Zhaomiao Liu

Advances in Micro-Droplets Coalescence Using Microfluidics

Abstract Recently, with the development of lab-on-a-chip researches, the use of microfluidics to precisely control the droplet behaviors in microchannel has received more and more research attention. This article firstly introduces the basic theory and mechanism of the droplets coalescence. Then, it carefully reviews the passive methods of droplets coalescence used in current researches by changing microchannel geometry and adding surfactants. The active methods to trigger droplet fusion such as applied electric field, magnetic fields, temperature gradients, surface acoustic wave and focused lasers are also introduced briefly. We also introduce the fluid dynamics of the droplets coalescence and forecast its progress at last to provide useful guidance for microfluidic devices design and wide use of the drop-based microfluidics.

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.

Effect of Pocket Geometry on the Performance of a Circular Thrust Pad Hydrostatic Bearing in Machine Tools

This article presents a three-dimensional numerical investigation and some experimental results supporting evidence on the flow structure and the pressure distribution in a circular thrust pad bearing of hydrostatic system in machine tools. The operational conditions include the feeding Reynolds number (Rein) and low sliding Reynolds number (Res). The motion of the thrust surface is assumed to be linear. The geometric parameters include pocket depth (H), pocket radius (r1), clearance height (h), inlet hole radius (rin), and pocket shapes (circular, elliptical, square, annular, and sector), which are optimized with respect to high load capability and high fluid film stiffness (K). A flow visualization experiment using a homemade pad bearing model and particle tracking method was used to visually reconstruct the flow pattern in the pocket for stationary conditions (Res = 0). Three-dimensional Navier-Stokes equations were employed to simulate the steady-state flow in hydrostatic pad bearings with incompressible Newtonian fluid using the finite volume method. The numerical results are qualitatively compared with the experimental flow patterns, finding good agreement for static cases. The numerical results show that vortexes driven by the inlet jet and the Couette effect generated by the thrust surface exist in the pocket and the flow field structure is very complicated. Flow in the pocket is nonuniform and significantly affected by the Rein and Res. The magnitude of the static pressure is much higher than the dynamic pressure and pressure distributions in the pocket are almost uniform. Pressure increases obviously as Rein increases and decreases slightly with the increase in Res. H has an important influence on flow patterns and no obvious effect on the pressure distribution. The pressure is highest with r1 = 25 mm and the fluid film stiffness is highest with r1 = 30 mm. The pressure of a pocket with rin = 3 mm is about two times that of a pocket with rin = 1.5 mm. The annular-shaped pocket has the highest maximum pressure (Pmax), whereas the circular-shaped pocket has the highest fluid film stiffness. The results will be useful for improving the load capability and fluid film stiffness of the thrust pad bearing of hydrostatic systems in machine tools with low rotating speed.

Single-particle trapping, orbiting, and rotating in a microcavity using microfluidics

With the aim of deepening the fundamental understanding of particle flow behaviors in inertial microfluidics, a mechanism of collision-triggered particle trapping in a confined rectangular microcavity (400 × 400 µm2) using microvortices is proposed, and an intriguing phenomenon that the orbit area of large particles is larger than that of small particles (diameter range of 22–38 µm) under the same flow conditions (Reynolds number = 178) is observed, which is in contrast to that indicated in previous reports. Moreover, the flow field structures of the microvortices are studied by micro-particle image velocimetry (micro-PIV), and the rotating behavior of a single particle (diameter = 40 µm) during orbiting is first measured experimentally. The results are expected to provide useful guidelines for the applications of microcavity-based microfluidics.

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.

Three-dimensional pressure- and shear-driven flow phenomena in a circular recess of a hydrostatic rotary table

The three-dimensional pressure- and shear-driven flow phenomena in a circular recess of hydrostatic rotary table in heavy-duty computer numerical control machines is very complicated and has not been fully explored. Navier–Stokes equations have been applied through the whole flow region using a finite volume approach to explore this complicated flow phenomena, including the influences of feeding Reynolds number (Rei), sliding Reynolds number (Res) and the recess geometry on flow behaviors. A test rig based on a particle image velocimetry was built to compare experimental and numerical results, finding a good agreement for stationary cases. The results show that the flow patterns in the recess are very complex and four three-dimensional vortices exist at Rei = 448 and Res = 74.6. Four flow states are defined according to the structure of the vortices. Different sectional profiles of the streamlines and velocity vector fields are examined to reveal the mechanism of pressure- and shear-driven flow interactions. The results of influences of recess geometry on flow states and pressure patterns are intended to contribute to represent a database in view of the hydrostatic rotary table theoretical modeling.

Numerical and Experimental Study of the Flow Field Structure Evolution in the Circular Recess of Oil Cavity

The laminar radial flow in the oil cavity of heavy-duty computer numerical control (CNC) machines is very complicated and has not been fully explored. Navier-Stokes equations have been applied through the whole flow region using finite volume approach to explore this complicated flow phenomenon, including the influences of the clearance height (), inlet nozzle Reynolds number (Re), and geometrical aspect ratio () on flow behaviors. A fluid dynamic experiment has been conducted to study the flow structure by using particle image velocimetry (PIV). Numerical simulation results have been compared with the experimental results, finding a good agreement with the studied cases. The results suggest that there are complex vortices in the oil cavity. Flow field structure of the oil cavity largely depends on , Re, and . Re and have a great influence on the size and amount of vortices, and has slight effects on the size of the vortices. The lengths of primary, secondary, and tertiary isolated vortices have a linear relationship with . The lengths of the primary and secondary isolated vortices increase linearly with ascending as is small. But when Re and are large enough, the size of the three vortices decreases.