Yan Pang

Concentration gradient generation methods based on microfluidic systems

Various concentration gradient generation methods based on microfluidic systems are summarized in this paper. The review covers typical structural characteristics, gradient generation mechanisms, theoretical calculation formulas, applicable scopes, and advantages and disadvantages of these approaches in detail. According to the type of reagents involved, these methods are classified into mono-phase methods and multi-phase methods, both of which can be implemented by alternative protocols, while the latter methods particularly refer to droplet-based platforms. For mono-phase methods, the shearing effect would be presented if there are flowing streams in the gradient generation channel. Therefore, the generation speed of channels with moving liquids is relatively fast, which is suitable for dynamic gradients but accompanied by shearing as well, while channels without flowing streams would avoid shearing but are prone to static gradient generation determined by the low speed. Newly developed droplet-based generation systems could provide isolated droplets to avoid the disturbances from the outside continuous phase, however, they require precise droplet generation and control modules. Thereby the most suitable platform can be chosen according to the specific application, while the advantages of different methods could be combined to evade the defects and improve the precision of a single structure.

Trapping a moving droplet train by bubble guidance in microfluidic networks

Trapping a train of moving droplets into preset positions within a microfluidic device facilitates the long-term observation of biochemical reactions inside the droplets. In this paper, a new bubble-guided trapping method, which can remarkably improve the limited narrow two-phase flow rate range of uniform trapping, was proposed by taking advantage of the unique physical property that bubbles do not coalescence with two-phase fluids and the hydrodynamic characteristic of large flow resistance of bubbles. The flow behaviors of bubble-free and bubble-guided droplet trains were compared and analyzed under the same two-phase flow rates. The experimental results show that the droplets trapped by bubble-free guided trapping exhibit the four trapping modes of sequentially uniform trapping, non-uniform trapping induced by break-up and collision, and failed trapping due to squeezing through, and the droplets exhibit the desired uniform trapping in a relatively small two-phase flow rate range. Compared with bubble-free guided droplets, bubble-guided droplets also show four trapping modes. However, the two-phase flow rate range in which uniform trapping occurs is increased significantly and the uniformity of the trapped droplet array is improved. This investigation is beneficial to enhance the applicability of microfluidic chips for storing droplets in a passive way.

Droplet Breakup in an Asymmetric Bifurcation Consisted of Two Branches

Extended Abstract Due to the various advantages, droplet-based microfluidics has found applications in many chemical and biochemical reactions. On most occasions, the size and frequency of droplets are two input parameters that determine the outcomes of the droplet-based systems, therefore, they need to be controlled precisely. Droplet breakup is one of the most effective method to tune the droplet size and frequency, and it can be divided into active and passive type. The former are achieved by additional energy fields, such as the heater and electrodes. Complex structures are usually involved and the outcomes may be disturbed. For the passive type, the breakup process depends mainly on the splitting junctions and flow conditions, and the size of daughter droplets could be reduced to very small values and the throughput of droplets is largely increased meanwhile. At the splitting junction, the droplet is stretched with the viscous shear stress of the continuous phase. In order to reduce its surface energy, the droplet would completely flow into one of the branches or split into daughter droplets, determined by the capillary number. In this paper, the passive droplet breakup in an asymmetric bifurcation with two branches is experimentally studied. Sunflower oil and deionized water are used as the continuous and dispersed phase, respectively. In order to separately tune the initial droplet size and droplet velocity, a diluting channel is introduced at the downstream of the generation unit and its influence on droplet generation is confirmed. Effects of the droplet parameters, including the initial droplet length, the droplet velocity, the interfacial tension between the dispersed phase and the continuous phase, on breakup characteristics are analyzed. The splitting ratio is largely influenced by the initial droplet length and the droplet velocity, which proves that the breakup process is an inter-dependent process between the splitting junction and droplets themselves. During droplet breakup, locations of the maximum velocity inside the droplet correspondingly change. Besides, the breakup time is dominated by the velocity of droplets, while only varied a little with the droplet length for short droplets and nearly not dependent on the interfacial tension. Strong similarity is also found in the breakup time between the droplet and the bubble.

Effects of Geometry on the Liquid Flow in Microchannel

The geometric effects on the liquid flow were investigated in microchannel based on the numerical simulation method. Effects of geometry were shown obviously on the microflow by comparing the results of different geometric characteristics covering hydraulic diameter, length and aspect ratio. Thermal boundary conditions are the same for different cases in the numerical simulation.The geometric parameters except the aspect ratio have little effect on the friction factor but all of them obviously make the Nusselt number changed.