I-Da Yang

Fluid filling into micro-fabricated reservoirs

Abstract This study reports that the success of reservoir-filling strongly depends on the designs of the hydrophilic wall surface and the well shape/size of the flow network. The idea is illustrated both by experiments and numerical simulations: micro-particle-image-velocimetry (μ-PIV) system is setup to monitor the process of a liquid slug moving in and out of the micro-reservoir and numerical computations are performed by solving first principle equations to provide the details of the flow process. The cross-check between measurements and computations validate the computations. Numerical computations solve conservation equations similar to homogenous flow model used in two phase flow calculation in co-operation with volume-of-fluid (VOF) interface tracking methodology and continuum surface force (CSF) model. The simulations show that wall surface property as hydrophilic/hydrophobic is a dominating factor in filling processes of reservoirs of various shapes. A flow system consisting of micro-channels and micro-wells is fabricated using MEMS technology to demonstrate the filling process and validate numerical simulation. The agreement between measurement and computation helps to fully understand the process.

Surface tension driven and 3-D vortex enhanced rapid mixing microchamber

This paper proposes a novel passive micromixer design for mixing enhancement by forming a large three-dimensional (3-D) flow vortex in a counterflow microfluidic system. The counterflow fluids are self-driven by surface tension to perform mixing in an open chamber. The chamber design consists of two rectangular bars to house the chamber and to form two opening inlets from opposite directions. The best design is selected from various versions of mixing chambers. The mixing effectiveness is tremendously increased by folds of contacting surface between two fluids induced and enhanced due to the stretching of two fluid contacting interfaces by the formation of a 3-D large size vortex structure inside the mixing chamber itself with unaccountable numbers of fluid layers. Both numerical simulations and experiments are performed and compared to identify the design parameters for maximum utilization in this microfluidic system, such as the length of rectangular bar, microchannel wall height, and mixing chamber size. Compared to traditional micromixers operated by two-dimensional (2-D) vortex, this passive mixer can greatly enhance mixing efficiency and reduce mixing time by tenfold from around 10 s to less than 10 ms by 3-D effective chaotic flow structures in a more compact size. This mixing chamber is also suitable for an H-shape digital fluidic system for parallel mixing process in different mixing ratio simultaneously as a lab-on-a-chip system.

Bubble Dynamics for Explosive Microthermal Dual Bubbles

Bubble dynamics of explosive microthermal dual bubbles including the growth and collapse process are investigated experimentally in detail. The dual bubbles are generated with supplied heat flux of 1.61 GW/m2 on two 30 times 60 mum2 heaters. The heaters are separated by different distances ranging from 25 to 125 mum. As the heaters get closer in x-direction, higher anisotropic degree in directional extensions of dual bubbles is obtained with faster bubble expansion velocity during collapse in x-, y-, and z-directions. In addition, the bubble lifetime is shorter as the two heaters are separated more although the supplied heat flux is the same. Furthermore, one application of dual bubble system actuating the droplet formation demonstrates that the droplet formation process with no satellites can be achieved by adjusting the expansion rate of bubbles or the spacing between two heaters.

Microbubble Formation Dynamics Under High Heat Flux on Heaters with Different Aspect Ratios

Extensive studies of vapor bubble in water on microbubble growth and collapse process on microheaters with heat fluxes of 1.2, 1.4, and 1.6 GW/m 2 are performed. The 4 μs heat pulse is applied to 30 × 30 μ m 2 , 30 × 60 μ m 2 , and 20 × 60 μ m 2 heaters for undergone the nucleate boiling. Sequential images during the micro bubble growth process are recorded by the microscopic flow visualization method coupled with phase-averaged technique. The analysis of the top view images show that the increase of aspect ratio of rectangular shape heater changes the bubble shape into ellipsoid with larger ratio of axes, slower rates of bubble growth and collapse, longer bubble lifetime, later time to reach maximum size but play small roles on incipience nucleation, and vapor formation temperature. While the change of high heat flux shifts the incipience time at nucleation temperature, time to vapor sheet formation, time to reach maximum size and thus the bubble lifetime. With side view of bubble images, not only histories of extension of bubble in height but also histories of the contact angle can be provided. Therefore, the correlations of contact angle/surface tension force and boundary moving speed of bubble boundary as well as the determination of major driving mechanism in bubble growth/collapse process can be identified.

