Winky Lap Wing Hau

Surface-chemistry technology for microfluidics

A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. A positively charged poly(allylamine hydrochloride) (PAH) layer is coated onto a negatively charged silicon oxide surface by electrostatic self-assembly (ESA). Combined with photolithography in a lift-off-based process, several different surface charge patterns were successfully produced. Due to definition of the pattern by photolithography, no limitations in the pattern geometry exist. Any surface charge pattern can be created to enable fine control of fluid motion in microfluidic devices. Physical properties of this PAH layer were characterized. The generation of a bi-directional shear flow was demonstrated by using alternating longitudinal surface charge pattern with a single driving force, i.e. an externally applied electric field inside a microchannel.

Electrokinetic generation of microvortex patterns in a microchannel liquid flow

The technology developed for micropatterning the electric surface charge to be negative, positive or neutral enables the realization of complex liquid flows in simple microchannels. A commercial CFD code is utilized to numerically simulate a variety of electrokinetically-generated liquid flows in a straight and uniform microchannel due to non-uniform surface charge distribution under an externally applied, steady electric field. We present design methodologies to electrokinetically drive vortical flows in any desired direction. In particular, we investigate surface charge patterns required to generate single or multi, co-rotating or counter-rotating, in-plane or out-of-plane vortices. Finally, in view of its potential application to microscale mixing, we discuss a surface charge pattern that can give rise to streamwise vorticity.

In-plane vortex flow in microchannels generated by electroosmosis with patterned surface charge

Electrokinetically driven in-plane vortex flows in a microchannel are studied utilizing a patterned surface charge technique requiring both positively and negatively charged regions on the same substrate. In the first part, a periodic flow pattern consisting of counter-rotating vortex pairs is analyzed experimentally and numerically; this is a relatively easy flow to experimentally realize in the lab since no charge-free region is necessary. The good agreement between the measured and computed flow fields demonstrates that: (i) the surface charge patterning technique can be used for driving electrokinetically complex vortex flow patterns in microchannels, and (ii) the applied CFD code can be used for calculating reliably such flow fields. In the second part, the numerical scheme is utilized to study a single, in-plane vortex in order to reveal the proper length and velocity scales as well as the dominant control parameters. This flow field, although simpler, is very difficult to realize experimentally due to the need for a large surface area carrying no charge. The resulting 3D flow field features a coherent vortex with its axis perpendicular to the symmetrically charged regions on the top and bottom surfaces of the microchannel. Three length scales, the active-region length and width as well as the channel height, and a velocity scale, the speed of the electroosmotic flow, have been identified as the relevant variables. The strength of the in-plane vortex along with several flow patterns has been characterized on the basis of these four independent variables.

A new stress chip design for electronic packaging applications

Stress sensing chips are invaluable for structural analysis of electronic packages, and can be used for in-situ real-time measurement of thermally induced die surface stress. Four-point-bending calibration, wafer-level calibration and hydrostatic calibration have been used to calibrate the stress sensing chip. These methods are used to calibrate different piezoresistive coefficients B/sub 1/, B/sub 2/ or their combinations (B/sub 1/-B/sub 2/), (B/sub 1/+B/sub 2/) and (B/sub 1/+B/sub 2/+B/sub 3/). Hydrostatic calibration is a bottleneck to quick calibration. Only one chip can be calibrated at a time and it must be wire bonded. Also, the value of coefficients (B/sub 1/+B/sub 2/+B/sub 3/) obtained is not temperature compensated and precise temperature measurement is required. Thus, a quick and accurate calibration method is key to making stress sensing chips more acceptable for electronic packaging use. In this paper, a new (111) stress sensing chip design is presented, and an innovative calibration scheme is proposed. Calibration can be done at wafer level, giving large savings in calibration time over the die form process. The mechanism is based on thermal expansion of the Al micro-beams which produce out-of-plane shear stresses on the sensing elements under temperature change. The relationship of normalized resistance change against temperature was found experimentally. Stresses produced by the Al micro-beam are high enough for calibration, with 4.6/spl sim/5.4% difference of normalized resistance changes between resistors with and without micro-beam. Also, a simple 1D model was proposed to estimate the order of magnitude of the stress under temperature change.

