Micro and Nano Particle Manipulation Using Optically Modulated Electrokinetic Flows

Recently, we have demonstrated an optically induced AC electrokinetic technique that rapidly, continuously and selectively concentrates colloids on an electrode surface [1–3]. This is demonstrated with a highly focused near-infrared (1,064 nm) laser beam applied to parallel plate electrodes separated by 50 μm without any additional surface modification or patterning of the electrodes. This dynamic optically-induced technique can be applied towards a variety of lab-on-a-chip applications. This paper will explain its physical mechanisms and showcase recent results regarding its particle sorting capabilities. This dynamic, optically induced fluid and particle manipulation technique could be used for a variety of lab-on-a-chip applications.

Numerical Study on Mixing of Two Fluids With Modified Tesla Structure

In this study, a parametric investigation on mixing of two fluids in a modified Tesla microchannel, has been preformed. Modified Tesla micromixer applies both flow separation and vortices string principles to enhance the mixing. The fluid stream splits into two sub-streams and one of them mixes with the other again at the exit of the Tesla unit. Analyses of mixing and flow field have been carried out for a wide range of Reynolds number from 0.05 to 40. Mixing performance and pressure drop characteristics with two geometrical parameters, i.e, ratio of the diffuser gap to channel width (h/w) and ratio of the curved gap to the channel width (s/w), have been analyzed at six different Reynolds numbers. The vortical structure of the flow has been analyzed to explain mixing performance. The sensitivity analysis reveals that mixing is more sensitive s/w, than the h/w.

A 2D Chamber-Free Micro Droplet Generator Array Controlled by Dynamic Virtual Walls

This paper presents an innovative micro droplet generator array controlled by dynamic virtual walls. The heaters for bubble generation can be arranged into a dense two-dimensional (2D) array by removing the chamber wall structures. The micro droplet generator array was fabricated by heater lithography and direct nozzle formation on a laminated SU-8 dry film without any solid chamber wall among heaters. With precise time-delayed control among microheaters, droplets could be ejected out by the thrust generated from the bubbles around the ejection site under the specific configuration. The volume of the droplet was about 3.8 pL and the initial speed can approach 15 m/s, meeting the standard of commercial printheads. In addition, we carried out the meniscus control by precise sequential control of bubble generations, so the micro droplet generator is free of satellite droplets throughout the whole traveling distance. It showed that the meniscus undergoes a “push-pull-push” progress which can effectively cut the liquid column short. To summarize, a 330 dpi chamber-free micro droplet generator prototype has been realized, and it has the potential for the application to large 2D format, high frequency, high resolution printing.

Simulation of Gas Flow Over a Micro Cylinder

Gas flow over a micro cylinder is simulated using both a compressible Navier-Stokes solver and a hybrid continuum/particle approach. The micro cylinder flow has low Reynolds number because of the small length scale and the low speed, which also indicates that the rarefied gas effect exists in the flow. A cylinder having a diameter of 20 microns is simulated under several flow conditions where the Reynolds number ranges from 2 to 50 and the Mach number varies from 0.1 to 0.8. It is found that the low Reynolds number flow can be compressible even when the Mach number is less than 0.3, and the drag coefficient of the cylinder increases when the Reynolds number decreases. The compressible effect will increase the pressure drag coefficient although the friction coefficient remains nearly unchanged. The rarefied gas effect will reduce both the friction and pressure drag coefficients, and the vortex in the flow may be shrunk or even disappear.

Aerosol Flow Through a Micro-Capillary

A combined theoretical/experimental study of micron size aerosol flows through micro-capillaries of diameter about 100 μm and length about 1 cm is presented. It is shown that under proper conditions at a relatively high velocity of about 100 m/s such an aerosol flow reveals a new manifestation of microfluidics: the Saffman force acting on aerosol particles in gas flowing through a micro-capillary becomes significant thereby causing noticeable migration of particles toward the center line of the capillary. This finding opens up new opportunities for aerosol focusing, which is in stark contrast to the classical aerodynamic focusing methodologies where only particle inertia and the Stokes force of gas-particle interaction are typically used to control particle trajectories. A mathematical model for aerosol flow through a micro-capillary accounting for complicated interactions between particles and carrier gas is presented. This model describes the experimental observables obtained via shadowgraphy for aerosol beams exiting micro-capillaries. It is further shown that it is possible to design a micro-capillary system capable of generating a Collimated Aerosol Beam (CAB) in which aerosol particles stay very close to a capillary center line. The performance of such a CAB system for direct-write fabrication on a substrate is demonstrated. The lines deposited by CAB for direct-write fabrication are shown to exhibit widths of less than 5 μm — superior to ink-jet. Materials deposition based upon directed aerosol flow has the potential of finding application in the fields of flexible electronics, sensors, and solar cells. In this paper, the genesis of a new materials deposition method termed Collimated Aerosol Beam Direct-Write (CAB-DW) is discussed.

