Microphotometric characterization of fluid segment populations generated in different simple microfluidic networks

The stability of fluid segments is limited by deformation stress and by coalescence events. Both factors are typical for the passage of fluid segments through microfluidic networks. Therefore, the coalescence behaviour of micro fluid segments in simple network structures in dependence of flow rate ratios was investigated and characterized by the composition of obtained segment populations. Series of segments of different size and distance were generated either in a double T- or in a triple T-arrangement. PTFE elements were used for the micro fluid network. Nearly pulsation-free fluid actuation was realized by syringe pumps. The flow conditions in the input streams of carrier liquid and injected solutions remained constant during the experiments. Segment sequences become divers by different injection, stacking and coalescence events. The resulting segment sequences were characterized by on-line micro photometry. The populations of obtained micro fluid segments during each experiment were characterized by the distribution of segment size and segment distance or segment period, respectively. Microfluidic experiments as well as the numerical simulations support the assumption that the character of segment populations is mainly determined by the flow rate ratios and by the coalescence sensitivity beside the topology of the fluidic network.     

Decentralized and adaptive control of nonlinear fluid flow networks

In this paper we solve a decentralized nonlinear control problem where actuator valves and flow rate sensors are collocated in individual branches and do not exchange information. This is in contrast to our previous paper where a centralized controller required measurements from all the branches of the network. We solve both regulation (constant references) and tracking (time varying reference signals) problems. To eliminate conservativeness in choosing the gains of the controllers, we employ adaptation. We illustrate the results with an example reminiscent of a blood flow network.     

Droplet coalescence by geometrically mediated flow in microfluidic channels

Microfluidic flow is geometrically mediated at a trifurcating junction allowing periodically formed, equally spaced out emulsion droplets to redistribute and fuse consistently. This is achieved by controlling the ratio between the droplet transport time across the trifurcating junction and the drainage time of the fluid volume separating the droplets t _r/t _d. Three different microfluidic trifurcation geometries have been designed and compared for their droplet fusion efficiencies. Fusion of up to six droplets has been observed in these devices. The fusion of two droplets occurs when t _r/t _d is equal to 1.25 and the number of fused droplets increases with t _r/t _d. When the junction length (d) is 216 μm fusion of 2–6 six droplets are possible however when the junction length is increased to 360 μm fusion of only two droplets is observed.     

Manipulation of microfluidic flow pattern by optically controlled electroosmosis

Light is used to dynamically control the pattern of electroosmotic flow in a microfluidic channel. One wall of the channel is formed by a photoconductor film in a specific geometry. Illumination of this surface results in an increase of the conductivity that modifies the structure of the electric field inside the channel. A dramatic change of the electroosmotic flow pattern can be achieved. This approach provides useful capabilities for the manipulation of fluids in microfluidic systems like directing flow and mixing.     

Measuring the force of adhesion between multiple kinesins and a microtubule using the fluid force produced by microfluidic flow

We developed a method to measure the adhesion force between the motor protein, kinesin, and a microtubule. Compared with conventional methods that use optical tweezers, our method employs the fluid force that acts on the interaction between a kinesin-coated microbead and a microtubule in a microfluidic channel. When the fluid force just exceeds the kinesin-microtubule adhesion force, the beads are released from the microtubules. Having modeled the kinesins that are bound to the microtubules and the beads as mechanical springs, adhesion forces were measured as 31.3 or 362.9 pN for fluid containing 1 mM ATP or 0 M ATP, respectively. These forces are much larger than those measured when optical tweezers were used to measure the adhesion force between a single kinesin and a microtubule. For our multi-kinesin system we elucidated the relationship between the binding force of a single kinesin molecule and that of all kinesin molecules in a contact area by varying one of two parameters: either the contact area length or the kinesin density on a bead. This study provides insight into the behavior of a bead that is supported by several kinesins in a microfluidic system, which is essential knowledge if a motor protein is to be used as a nanoactuator for in vitro molecular transport.     

Design of microfluidic channel networks with specified output flow rates using the CFD-based optimization method

We report an effective, easy-to-use, computational fluid dynamics-based optimization method for designing purely resistive microfluidic networks with desired flow rates at user-specified outlets. The detailed topology and shape of the microchannel networks are obtained by minimizing the fluidic resistance of channels under a fixed driving flow rate at the inlet. This proposed method allows flexibility in setting up the relative positions among the inlet and outlets so that the layout of channel networks can be compactly adjusted based on the specific design requirements.     

