### Session HC: Microfluidics: Mixing II

Chair: Igor Mezic, University of California, Santa Barbara
Room: Hilton Chicago Grand Ballroom

 Monday, November 21, 2005 1:20PM - 1:33PM HC.00001: Nonlinear Dynamics of Electrokinetic Instabilities Jonathan Posner , Juan Santiago Electrokinetic instabilities (EKI) are caused by a coupling of electric fields and ionic conductivity gradients. Electrokinetic flows become unstable when the advection of conductivity fields due to electric body forces dominates over the dissipative effects of viscosity and molecular diffusion. EKI's are relevant to on-chip electrokinetic flows with conductivity gradients such as FASS, multi-dimensional assay devices, and sample streams with poorly specified chemistry. Here, we present an experimental study of the nonlinear fluid flow and mixing dynamics of convective electrokinetic instabilities. This experimental study was performed in a cross-pattern chip with 50~$\mu$m wide microchannels. We obtained instantaneous concentration fields of electrically neutral, rhodamine B dye using epifluorescence microscopy and DC applied electric fields. When a critical electric Rayleigh number is exceeded, sinuous scalar patterns develop and advect downstream resulting in a single spectral peak at \textit{$\nu$}~=~42~sec$^{-1}$. As the applied electric field increases above a critical value, the flow exhibits sub-harmonics and higher-order harmonics, frequency bifurcations, and continuous power spectra. We will also show spatiotemporal concentration maps that exhibit strong aperiodicity. These flow phenomena are consistent with strong nonlinear dynamics and chaos. Monday, November 21, 2005 1:33PM - 1:46PM HC.00002: A modular micro impedance pump: a scalable concept for driving fluid and mixing in integrated microfluidic systems Derek Rinderknecht , Mory Gharib Functional microfluidic systems require architectures which allow for precise control and mixing of very small volumes of fluid. In order for these systems to become effective and as the need becomes greater to fit more functionality on a single chip, pumps will remain fundamental components of microfluidic systems. Exploiting the basic principles of impedance based pumping, we have shown that the impedance pump can be constructed in a wide variety of configurations and is scaleable to dimensions of a few hundred microns. Here we present experimental studies of the flow performance of a valveless micro impedance pump as well as explore its use in the context of a specific application, micro mixing. The pump, formed by a closed rectangular channel that is 200 microns in depth, 3 mm wide and 15 mm in length, has no complex parts and is easily adaptable to patterning by soft lithography. Flow rates of up to 16 microliters/min have been measured using high speed micro-PIV. Imaging of impedance pump driven micro flows has revealed that the flow is pulsatile at frequencies in excess of 100 Hz. The microfluidic system demonstrated in this work utilizes the flow unsteadiness combined with the interaction between the fluid and structures placed in the flow to enhance mixing efficiency at low Reynolds number. Monday, November 21, 2005 1:46PM - 1:59PM HC.00003: Numerical Investigation of Enhanced Mixing in a Microchannel Using Ferrofluid Using The Lattice Boltzmann Method Anindya De , Ishwar Puri Ferrofluids are colloidal suspensions of magnetic nanoparticles in carrier liquids which can be readily maneuvered from a distance using magnetic fields. When functionalized with different antibodies or medicinal compounds the ferrofluid nanoparticles can be used for various purposes, e.g., to detect bacteria or as a carrier of chemotherapeutic agents for targeted drug delivery. Localized magnetic nanoparticle agglomerates can also be remotely moved to create perturbations within a microchannel flow, thereby resulting in better mixing of various fluids. We have numerically investigated ferrofluid agglomeration and its influence on enhancing local mixing in microchannels by using the lattice Boltzmann method. Employing this method, we solve for the one-particle probability distribution function $f$ which denotes the probability density of finding a particle at time $t,$ at the location \textbf{x,} moving with velocity \textbf{v }when a force \textbf{F} is acting on it. (A Chapman-Enskog expansion recreates the continuum relation and the Navier-Stokes equation for weakly compressible flows.) We have simulated ferrofluid agglomeration near a magnetic dipole for flow through a rectangular microchannel. When a number of such magnets are placed across the channel and activated in sequence, they locally perturb the fluid flow to produce better mixing in two initially unmixed fluids. Monday, November 21, 2005 1:59PM - 2:12PM HC.00004: Scalar mixing and flow field measurements in curved-duct micro mixers Dimitrios Kyritsis , S. Pratap Vanka , Kenneth Christensen , Kevin Joyce Measurements of scalar mixing and velocity were performed in a spiral-shaped duct in order to determine the potential of secondary (Dean) flows to enhance mixing and therefore yield an effective micromixer design. Two inlet flow streams were mixed in a curved channel mixer of a 560 x 790 um cross section. One inlet flow was marked with Rhodamine 6G fluorescent dye. The three dimensional flow was imaged using confocal microscopy. Acquisition of frames parallel to the mixer plane at multiple depths in the channel allowed the reconstruction of the three-dimensional flow pattern for a series of stations downstream in the mixer. The effects of secondary flows induced by channel curvature were studied as a function of Re number for Re values between 1 and 12. In general, the secondary flows reduced the mixing time. Higher Reynolds number flows (Re=12) demonstrated more vigorous secondary flows and proportionally shorter mixing times than lower Reynolds number flows (Re=1). These results were complemented by micro - Particle Imaging Velocimetry measurements in order to measure the in-plane velocity field. On the basis of our scalar mixing and velocity data, we define and discuss several measures of mixedness which were used in order to characterize the process. Monday, November 21, 2005 2:12PM - 2:25PM HC.00005: Numerical Simulation Applied to Micromixer Design David Mott , Daniel Markus , Peter Howell , Joel Golden We describe the development of new microfluidic components designed to optimize mixing. The components use pressure-driven flow and consist of a microchannel with surface features (such as diagonal grooves, chevron-shaped grooves, and herringbone-shaped grooves) cut into the top and/or bottom of the channel. We describe a fast advection routine used to predict the transport of passive scalars through the components and the various metrics used to characterize the effectiveness of a proposed design. We also present mixer designs optimized for our chosen metrics over our feature set and compare the numerical predictions with experimental results.