Search

The flow pattern and mixing performance in different geometries of microchannels are investigated. A study for the zigzag shape channel under both stationary and rotation condition was investigated. The results are compared to the straight channel. Both numerical model via CFD and experiments are agreeing with each other. It is found when the channel is under rotation, the mixing performance is better than the stationary condition due to rotation induced Coriolis effect. As the rotation speed increases from 300rpm to 900rpm, an improved mixing performance can be further obtained. Our study results also demonstrate that zigzag channel can provide relatively better mixing, because the successive bending design can lead to secondary flow in the cross sectional directions due to Gortler vortices generated from centrifugal effect as the flow negotiates the “turns” or “bends” of the channel, and this adds to mixing in addition to that due to aforementioned Coriolis effect. An analysis reveals that zigzag shape channel requires more pressure drop when compared with the straight channel in order to maintain the continuous throughflow due to the pressure/head loss as the flow encounters each bend and these losses become additive. This adds to the cost for the improved mixing from the Gortler vortices.Copyright © 2011 by ASME

Reduction of contaminants in polluted water can be effectively enhanced by a suitable photocatalyst that utilizes an extended light adsorption spectrum, presents a large surface-to-volume ratio to enhance contact with the contaminants, and reduces the inefficiency from recombination of electron–hole pairs. Based on these objectives, TiO 2 /ZnO nanofibers are synthesized with small diameter having 10-nm poly-crystallites of large surface area by electrospinning followed by calcination. With different concentrations of zinc acetate in the precursor solutions between 0.15%–0.6%, the diameter of the finished nanofibers ranges from 80 to 130 nm. The photocatalytic activities of nanofibers are studied systematically by the degradation of Rhodamine B dye under the 420-nm visible-light irradiation. Photocatalytical activity can be optimized by introducing an appropriate amount of Zn (0.30 wt.% ZnAc in precursor) to maximize the available hydroxyl radicals for oxidation of the dye. Also, the optimal catalyst loading has been determined to be 2 g of TiO 2 /ZnO nanofibers per liter of water treated after balancing the enhanced active sites with increasing catalyst and catalyst over-dosage that results in solution opacity and catalyst agglomeration. Therefore, enhanced photodegradation of dyes with photocatalyst nanofiber under visible irradiation can be realized. A kinetic model has been developed to describe successfully the photodegradation of RhB.

The fluid mechanics of mixing different species in a closed rotating microfluidics chamber was investigated. Complicated secondary flow, in form of vortices, in a rotating chamber was generated for mixing different species by continuous changing the rate of rotation over time until uniform mixing of the different species in the chamber has been attained. Three vortices are observed when reference to the rotating chamber — one main vortex in the R-θ planes, generated from the d’Alembert force, was responsible for momentum and mass transfer while two pairs of toroidal vortices, generated from the Coriolis force, is responsible for momentum and mass transfer in the direction parallel to the rotational axis. These secondary flow and vortices help to reduce the mixing length between species of a given concentration. With a much smaller mixing length, diffusion can further effect the remaining mixing in a more reasonable time for the mixture to attain a more uniform or homogenous condition of the species in the microfluidic chamber. Numerical simulation using ANSYS-CFX was used to solve the transient Navier-Stokes equation in the rotating frame simulating the Newtonian laminar flow pattern for linear acceleration and deceleration (in rotation) schemes. Mass transfer was also modeled using the convective-diffusion equation governing movement of the different species in the chamber after the NS and continuity equations have been solved for the velocity field. Using the numerical model, parametric study of the rotating truncated pie-shaped chamber (radius and angular span) have been carried out to investigate the effect of momentum and mass transfer. An index, the mixing quality based on concentration distribution for the whole domain, was used respectively to quantitatively evaluate the mixing performance.Copyright © 2010 by ASME

Mixing in rotational micro-chamber has been carried out both experimentally and numerically with intent of improving mixing in viscous dominated microfluidics. In experiment, an enclosed chamber made of machined PMMA cut-out with a PDMS cover was set-up on the rotational platform. Different geometries of the chamber (on the order of several mm) with constant height yet different angular span 5, 10, 15, 20 deg. and radial extents 1.5 and 3 mm were used in the tests. While the two different dyes each occupying half of the chamber took a long time (e.g. nearly 40 minutes for a 20-degree span chamber with inner and outer radii, respectively, 35 and 36.5 mm and height 0.5 mm) for mixing by molecular diffusion in the chamber, faster mixing (about 2.5 minute) can be achieved under continuous acceleration and deceleration rotation with a linear rate of 25 rad/s2. The time for mixing per unit volume (Specific Mixing Time - SMT) was determined experimentally as a function of geometry. The SMT increases with increasing vorticity strength as a result of increasing size of the rotating chamber and/or increasing the magnitude of the linear acceleration and deceleration rates. Mixing is also studied by numerical simulation of the tested geometry. Comparing with the experiments, we found good agreement for the SMT between test results and numerical model for small-angled chamber while the numerical model seems to under-estimate the SMT for large-angled chamber. Further, we have also verified qualitatively in the experiment the flow pattern of the primary vortex in the radial-azimuthal plane responsible, to a large extent, for mixing in the chamber. Two approaches have been taken to observe and confirm the primary vortex being set up by continuous acceleration-and-deceleration of the chamber. The first approach adopted color tracing with two miscible dyed liquids, and the second approach used neutrally buoyant particles to trace fluid particle flow in the rotating chamber.