Two-Dimensional Analysis of Electrokinetically Driven Out-of-Plane Vortices in a Microchannel Liquid Flow Using Patterned Surface Charge

The technology developed for photolithographically patterning the electric surface charge to be negative, positive, or neutral enables the realization of complex liquid flows even in straight and uniform microchannels with extremely small Reynolds number. A theoretical model to analyze a steady incompressible electrokinetically driven two-dimensional liquid flow in a microchannel with an inhomogeneous surface charge under externally applied electric field is derived. The flow field is obtained analytically by solving the biharmonic equation with the Helmholtz-Smoluchowski slip boundary condition using the Fourier series expansion method. The model has been applied to study three basic out-of-plane vortical flow fields: single vortex and a train of corotating and a series of counterrotating vortex pairs. For model verification, the solution for the single vortex has been tested against numerical computations based on the full Navier-Stokes equations revealing the dominant control parameters. Two interesting phenomena have been observed in out-of-plane multivortex dynamics: merging of corotating vortices and splitting of counterrotating vortices. The criteria for the onset of both phenomena are discussed

Experimental investigation of electrokinetically generated in-plane vorticity in a microchannel

Electrokinetic generation of micro-flow patterns has advanced in recent years and received significant attention due to promising applications in biotechnology. Basic flow fields like bi-directional shear and out-of-plane vortices have been generated electrokinetically in microchannel liquid flow using various surface-charge patterns. In-plane vortex flows present a higher challenge since positive and negative charge regions on the same surface are required. Utilizing a newly-developed polymer-coating technology, the fabrication and characterization of microchannel devices with a variety of charge patterns are reported. Pairs of in-plane counter-rotating vortices or serpentine-like vortical motion have been observed depending on the absence or presence of a mean flow. The experimental results have been found to be consistent with CFD computations using a commercial code.

Micro flow patterns on demand using surface-chemistry technology

A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. Unlimited flow patterns can be generated under an externally applied electric field by electro-osmotic and electrophoretic driving forces to enable fine control of fluid motion in microfluidic devices. Two basic flows, shear and vortical, have been realized experimentally to demonstrate the tremendous potential of this technology, especially in analytical microsystems for genomics or cell biology.

Unsteady in-plane vortex motion in a microchannel liquid flow

An unsteady flow can dramatically enhance the mixing efficiency in a highly localized region, as the flow would become chaotic if time is an independent variable. In this work, the response of uniform electroosmotic flow to an oscillating electric field is first examined experimentally and numerically as a function of the driving frequency. Then a steady in-plane micro vortex flow pattern, traced by microparticles, is realized and compared to numerical simulations. Upon confirmation of the simulations for uniform but unsteady and steady but non-uniform flows, the CFD code has finally been applied to study unsteady non-uniform flow field, for which it is difficult to measure flow properties. The time evolution of liquid vortex motion in a microchannel, due to either sinusoidal or sudden electric field reversal, is numerically investigated revealing the relationship between length and time scales dominating momentum transfer in electrokinetically-driven unsteady liquid flow.

Electrokinetically-driven micro mixer with a novel surface-charge pattern

An electrokinetically-driven micro mixer with a special surface-charge pattern was designed, fabricated and characterized using fluorescence video microscopy. Zeta potential of the working fluids was measured to facilitate the numerical simulation and optimization of the proposed mixer. The mixing flow of the electrolyte with and without microparticle was digitally recorded and analyzed in terms of concentration profile and mixing index. The experimental mixing enhancement is consistent with the simulation result

Numerical simulations of electrokinetically generated micro vortices in microchannels

The technology developed to micro-pattern the electric surface charge to be negative, positive or neutral enables the realization of complex liquid flows in simple microchannels. A commercial CFD code is utilized to numerically simulate a variety of electrokinetically-generated liquid flows in a straight and uniform microchannel due to non-uniform surface charge distributions under a single, externally applied electric field. Design methodologies to generate vortical flow in any desired direction are presented. In particular, surface charge patterns required to generate single or multi, co-rotating or counter-rotating, in-plane or out-of-plane vortices are investigated.