An integrated microfluidic device for characterizing chondrocyte metabolism in response to distinct levels of fluid flow stimulus

In this work, we presented a novel integrated microfluidic perfusion system to generate multiple parameter fluid flow-induced shear stresses simultaneously and investigated the effects of distinct levels of fluid flow stimulus on the responses of chondrocytes, including the changes of morphology and metabolism. Based on the electric circuit analogy, two devices were fabricated, each with four chambers to enable eight different shear stresses spanning over four orders of magnitude from 0.007 to 15.4 dyne/cm² with computational fluid dynamics analysis. Chondrocytes subjected to shear stresses (7.5 and 15.4 dyne/cm²) for 24 h reoriented their cytoskeleton to align with the direction of flow. Meanwhile, the collagen I, collagen II and aggrecan expression of chondrocytes increased in different ranges, respectively. Furthermore, interleukin-6 as a proinflammatory cytokine can be detected at shear stress of 7.5 and 15.4 dyne/cm² in mRNA level. These results indicated that fluid flow was beneficial for chondrocyte metabolism at interstitial levels (0.007 and 0.046 dyne/cm²), but induced an increase in fibrocartilage phenotype with increasing magnitude of stimulation. Moreover, a moderate level of flow stimulus (7.5 dyne/cm²) could also result in detrimental cytokine release. This work described a simple and versatile way to rapidly screen cell responses to fluid flow stimulus from interstitial shear stress level to pathological level, providing multi-condition fluid flow-induced microenvironment in vitro for understanding deeply chondrocyte metabolism, cartilage reconstruction and osteoarthritis etiology.     

Fabrication of microfluidic networks with integrated electrodes

In this paper a method is presented for the fabrication of micro-channel networks in glass with integrated and insulated gate electrodes to control the zeta-potential at the insulator surface and therewith the electro-osmotic flow (EOF). The fabrication of the electrodes is a sequence of photolithography, etching and thin film deposition steps on a glass substrate, followed by chemical mechanical polishing (CMP) and subsequently direct thermal bonding to a second glass plate to form closed micro-channels. Plasma enhanced chemical vapor deposition (PECVD) SiO₂-layers as insulating material between the electrodes and micro-channels and different electrode materials are examined with respect to a high bonding temperature to obtain an optimal insulating result. A CMP process for the reduction of the SiO₂ topography and roughness is studied and optimized in order to obtain a surface that is smooth enough to be directly bondable to a second glass plate.     

Reproducing chaos by variable structure recurrent neural networks

In this paper, we present a new approach for chaos reproduction using variable structure recurrent neural networks (VSRNN). A neural network identifier is designed, with a variable structure that will change according to its output performance as compared to the given orbits of an unknown chaotic systems. A tradeoff between identification errors and computational complexity is discussed.     

Transient flow and heat transfer mechanism for Williamson-nanomaterials caused by a stretching cylinder with variable thermal conductivity

The utilization of nanometre-sized solid particles in working fluids has been seriously recommended due to their enhanced thermal characteristics. This suspension of solid particles in base fluids can significantly enhance the physical properties, such as, viscosity and thermal conductivity. They are widely used in several engineering processes, like, heat exchangers, cooling of electronic equipment, etc. In this exploration, we attempt to deliver a numerical study to simulate the nanofluids flow past a circular cylinder with convective heat transfer in the framework of Buongiorno’s model. A non-Newtonian Williamson rheological model is used to describe the behavior of nanofluid with variable properties (i.e., temperature dependent thermal conductivity). The leading flow equations for nanofluid transport are mathematical modelled with the assistance of Boussinesq approximation. Numerical simulation for the system of leading non-linear differential equations has been performed by employing versatile, extensively validated, Runge–Kutta Fehlberg scheme with Cash–Karp coefficients. Impacts of active physical parameters on fluid velocity, temperature and nanoparticle concentration is studied and displayed graphically. It is worth to mention that the temperature of non-Newtonian nanofluids is significantly enhanced by higher variable thermal conductivity parameter. One major outcome of this study is that the nanoparticle concentration is raised considerably by an increasing values of thermophoresis parameter.

