circular cylinder in the presence of thermally stratified medium. To remove the difficulties in illustrating the coupled PDE’s, the finite-difference scheme with efficient primitive-variable formulation is proposed to transform dimensionless equations. The numerical simulations of coupled non-dimensional equations are computed in terms velocity of fluid, temperature and magnetic field which are computed to examine the fluctuating components of skin friction, heat transfer and current density for various emerging parameters. The governing parameters namely, thermally stratification parameter     

Cytokine Secreting Microparticles Engineer the Fate and the Effector Functions of T‐Cells

T‐cell immunotherapy is a promising approach for cancer, infection, and autoimmune diseases. However, significant challenges hamper its therapeutic potential, including insufficient activation, delivery, and clonal expansion of T‐cells into the tumor environment. To facilitate T‐cell activation and differentiation in vitro, core–shell microparticles are developed for sustained delivery of cytokines. These particles are enriched by heparin to enable a steady release of interleukin‐2 (IL‐2), the major T‐cell growth factor, over 10+ d. The controlled delivery of cytokines is used to steer lineage specification of cultured T‐cells. This approach enables differentiation of T‐cells into central memory and effector memory subsets. It is shown that the sustained release of stromal cell‐derived factor 1α could accelerate T‐cell migration. It is demonstrated that CD4+ T‐cells could be induced to high concentrations of regulatory T‐cells through controlled release of IL‐2 and transforming growth factor beta. It is found that CD8+ T‐cells that received IL‐2 from microparticles are more likely to gain effector functions as compared with traditional administration of IL‐2. Culture of T‐cells within 3D scaffolds that contain IL‐2‐secreting microparticles enhances proliferation as compared with traditional, 2D approaches. This yield a new method to control the fate of T‐cells and ultimately to new strategies for immune therapy.     

Particle Transport and Deposition in a Duct Flow with a Rectangular Obstruction

The airflow field and particle trajectory and deposition in a duct with a rectangular obstruction were studied. The governing conservation equations of mass and momentum were discretized using a finite volume method, and the corresponding velocity vector and pressure fields were evaluated. The particle trajectories were evaluated by solving the Lagrangian equation of motion that included the drag, Saffman's lift, and gravity forces. Effects of different forces as well as the blockage and the obstruction aspect ratios on particle trajectory and deposition were analyzed for a Reynolds number of 200. The simulation results showed that with the increase of Stokes number, particle deposition efficiency on the front side of the obstruction increased and also the presence of the gravitational force in the span-wise direction caused the particles to be deposited on the channel lower wall. The presence of gravity in the stream-wise direction increased the deposition efficiency and in the counter-stream-wise direction decreased the deposition efficiency. Changing the obstruction aspect ratio had no noticeable effect on the deposition but increasing the blockage ratio increased the deposition efficiency. The presence of a lift force had different effects for different blockage ratios and Stokes numbers. But the lift force generally increased the deposition rate, especially at large Stokes numbers and large blockage ratios.     

Efficient One‐Step Production of Microencapsulated Hepatocyte Spheroids with Enhanced Functions

Hepatocyte spheroids microencapsulated in hydrogels can contribute to liver research in various capacities. The conventional approach of microencapsulating spheroids produces a variable number of spheroids per microgel and requires an extra step of spheroid loading into the gel. Here, a microfluidics technology bypassing the step of spheroid loading and controlling the spheroid characteristics is reported. Double‐emulsion droplets are used to generate microencapsulated homotypic or heterotypic hepatocyte spheroids (all as single spheroids <200 μm in diameter) with enhanced functions in 4 h. The composition of the microgel is tunable as demonstrated by improved hepatocyte functions during 24 d culture (albumin secretion, urea secretion, and cytochrome P450 activity) when alginate‐collagen composite hydrogel is used instead of alginate. Hepatocyte spheroids in alginate‐collagen also perform better than hepatocytes cultured in collagen‐sandwich configuration. Moreover, hepatocyte functions are significantly enhanced when hepatocytes and endothelial progenitor cells (used as a novel supporting cell source) are co‐cultured to form composite spheroids at an optimal ratio of 5:1, which could be further boosted when encapsulated in alginate‐collagen. This microencapsulated‐spheroid formation technology with high yield, versatility, and uniformity is envisioned to be an enabling technology for liver tissue engineering as well as biomanufacturing.     

One‐Step Fabrication of Triple‐Layered Polymeric Microparticles with Layer Localization of Drugs as a Novel Drug‐Delivery System

Particulate systems have tremendous potential to achieve controlled release and targeted delivery of drugs. However, conventional single‐layered particles have several inherent limitations, including initial burst release, the inability to provide zero‐order release, and a lack of time‐delayed or pulsatile release of therapeutic agents. Multilayered particles have the potential to overcome these disadvantages. Herein, it is shown how triple‐layered polymeric microparticles can be fabricated through a simple, economical, reliable, and versatile one‐step solvent evaporation technique. Particle morphologies and layer configurations are determined by scanning electron microscopy, polymer dissolution tests, and Raman mapping. Key fabrication parameters that affect the formation of triple‐layered polymeric microparticles comprising poly(DL ‐lactide‐co‐glycolide) (50:50), poly(L ‐lactide), and poly(ethylene‐co‐vinyl acetate) (40 wt% vinyl acetate) are discussed, along with their formation mechanisms. Layer thickness and the configurations of these microparticles are altered by changing the polymer mass ratios. Finally, it is shown that drugs can be localized in specific layers of the microparticles. This fabrication process can therefore be used to tailor microparticle designs, thus allowing such “designer” particulate drug‐delivery systems to function across a wide range of applications.