Session AD: Microfluidics II: Applications and Actuations

Free-surface microfluidics for detection of airborne explosives

A novel microfluidic, remote-sensing, chemical detection platform has been developed for real-time sensing of airborne agents. The key enabling technology is a newly developed concept termed Free-Surface Fluidics (FSF), where one or more fluidic surfaces, confined by surface tension forces, are exposed to the surrounding atmosphere. The free-surface fluidic architecture can be combined with Surface-Enhanced Raman Spectroscopy (SERS) to allow the real-time profiling of atmospheric species and detection of airborne agents. Results indicate that 4-aminobenzenethiol, a chemical species similar in size and structure to trinitrotoluene (TNT), is readily detected by the SERS system which employs Free-Surface Fluidics to continuously detect the presence of gas-phase species.   

Retention of polymer molecules in a grooved channel

Previous studies have shown that for small Reynolds number flow in a smooth channel, polymer molecules tend to migrate towards the channel center due to the wall effects on the hydrodynamic interactions. In this work, we consider instead a structured channel. A bead-spring chain model of a dissolved polymer molecule confined between two solid walls with a cavity on one of them is considered. The solvent is simulated using the Lattice-Boltzmann method. From the simulation, we obtain the probability histogram of the chain center of mass and chain stretch as a function of the spatial position. The effects of Weissenberg number and the Peclet number are discussed in details. We artificially turned the hydrodynamic interactions on and off to reveal their importance. The effect of the flow strength, the geometry, and concentration are summarized. This study clarifies the origin of the retention of polymer molecules in structured microchannels.   

Dispersion in channels with adsorption and desorption at walls

The transport of a species, in a microfluidic capillary of arbitrary but axially-invariant cross-sectional shape  in the presence of an adsorption-desorption reaction on the capillary wall is studied, motivated by applications to capillary electrophoresis and open-tubular capillary electrochromatography. An asymptotic approach based on the long time limit is adopted. Numerical simulations are conducted for (a) an axially homogeneous flow with arbitrary cross- sectional variation of the axial velocity and (b) an electroosmotic flow rendered temporally and axially variable by the adsorbed species concentration on the wall. The  predictions from asymptotic theory are tested against numerical results obtained from the full three-dimensional solution of the governing equations.   

Topology optimized catalytic microfluidic reactors

We show that catalytic microfluidic reactors, when optimally structured, share underlying scaling properties. The scaling is predicted theoretically and verified numerically. Analytically we show that the reaction conversion for optimal porosity scale with respect to $\sqrt{\tau^{{}}_A\,\tau^{{}}_R}/\tau^{{}}_D$, where $\tau^{{}}_A$, $\tau^{{}}_R$ and $\tau^{{}}_D$ correspond to the characteristic timescales for advection, reaction, and diffusion, respectively. Furthermore, we show how to increase the reaction rate significantly by distributing the active porous material within the reactor~[1] using a high-level implementation of topology optimization~[2]. This optimization leads to a 20-fold increase of reaction rates as compared to corresponding optimal uniform reactors; an increase mainly due to a more efficient transport and distribution of the reactant by the pressure-driven buffer fluid. Our work points out a new, general, and potentially very powerful method of improving microfluidic reactors.\\[5mm] {\small {}[1] F.~Okkels and H.~Bruus, http://www.arxiv.org/abs/physics/0608020\\ {}[2] L.H.~Olesen, F.~Okkels, and H.~Bruus, Int.~J.~Num.~Meth.~Eng.~\textbf{65}, 975 (2006)}   

Flow and polymerization in small cracks with application to self-healing composites

The flow of a fluid undergoing a ring opening metathesis polymerization has been used for autonomic healing of cracks in self-healing materials under development. A monomer healing agent flows into a crack bringing it into contact with a healing agent catalyst which induces polymerization. Under the right conditions this has been demonstrated to heal the material. We have designed a simulation tool to investigate the coupled micro-fluid mechanics, the advection-diffusion of the healing agent, and the solidification in the crack. The polymerization is approximated by a large increase in the viscosity of the fluid. Lagrangian particles within the finite-element mesh are used to track the contact of monomer with the local catalyst concentration and integrate the rate equation for polymerization. The simulation tool is used to construct ``phase diagrams'' which identify the regimes in which healing occurs for various ranges of parameters of the system. These are useful for assessing the robustness of designs and optimizing the healing system.   