Inkjet Printhead Arrays with No Separating Wall Between Bubbles

Present work proposes an innovative design of microbubble generator with no separating wall between bubbles, i.e. no manifold and no chamber, so that faster refilling rate and higher resolution can be achieved. This design concept is illustrated by operating three micro heaters in parallel with different delay times of the central heater. The bubble images and induced flowfield in growth/collapsed processes are recorded by CCD camera and measured by micro particle image velocimetry (PIV). In addition, the measured flowfields are compared with numerical simulations based on conservation equations of mass and momentum. This study proposes the use of delay time between adjacent bubbles to mimic the chamber barrier and build pressure of different strength correlating to delay time.

Energy cascading by triple-bubble interactions via time-delayed control

The triple-bubble interaction controlled by a precise time-delayed technique was investigated in detail with respect to different ignition times, heater spaces and sequential firing modes to promote efficient energy cascading and concentration. The target bubble, which was generated under a specific delay time with two auxiliary bubbles, can have a volume that is two or almost three times larger than that of a single bubble. This result overcomes the limitation of energy usage on an explosive microbubble under a constant heat flux. As the heater space decreases, stronger bubble–bubble interactions were obtained due to the hydrodynamic effect and the intensive pressure wave emission, resulting in highly enhancing and depressing bubble dynamics. Other interesting phenomena, such as bubble shifting, mushroom-shape bubble, rod-shape bubble and bubble extension among heaters, were also recorded by a high-speed phase-averaged stroboscopic technique, displaying special non-spherical bubble dynamics. Artificial manipulation of bubble behavior was further conducted in a two-level sequential firing process. Using various volumetric combinations, the adjustable multi-level fluid transportation can be realized by a digital time-delayed control. The above-mentioned information can be applied to not only the design and operation of inkjet printheads but also cavitation research and fluid pumping in microdevices.

A 2D chamber-free micro droplet generator array controlled by dynamic virtual walls

A chamber-free two-dimensional-array micro droplet generator has been realized by precise time-delayed control of micro bubble arrays as virtual chamber walls. Droplets can be ejected out by the bubbles around the ejection site in specific configuration of excitation, thus replacing physical chamber walls for pressure preservation. The volume, initial speed, and frequency response of the generated droplets was 3.6∼3.8 pL, 15 m/s, and 20 kHz, respectively, meeting the standard of commercial printheads. The micro droplet generator is free of satellite droplets due to the precise control of meniscus that undergoes a “push-pull-push” progress which effectively cuts the liquid column short.

Nano/micro lipid monodispersed droplets formation based on flow instability

In this paper, we report a novel method to generate sub-micron size droplet. By suitably designing the micro-channel device and controlling the fluids, we can induce the occurs of Kelvin-Helmholtz instability to make a lot of droplet forming with a sub-micron diameter. The size is much smaller than droplet generated by most physical or mechanical method. Further, we observe the process of sub-micron droplet formation with high speed camera and high magnification optical microscope.

Growth/collapse and the induced flow fields for explosive micro dual bubbles

This paper investigates the growth/collapse process and the induced flow fields for explosive microthermal dual bubbles by experimental and numerical approaches quantitatively. The study reveals the decrease of pressure and velocity induced with decreasing spacing between bubbles and thus influence of the actuated momentum.

Shape Effect on Fluid Filling for Microfabricated Reservoir

This study demonstrates the shape effect on filling process of protein solution/water into micro-wells for microfabricated microchips. Two approaches are employed (1) high speed camera system is set up to record the liquid slug moving in and out of the micro-well and (2) numerical simulations by solving first principle equations provide the details of the flow process.