Jing-Tang Yang

Geometric effects on fluid mixing in passive grooved micromixers

The effects of geometric parameters on the mixing performance of a staggered herringbone mixer (SHM) with patterned grooves are numerically investigated. Combining use of the software package CFD-ACE+ and the Taguchi method provides a powerful and systematic approach for research on microfluidics. An orthogonal array L9(3(4)) is established for parameters introduced by the groove geometry; in total 9 cases are simulated. Analyses of the mixing phenomena, geometric parameter, pressure loss and flow rate through grooves are conducted. The modes of fluid motion and dominant mechanisms of mixing within the SHM are observed and ascertained. Although the depth ratio and the asymmetry index of the groove are found to be dominant geometric parameters, the rate of flow within the groove is verified to be the most significant factor that affects the mixing performance of a SHM. To date, the effects of the parameters are evaluated within specified ranges, and the true optimum design has yet to be discovered.

Conversion of Surface Energy and Manipulation of a Single Droplet across Micropatterned Surfaces

To clarify a driving mechanism for the self-movement of a droplet across hydrophobic textured surfaces in series and to develop applications for a microfluidic device, we report a theoretical model, a microfabrication technique, and experimental measurements. The contact angle of a droplet on a composite surface, the stable surface energy level, and the energy barrier caused by hysteresis were investigated. With increasing patterned density of the microstructure, the contact angle and stable surface energy decreased gradually, but the energy barrier increased. Both the analytical results and the experimental measurements show that the surface energy for a suspended status is greater than that for a collapsed status, which produces increased energy to generate the movement of a droplet. An analysis of interactions between actuation force, resistive force, and viscous force during the motion of a droplet is based on the equilibrium between forces. From the perspective of energy conversion, the difference in surface energy between a higher state and a lower state would drive a single droplet and make it move spontaneously if it could overcome the static friction force resulting from hysteresis and the kinetic friction force under droplet movement. The mean velocity in the present device, measured to be 62.5 mm s (-1), agrees satisfactorily with the theoretical prediction. The model developed for the energy levels enables us to assess the contact mode of a droplet placed on the patterned surface. For a prediction of the transport capability of the designed devices, a theoretical interpretation of the conversion between the surface energy and the kinetic energy of the droplet establishes a criterion that the pattern density of a textured surface should be less than 0.76. The effective rate of energy conversion is estimated to be 20.6%.

Droplet manipulation on a hydrophobic textured surface with roughened patterns

A novel concept is proposed and verified, experimentally and theoretically, to manipulate droplets without external power sources. The proposed device is a hydrophobic surface containing specific roughness gradients, which is composed of several textured regions with gradually increased structural roughness. Hydrophobic materials of four types, photoresist AZ6112, Teflon, Parylene C, and plasma polymerization fluorocarbon film (PPFC)-are adopted to fabricate the textured surfaces, and are tested. Actuating forces come from the different Laplace pressures exerted on a droplet across various hydrophobic surfaces, whereas resistance forces come from the contact-angle hysteresis. Two patterns of devices are shown in this article-chain-shaped and concentric circular. The former functions as a droplet transport route and the latter provides both transport and orientation functions. Theoretical estimation and experimental verification of the droplet motion, including actuation and resistance forces, on the device are conducted. Optimal design is achieved based on accurate estimations of the acting forces. The proposed device provides a simplified fabrication process and shows superior biocompatibility for droplet manipulation in microfluidic systems.

Flame lift-off and stabilization mechanisms of nonpremixed jet flames on a bluff-body burner

A detailed regime diagram for bluff-body stabilized flames is proposed for the flame lift-off and stabilization limits. At low fuel velocities, the flame structure is classified into three stable modes: recirculation zone flames, jet-dominated flames, and jet-like flames according to the velocity ratio of annular to central jets. Two different flame stability limits can be identified between cold and combusting recirculation zones. For the former case, local flame extinction dominates lifting of the jet-like flames due to a strong interaction between the recirculating air flows and the jet flame front. A critical annulus Reynolds number is found at which the jet-like flame is least probable to lift off, whereas, for the latter case, partial quenching of the blue neck flame in jet-dominated flames is retarded due to the presence of a reignition source, the combusting recirculation zone. Thus, flame stability can be improved. It is further shown that stabilization of lifted flames is more sensitive to the co-flow air than the fuel jet velocity at the inception of flame lift-off, indicating the importance of diffusion flamelet quenching. At high fuel velocities, the annular air flows have little effect on the lift-off heights and premixed flame propagation becomes dominating. In the hysteresis region, the base of lifted flames is elevated with decreasing fuel velocities and the circular ring-shaped premixed flame in the leading front becomes more fragmented. When approaching the maximum lift-off height, the flame base consists mainly of separated, broken flamelets, suggesting an inhomogeneous fuel/air premixing, due to interaction with large-scale vortical structures. Some isolated flamelets with an arrow-headed structure, typical for a triple flame, can be observed at the flame stabilization position.