Mixing of liquids in rotating micro-channel is studied as it has important applications in portable diagnostic devices in rotating lab-on-a-chip. The latter utilizes rotation to pump liquid samples and reagents, and to actuate mixing between different liquids during transport in the channel using secondary flow generated from rotation. Experiments have been carried out on a rotating platform wherein two liquids are directed from their respective reservoirs through small capillaries into a common channel when the centrifugal force generated from rotation exceeds the capillary force which initially traps the liquid in their respective reservoirs. As two liquids are combined to flow through a common channel directed radially outward, Coriolis acceleration which is proportional to the rotation speed and relative radial velocity of the fluids, induces a tangential acceleration and thus velocity component that directs opposite to the direction of rotation. As this cross-flow, predominantly with maximum effect at the center of the channel, intercepts the far wall (i.e. pressure face) it splits up into two streams returning the flow back to the leading (in direction of rotation) face of the channel. This produces two main vortices/circulations for the channel and smaller vortices at the four corners for a channel with a rectangular cross-section. These vortices specifically the two main vortices are responsible for mixing of momentum, mass and energy in the cross-flow direction as the throughflow moves radially outward along the channel from small to a large radius. In the reference frame of the rotating channel, the combination of throughflow and crossflow results in two cork screw-shaped helical flow pattern streaming down the rotating channel. Connected with a microscope to view in details of the mixing in a small area, a rotating camera, mounted to the rotating platform, is used to take video of the mixing at various locations along a radial channel 30-mm in length and 1 mm width. This test is repeated for other observation locations along the channel. All the tests under identical rotation speed are grouped together to provide a full record of the mixing in the entire channel. These results can also be examined as mixing under same rotation speed but different channel lengths. Obviously, increasing length enhances mixing especially at small channel lengths under 5–10 mm. However, between 10–30 mm downstream mixing is diminished. It was also found that higher rotation speed can indeed produces better mixing with a modest increase.Copyright © 2010 by ASME

The acceleration of a continuous feed liquid stream in a film “down” the rotating cone and disk is of great interest in centrifuges [1, 2], thin-film reactors and process intensifiers. The mechanism of feed acceleration is determined by an interaction of several different effects. Circumferential viscous forces act to increase the angular momentum. The centrifugal field thus produced establishes a body-force component along the cone/disk surface, thereby driving the flow “down” toward larger radius. The longitudinal flow is however impeded by longitudinal resistance forces. These different effects compete with each other as the flow proceeds, never quite coming to an unchanging equilibrium state. An approximate integral method which was used to explore the “near-equilibrium” flow behavior in earlier work has been extended to investigate the case with large departure from equilibrium. The latter exhibits complicated highly nonlinear effect. Despite this, useful information can be obtained from the theoretical analysis. Experimental results on feed acceleration of liquid streams at various feed rates and rotation speeds in a rotating cone have been used to validate the study. The theoretical study with complementary experimental tests provides insights into how continuous liquid stream in form of a thin film is being accelerated using rotating cones and disks, and the associated shear rates involved. The latter has important bearing in processing shear-sensitive mammalian cells in biopharmaceutical separation with centrifuges and mass transfer in thin-film reactors.Copyright © 2008 by ASME

The objective is to develop improved microvalve and micropump designs to simplify fabrication process, decrease manufacturing cost and increase efficiency. In this paper, a new prototype model of a micropump using piezoelectric (PZT) disc as the actuator is designed. This paper describes the structure and the working principle of new micropump using PZT disc as an actuator. The main basic structure includes a pump body with valveless structure and piezoelectric disc. The overall size of this prototype is the same as the diameter of a PZT disc about 4cm × 5cm. The working principle relies on the vibration of a membrane by applying a.c. voltage to the PZT actuator. The fabrication process is simplified, comparing with traditional manufacturing method and save time and cost. While fabrication and design are discussed herein, experimental results and pump performance will be reported separately. The developed micropump is able to produce a micro-flow rate suitable for medical applications such as drug delivery system.Copyright © 2008 by ASME

A new micro-electromechanical system (MEMS) gas sensor has been developed using quartz crystal microbalance (QCM) with adsorbent coated in form of nanofibers on the QCM sensor. The nanofibers with fiber diameter typically around 200–300 nm increases the specific surface area to enhance adsorption. The QCM is made to oscillate at its natural resonance frequency. Upon exposure of the gas sensor to a given gas, the adsorbed gas onto the nanofibers adds a small mass which changes the natural frequency of the oscillation. By detecting the frequency shift due to adsorption of a given gas, the presence of the gas is detected, and by measuring the frequency shift, the amount of gas being adsorbed at a given pressure and temperature is quantified via the Sauerbrey equation [1]. A circuit has been developed to read the frequency shift due to the energy dissipation in the QCM coated with Polydimethylsiloxane (PDMS) nanofibers under the environment of several solvent vapors. The developed circuit includes two crystal oscillator circuits, two QCM’s which are respectively 1MHz reference QCM and a coated QCM, RC filter and AND gates. The results of the frequency shift between the reference QCM and the coated QCM were recorded on the oscilloscope so as to investigate the relationships between the frequency shift and the amount of vapor adsorbed for different gases. Ultimately, Volatile Organic Compounds (VOCs) are the target to be monitored and a MEMS based sensor will be developed similar to the present QCM gas sensor discussed herein. This work provides the feasibility study for using nanofiber coating to enhance the adsorbent specific area and a stand-alone QCM sensor for making measurement.Copyright © 2008 by ASME