Finite element analysis of the lift on a slightly deformable and freely rotating and translating cylinder in two-dimensional channel flow

Analysis of Hydrodynamic Mechanism on Particles Focusing in Micro-Channel Flows

In this paper, the hydrodynamic mechanism of moving particles in laminar micro-channel flows was numerically investigated. A hydrodynamic criterion was proposed to determine whether particles in channel flows can form a focusing pattern or not. A simple formula was derived to demonstrate how the focusing position varies with Reynolds number and particle size. Based on this proposed criterion, a possible hydrodynamic mechanism was discussed as to why the particles would not be focused if their sizes were too small or the channel Reynolds number was too low. The Re-λ curve (Re, λ respectively represents the channel-based Reynolds number and the particle’s diameter scaled by the channel) was obtained using the data fitting with a least square method so as to obtain a parameter range of the focusing pattern. In addition, the importance of the particle rotation to the numerical modeling for the focusing of particles was discussed in view of the hydrodynamics. This research is expected to deepen the understanding of the particle transport phenomena in bounded flow, either in micro or macro fluidic scope.

Inertial microfluidics

Despite the common wisdom that inertia does not contribute to microfluidic phenomena, recent work has shown a variety of useful effects that depend on fluid inertia for applications in enhanced mixing, particle separation, and bioparticle focusing. Due to the robust, fault-tolerant physical effects employed and high rates of operation, inertial microfluidic systems are poised to have a critical impact on high-throughput separation applications in environmental cleanup and physiological fluids processing, as well as bioparticle focusing applications in clinical diagnostics. In this review I will discuss the recent accelerated progress in developing prototype inertial microfluidic systems for a variety of applications and attempt to clarify the fundamental fluid dynamic effects that are being exploited. Finally, since this a nascent area of research, I will suggest some future promising directions exploiting fluid inertia on the microscale.

A microfluidic device for continuous, real time blood plasma separation

A microfluidic device for continuous, real time blood plasma separation is introduced. The principle of the blood plasma separation from blood cells is supported by the Zweifach-Fung effect and was experimentally demonstrated using simple microchannels. The blood plasma separation device is composed of a blood inlet, a bifurcating region which leads to a purified plasma outlet, and a concentrated blood cell outlet. It was designed to separate blood plasma from an initial blood sample of up to 45% inlet hematocrit (volume percentage of cells). The microfluidic network was designed using an analogous electrical circuit, as well as analytical and numerical studies. The functionality of this device was demonstrated using defibrinated sheep blood. During 30 minutes of continuous blood infusion through the device, all the erythrocytes (red blood cells) traveled through the device toward the concentrated blood outlet while only the plasma was separated at the bifurcating regions and flowed towards the plasma outlet. The device has been operated continuously without any clogging or hemolysis of cells. The experimentally determined plasma selectivity with respect to blood hematocrit level was almost 100% regardless of the inlet hematocrit. The total plasma separation volume percent varied from 15% to 25% with increasing inlet hematocrit. Due to the device's simple structure and control mechanism, this microdevice is expected to be used for highly efficient continuous, real time cell-free blood plasma separation from blood samples for use in lab on a chip applications.

Bacteria sorting by field-flow fractionation. Application to whole-cell Escherichia coil vaccine strains

Sorting and quantification of deactivated bacteria is an important way of quality control for whole-cell bacterial vaccines. In general, surface features of deactivated bacteria used for whole-cell bacterial vaccines affect the immunoresponse to bacteria-associated antigens. Enumeration of bacteria is also an important process development parameter for these vaccines. Field-flow fractionation (FFF) was previously applied to the separation of bacteria. For the first time, FFF is used for sorting bacteria strains of the same species on the basis of differences in bacterial membrane characteristics. Two FFF techniques, gravitational FFF (GrFFF) and asymmetrical flow FFF (AsFIFFF), are shown to be able to fractionate, distinguish, and quantify different deactivated Escherichia coli strains used for vaccines. E. coli can differ in the presence of fimbriae on the bacterial membrane. Fimbriae affect E. coli pathology and thus the use of E. coli for vaccines. GrFFF and AsFIFFF are able to fractionate fimbriated/ nonfimbriated cells in mixtures of different strains. While GrFFF is characterized by low cost and simplicity, As-FIFFF shows a higher performance in size fractionation with a high-speed separation. Coupled, on-line UV/visible turbidimetry yields the relative numbers of fractionated cells and sample recovery. Scanning electron microscopy and quasi-elastic light scattering are employed as uncorrelated techniques for size and morphology analysis of the E. coli strains.

Stability of elliptical cylinders in two-dimensional channel flow

The flow around rigid cylinders of elliptical cross section placed transverse to Poiseuille flow between parallel plates was simulated to investigate issues related to the tumbling of red blood cells and other particles of moderate aspect ratio in the similar flow in a Field Flow Fractionation (FFF) channel. The torque and transverse force on the cylinder were calculated with the cylinder freely translating, but prevented from rotating, in the flow. The aspect ratios (long axis to short axis) of the elliptical cylinders were 2, 3, 4, and 5. The cylinder was placed transversely at locations of y0/H = 0.1, 0.2, 0.3, and 0.4, where y0 is the distance from the bottom of the channel and H is the height of the channel, and the orientation of the cylinder was varied from 0 to 10 deg with respect to the axis of the channel for a channel Reynolds number of 20. The results showed that equilibrium orientations (indicated by a zero net torque on the cylinder) were possible for high-aspect-ratio cylinders at transverse locations y0/H < 0.2. Otherwise, the net torque on the cylinder was positive, indicating that the cylinder would rotate. For cylinders with a stable orientation, however, a transverse lift forced existed up to about y0/H = 0.25. Thus, a cylinder of neutral or low buoyancy might be lifted with a stable orientation from an initial position near the wall until it reached y0/H < 0.2, whereupon it would begin to tumble or oscillate. The dependence of lift and torque on cylinder orientation suggested that neutral or low-buoyancy cylinders may oscillate in both transverse location and angular velocity. Cylinders more dense than the carrier fluid could be in equilibrium both in terms of orientation and transverse location if their sedimentation force matched their lift force for a location y0/H < 0.2.