An overlapping crisscross micromixer using chaotic mixing principles

A novel PDMS-based microfluidic system incorporating an overlapping crisscross entrance with patterned groove microchannels in sequence serves as a high-performance micromixer. Such a design is characterized as involving two chaotic mixing mechanisms: the split entrance streams through the first overlapping crisscross junction are stretched and folded about the parabolic point within the patterned channel, and the reoriented streams are merged and restretched about the hyperbolic point at the next junction. The multiplicative mixing quality is thus revealed to increase 21% as installed with the staggered bas-relief structures on the walls and 29% through the second intersection of the overlapping crisscross micromixer. This microfluidic device has a fairly constant mixing index for the Reynolds number through a wide range between 0.01 and 10; it is potentially capable of being extended to achieve a key element of a lab-on-a-chip. Use of both numerical analysis and a confocal microscope elucidates the detailed mixing pattern; the results of these approaches agree convincingly.

Separated-Reattaching Flow Over a Backstep With Uniform Normal Mass Bleed (Data Bank Contribution)

The effect of normal mass bleed into the separated-reattaching flow behind a backward-facing step has been experimentally investigated. Results of LDA measurements showed that normal mass bleed suppressed the reverse horizontal velocity, the reverse flow rate, turbulence intensity, and Reynolds shear stress within the whole recirculating zone. An analysis of the distributions of vertical velocity and turbulence intensity indicates that the interaction between the injected fluid and the main stream began at 0.4 step height and became significant after 0.8 step height behind the backstep.

Investigation and application of an ultrahydrophobic hybrid-structured surface with anti-sticking character

Hybrid-structured surfaces consisting of microgrooves and nanocrystals have been modified with a self-assembled monolayer (CF3(CF)7CH2CH2SiCl3) via low-cost, mass-production and highly integrated nano/microfabrication. The microgrooves decorated with nanocrystals were patterned and fabricated on a silicon substrate to yield an ultrahydrophobic surface with an anti-sticking property. The nanocrystals were etched by means of oxidation of the silicon surface. Contours of nanostructured surfaces were inspected with a SEM and an AFM; the surface roughness and level of hydrophobicity depended on the duration of etching. Comparison of contact angles for microliter droplets on those designed surfaces showed that the hydrophobicity of the solid surfaces became amplified with nanocrystals and accurately modulated with a pattern density (f1), ranging from 112° to 173.1°, to generate a much increased gradient of Gibbs surface energy that served to transport the droplet. To characterize the anti-sticking capability of those hybrid-structured surfaces in quantity, we measured the heights and frequencies of rebounding droplets on those test surfaces with varied roughnesses. Similar to the interfacial characteristics of a lotus leaf, our designed surfaces feature superior aqueous repellence, little hysteresis and slight adhesion, such that droplets hence roll off effortlessly and bounce off repeatedly.

Mixing and separation of two-fluid flow in a micro planar serpentine channel

Numerical simulation, micro-fabrication and flow visualization were performed for a micro planar serpentine channel to reveal the mixing and separation characteristics of two fluid streams with or without density variation. When the densities of the injected fluids are equal, the induced vortices laminate the interface and considerably increase the interfacial area in a spiral manner. It compensates the negative effect of the short residence period of fast flow for mixing. When the densities of the injected fluids slightly differ, the denser fluid distributes in regions of the outer corner of turning. Separation is the effect resulting from a density difference and a velocity difference in a flow field that can be promoted on flowing in a serpentine channel with a relatively rapid bulk flow. The design of a channel incorporating an alternation of cross-sectional areas improves the velocity difference. This hypothesis is verified by both numerical simulation and experimental observation. A green dye (density 2030 kg m−3) is separated from deionized water (density 1000 kg m−3) and double interfaces are formed significantly under conditions of flow in a micro planar serpentine channel at Reynolds number 16.

Planar Liquid Confinement for Optical Centering of Dielectric Liquid Lenses

In this letter, planar liquid confinement structures along with concentric electrodes are proposed for the optical centering of dielectric liquid lenses at the rest state and during actuation. Both the liquid confinement structures and electrodes that are photolithographically fabricated on glass substrates share the same geometric center, thereby minimizing the deviation of the optical axis at all operation modes. Tilt angles of mesa liquid confinement structure are experimentally found to be 0.11deg in maximum and below 0.03deg during actuation for a liquid lens with a droplet of initially 7 mm in diameter.

A passerine spreads its tail to facilitate a rapid recovery of its body posture during hovering.

We demonstrate experimentally that a passerine exploits tail spreading to intercept the downward flow induced by its wings to facilitate the recovery of its posture. The periodic spreading of its tail by the White-eye bird exhibits a phase correlation with both wingstroke motion and body oscillation during hovering flight. During a downstroke, a White-eyes body undergoes a remarkable pitch-down motion, with the tail undergoing an upward swing. This pitch-down motion becomes appropriately suppressed at the end of the downstroke; the birds body posture then recovers gradually to its original status. Employing digital particle-image velocimetry, we show that the strong downward flow induced by downstroking the wings serves as an external jet flow impinging upon the tail, providing a depressing force on the tail to counteract the pitch-down motion of the birds body. Spreading of the tail enhances a rapid recovery of the body posture because increased forces are experienced. The maximum force experienced by a spread tail is approximately 2.6 times that of a non-spread tail.