     

Engineering microfluidic papers: effect of fiber source and paper sheet properties on capillary-driven fluid flow

In the present study, we introduce a novel approach to control and modulate fluid transport inside microfluidic papers using lab-engineered paper sheets. Lab-sheets consisting of different fiber sources (eucalyptus sulfate and cotton linters pulp) and varying porosities were designed and further modified with small millimeter-scaled channels using hydrophobic barriers consisting of fiber-attached, hydrophobic polymers. The capillary-driven transport of an aqueous solution was monitored visually, and the influence of parameters such as fiber source, paper grammage, and channel width on the flow rates through the channel was investigated. The experimental results were compared with those obtained with commercially available filter papers. Our findings suggest that accurate control of fluid transport processes with standard filter papers is complex. Additionally, if the channel width is smaller than the mean fiber length, flow rates become dependent on the geometric parameters of the channel because of the formation of dead-end pores at the hydrophobic barriers. Finally, control of the paper sheets porosity, by varying the fiber density of the lab-made paper, affords the fabrication of chemically identical sheets whereby capillary flow is largely influenced and can be modulated accordingly by simple papermaking processes.     

网络流驱动的复杂输运网络演化模型

传统复杂网络演化模型在网络拓扑结构与边权演化的机制设计中,未考虑网络流对于输运网络演化的驱动作用。引入网络流的动态驱动机制,分析网络流的规模增长、空间距离的制约与最短路径的配流策略三种因素,在输运网络的演化过程中所发挥的作用。发现这三种因素并不足以改变复杂网络的无标度性;基于最短路径的配流机制是网络流分布不均的关键影响因素;空间距离抑制作用对于网络相配性具有关键影响作用。     

Intelligent material flow by decentralizedcontrol networks

This contribution presents a modular system for decentralized control of track-bound plant-floor material flow. The first main element of the system is a transport network of intelligent track and transport modules that autonomously execute transport orders. There is only a limited number of different standard module types, initially not bound to a specific topology. All modules can independently perform local operations and, in order to coordinate their actions, communicate throughout the network via an integrated data bus. The second pillar is a PC-based graphical user interface for automatically configuring modules with topology data, for passing transport orders to transport modules, and for optionally supervising all concurrent processes.     

Investigations on the flow behaviour in microfluidic device due to surface roughness: a computational fluid dynamics simulation

In the field of micro-fluidics device, as the cross section of micro-channel comes down to the scale of few tens of micro-meters, surface area to volume ratio increases significantly, and due to this, surface dependent phenomenon dominates during flow of the fluid. This surface dependent phenomenon is mainly governed by surface roughness as an important parameter which directly influences on flow and results in the loss of pressure head due to the building of localised pressure as well as eddy flow. To understand this mechanism, a computational fluid dynamics (CFD) simulation is carried out. In the present CFD simulation, fluid and solid interactions are modelled in two different types. The first is modelled as pure slip between them so that the effect of roughness can be investigated as a main source of friction factor. The second model consists the effect of the pure adhesion by maintain zero relative velocity on the surface of micro-channel. Behaviour of fluid flow and increase in pressure-drop are observed differently in the both types of model. It is observed that the rise in pressure-drop occurs exponentially as size of a channel reduces from 300 to 100 µm. This phenomenon reveals the science of the size effect on micro-channels. The surface roughness of micro-channel is simulated and it is also observed that the surface finish up to few tens of nanometers does not affect the fluid flow. However, the flow resistance increases as the surface roughness increases up to few hundreds of nanometers, and the pressure-drops along the channel length. In the present case, an elevated temperature of fluid mitigates the effect of surface roughness up to some extent for the efficient flow of fluid in a micro-fluidic device. Hence, micro-fluidic device with nano-finished micro-channel and elevated temperature of fluid is recommended for economic and efficient utilisation of the device.

     

On the influence of start-up costs in scheduling divisible loads onbus networks

Optimal distribution of divisible loads in bus networks is considered in this paper. The problem of minimizing the processing time is investigated by including all the overhead components that could penalize the performance of the system, in addition to the inherent communication and computation delays. These overheads are considered to be constant additive factors to the respective communication and computation components. Closed-form solution for the processing time is derived and the influence of overheads on the optimal processing time is analyzed. We derive a necessary and sufficient condition for the existence of the optimal processing time. We then study the effect of changing the load distribution sequence on the time performance. Through rigorous analysis, an optimal sequence to distribute the load among the processors is identified, whenever it exists. In case such an optimal sequence fails to exist, we present a greedy algorithm to obtain a suboptimal sequence based on some important properties of the overhead factors. Then, the effect of granularity of the data that is divisible is considered in the analysis for the case of homogeneous networks. An integer approximation algorithm capable of generating integer values of the load fractions in time O(m), where m is the number of processors in the network, is proposed. We then show that the upper bound on the suboptimal solution generated by our algorithm lies within a radius given by the sum of the computation and communication delays. Several numerical examples are presented to illustrate the concepts     