Numerical Simulation of Droplet Generation in Series Co-Flow Capillaries

We study the production and motion of monodisperse double emulsions in microfluidics comprising series co-flow capillaries. Both two and three dimensional simulations are performed. Flow was determined by dimensionless parameters, i.e., Reynolds number and Weber number of continuous and dispersed phases. The co-flow generated droplets are sensitive to the Reynolds number and Weber number of the continuous phase, but insensitive to those of the disperse phase. Because the inner and outer drops are generate by separate co-flow processes, sizes of both inner and outer drops can be controlled by adjusting Re and We for the continuous phase. Meanwhile, the disperse phase has little effect on drop size, thus a desirable generation frequency of inner drop can be reached by merely adjusting flow rate of the inner fluid, leading to desirable number of inner drops encapsulated by the outer drop. Thus highly monodisperse double emulsions are obtained. It was found that only in dripping mode can droplet be of high mono-dispersity. Flow begins to transit from dripping regime to jetting regime when the Re number is decreased or Weber number is increased. To ensure that all the droplets are produced over a wide range of running parameters, tiny tapered tip outlet for the disperse flow should be applied. Smaller the tapered tip, wider range for Re and we can apply.

Molecular Simulation of Electrokinetic Transport in Nanofluidics

Electrokinetic transport in nanofluidics has attracted many attentions in recent years due to its potential applications in biomedical analysis and energy conversion systems. The biggest challenge to model the electrokinetic transport using atomistic simulations is the extensive cost in calculation of the long-range electrostatic force between ions. In this work, we develop an efficient molecular approach to simulate electrokinetic transport in nano-scale. The long-range Coulombic interactions are treated using the Particle-Particle Particle-Mesh (P3M) scheme and the Poisson equation for electric potential is solved in physical space using an iterative multi-grid technique. We implement this approach to systematically investigate two examples: the electroosmotic flow in random rough channels and the electrowetting on dielectric (EWOD). Both cases pose important engineering applications as a mechanism for transporting small amount of liquid in micro and nano devices.

Two-Way Coupling Model for Fractal-Like Agglomerate-Laden Multiphase Flow

The disturbing of ultrafine particles on carrying phase is usually ignored in studies on dilute ultrafine particle systems. In a dense fractal-like agglomerate system, however, the effect of particle phase on carrier phase should be considered due to its unique heat and mass transfer while the corresponding mathematical model for this problem has not been established. By taking the agglomerate-laden suspension as a single pseudo fluid, we propose a two-way coupling model in which the developed governing equations for suspension and particle general dynamic equation (PGDE) for particle coagulation and breakage due to turbulence are simultaneously solved. The Taylor-expansion moment method has been applied to solve fractal-like aggregate process and dilute gas-to-particle conversion, and in this study it is further extended to close the PGDE equation by invoking fractal theory. The newly coupling model is not limited in dilute particle system, and thus it is expected to play important roles in studying dense fractal-like agglomerate synthesis or other particulate industrial applications.

The Directional Freezing Effects on Biological Cells in a Microfluidic Cell Culture System

Cryotherapy is a prospective also green method for malignant tumor treatment. At low temperature, the cell viability relates with the cooling rate, temperature threshold, freezing interface as well as ice formation. In this paper a series of directional freezing processes and cell responses in a culture microchip were experimentally investigated. The temperature in the microchip was manipulated by a thermoelectric cooler. The surviving cells, necrotic and apoptotic cells under different cryotreatment (duration of the freezing process, freeze-thaw cycle, post-culture et al) were stained and distinguished by PI and FITC-Annexin V. The locations of the ice front and cell death boundary were observed and recorded through an inverted microscopy. By controlling the cooling process in a microfluidic channel, it is possible to recreate a sketch of biological effect during the process of simulated cryosurgery.

Micro and Nanofabrication of GaN by Wet-Chemical-Assisted Femtosecond Laser Ablation

For micro/nanofabrication of Gallium nitride (GaN), we developed wet-chemical-assisted fs laser ablation in which the femtosecond (fs) laser beam focused using an objective lens was directed on the surface of a GaN substrate immersed in 35% hydrochloric (HCl) acid solution. Nanocrators with a diameter as small as 320 nm (FWHM) were successfully formed on surface of a single-crystal GaN substrate by a single pulse of the second harmonic fs-laser beam (λ = 387 nm, 150 fs) focused using an objective lens with NA of 0.5. Nano scale ablation is responsible for two-photon absorption of the fs laser. The ablated craters exhibit higher quality and better uniformity with little debris formation compared with those produced by fs-laser ablation in air followed by etching in HCl (two-step processing method). The high quality ablation is presumably due to photochemical or photothermal reaction of HCl solution with ablated materials, resulting in complete removal of debris and in sharp edge and smooth surface of craters. We have demonstrated formation of 140-μm-long straight and hollow channels embedded in GaN by scanning the laser beam inside the substrate. 3D micro and nano fabrication technique of GaN has great potential for manufacture of highly-functional micro devices. We have also tried to fabricate 2D periodic nanostructures on GaN surface by scanning the sample in x-y plane. Nanocrators with uniform size periodically arranged on GaN surface can act as a two-dimensional (2D) photonic crystal which is expected to enhance a light extraction efficiency of blue LED.