Binary ABR flow control over ATM networks with uncertainty using discrete-time variable structure controller

A binary available bit rate (ABR) scheme based on discrete-time variable structure control (DVSC) theory is proposed to solve the problem of asynchronous transfer mode (ATM) networks congestion in this paper. A discretetime system model with uncertainty is introduced to depict the time-varying ATM networks. Based on the system model, an asymptotically stable sliding surface is designed by linear matrix inequality (LMI). In addition, a novel discrete-time reaching law that can obviously reduce chatter is also put forward. The proposed discrete-time variable structure controller can effectively constrain the oscillation of allowed cell rate (ACR) and the queue length in a router. Moreover, the controller is self-adaptive against the uncertainty in the system. Simulations are done in different scenarios. The results demonstrate that the controller has better stability and robustness than the traditional binary flow controller, so it is good for adequately exerting the simplicity of binary flow control mechanisms.     

Binary ABR flow control over ATM networks with uncertainty using discrete-time variable structure controller

A binary available bit rate （ABR） scheme based on discrete-time variable structure control （DVSC） theory is proposed to solve the problem of asynchronous transfer mode （ATM） networks congestion in this paper. A discrete-time system model with uncertainty is introduced to depict the time-varying ATM networks. Based on the system model, an asymptotically stable sliding surface is designed by linear matrix inequality （LMI）. In addition, a novel discrete-time reaching law that can obviously reduce chatter is also put forward. The proposed discrete-time variable structure controller can effectively constrain the oscillation of allowed cell rate （ACR） and the queue length in a router. Moreover, the controller is self-adaptive against the uncertainty in the system. Simulations are done in different scenarios. The results demonstrate that the controller has better stability and robustness than the traditional binary flow controller, so it is good for adequately exerting the simplicity of binary flow control mechanisms.     

Human breast cancer cell enrichment by Dean flow driven microfluidic channels

Microsystem Technologies - Cell separation based on size by microfluidic devices has become a widely studied research area to facilitate the diagnosis of malaria and cancer, in particular....     

Transient flow behavior of complex fluids in microfluidic channels

The ongoing development of microfluidic devices involves the use of highly complex fluids, even of multiphase systems. Despite the great achievements in the development of numerous applications, there is still a lack in the complete understanding of the underlying physics of the observed macroscopic effects. One prominent example is the flow through benchmark contractions where micro- and even macroscopic explanations of some of the occurring flow patterns are still missing. Here, we study the development of the flow profiles of shear thinning semi-dilute polymer solutions in microfluidic planar abrupt contraction geometries. Flow profiles along the narrow channel part are obtained by μ-PIV measurements, whereby the pressure drop along the microfluidic channel as well as the local transient viscosities downstream to the orifice are computed. A relaxation process of the flow profiles from an initially parabolic shape to the flattened steady-state flow profile is observed and traced back to the polymer relaxation.     

Analytical and numerical solutions of the unsteady 2D flow of MHD fractional Maxwell fluid induced by variable pressure gradient

The nonlocal property of the fractional derivative can supply more precise mathematical models for depicting flow dynamics of complex fluid which cannot be modelled appropriately by normal integer order differential equations. This paper studies the analytical and numerical methods of unsteady 2D flow of Magnetohydrodynamic (MHD) fractional Maxwell fluid in a rectangular pipe driven by variable pressure gradient. The governing equation is formulated with Caputo time dependent fractional derivatives whose orders are distributed in interval (0, 2). A challenge is to firstly obtain the exact solution by combining modified separation of variables method with Mikusiński-type operational calculus. Meanwhile, the numerical solution is also obtained by the implicit finite difference method whose validity has been confirmed by the comparison with the exact solution constructed. Different to the most classical works, both the stability and convergence analysis of two-dimensional multi-term time fractional momentum equation are derived. Based on numerical analysis, the results show that the velocity increases with the rise of the fractional parameter and relaxation time. While an increase in the values of Hartmann number leads to a slower velocity in the rectangular pipe.