Morphology of C$_{60}$ crystals synthesized in microfluidic space

Since their initial discovery in 1985, Fullerene C$_{60}$ has attracted significant attention for their unique physical and chemical properties. In terms of technological issues, fabricating C$_{60}$ crystals with complex shapes for practical uses is a very important challenge. Recently, the liquid/liquid interfacial precipitation method for the synthesis of needle-like C$_{60}$ nanowhisker crystals was reported. However, the bulk structures of these C$_{60}$ crystals were all uniform and were single-dimensional. Here, we report unusual structures of C$_{60}$ crystals including tubes, trees, branches, hollow-ended columns, multiple pods, short prisms, and needles synthesized in a microfluidic device using a simple liquid/liquid interfacial precipitation method. The C$_{60}$ crystal morphology is categorized mainly by temperature, and is similar to the morphology of snow crystals. This simple method yields complex geometries of C$_{60}$ crystals quickly, and could be applied to all materials synthesis techniques that use liquid/liquid interfacial precipitation.   

Spiral Flow for a Rapid Micro-Particle Concentrator

A surface driven convection flow is shown to generate a spiral flow that can be used to rapidly concentrate micro-particles. In a point-plate electrode system, an ionic wind is generated at the tip of a needle under a high AC potential. This wind acts as a point source of momentum on the liquid, generating a surface driven electrohydrodynamic flow. At high Reynolds numbers, inertial effects arise, causing a secondary radial flow which is directed outwards near the free liquid surface and inwards near the substrate surface. Combining the azimuthal surface flow with the radial flow, a downward spiral is formed with a converging flow stagnation point at the center of the spiral, near the substrate surface. Suspended particles in the liquid follow the streamlines towards the stagnation point and are trapped at the substrate by a downward acting body force, such as DEP or gravitational force. A critical particle size and liquid velocity for trapping exists due to the balance between the resuspending and trapping forces exerted on the particle near the stagnation point. We quantify these observations and calculate the favorable conditions for particle trapping.   

Hydrodynamic loading on microcantilever in liquid near a solid surface

Recently, Green and Sader [J. Appl. Phys. \textbf{98}, 114913 (2005)] developed a theory predicting frequency responses of a microcantilever immersed in a fluid near a solid surface. This article presents an experimental investigation of the hydrodynamic loading effects on the frequency responses of microcantilever in liquid near a solid surface. Liquid viscosity and density are controlled by temperature change and the gap height between cantilever and a solid surface is controlled by piezoelectric actuator in atomic force microscope. It is found that the enhanced dissipative effect due to liquid viscosity near a solid surface is primarily source of hydrodynamic loading on vibrating microcantilever in liquid. The results show the physics of viscous dissipation in the micro-scale surrounding fluid and will be of value to microcantilever sensor communities.   

Exploiting the Stochastic Dynamics of Microscale Cantilevers for Single Molecule Measurements

The stochastic dynamics of micron scale cantilevers are investigated. The fluctuation-dissipation theorem is used to describe the stochastic dynamics using deterministic numerical calculations. A quantitative description of the fluid flow around experimentally realistic microcantilevers is presented to yield a baseline description of the fluid dynamics in the regime of small amplitude oscillations at high frequencies. It is shown that a correlated measurement technique using an array of two cantilevers is capable of obtaining real-time measurements of the dynamics of single proteins. The dynamics of a protein tethered between two cantilevers will correlate the cantilever motion with a magnitude smaller than that of thermal oscillations of a single cantilever, yet larger than the coupling due to hydrodynamic effects. The dynamics of a single microscale cantilever are also investigated as the cantilever is brought closer to a solid wall. The effect of the increased dissipation associated with the presence of the wall on the first and second peaks of the noise spectrum is discussed.   

Thermal fluctuations and the hydrophobic effect.

Hydrophobic effect typically implies three phenomena widely discussed in literature: depletion of liquid at hydrophobic surfaces, observation of slip at hydrophobic surfaces and the long range attraction between hydrophobic surfaces. The central feature that characterizes all these phenomena is considered to be the formation of thin vapor-like films next to hydrophobic surfaces. Prior work suggests that thermal fluctuations of the liquid/vapor-like film interface near hydrophobic surfaces, and correspondingly, fluctuations in the fluid density play an important role. Fluctuating interfaces are also implied from experiments with a liquid between a hydrophobic and a hydrophilic surface. Thus, it is necessary to develop a way to quantify the interfacial fluctuations to better understand this effect. We present a way to extract this information from molecular dynamic (MD) simulations. Ways to incorporate this information into fluctuating hydrodynamics (FHD) based mesoscale techniques will be proposed. FHD based methods can permit solutions of a variety of dynamic flow problems involving hydrophobic surfaces by using the usual tools available in hydrodynamics studies.   

Nature-inspired micro-fluidic manipulation using artificial cilia

One particular micro-fluidics manipulation mechanism ``designed'' by nature is that due to a covering of beating cilia over the external surface of micro-organisms (e.g. Paramecium). A cilium can be viewed as a small hair or flexible rod (in protozoa: typical length 10 $\mu $m and diameter 0.1 $\mu $m) which is attached to the surface. We have developed polymer micro-actuators, made with standard micro-technology processing, which respond to an applied electrical or magnetic field by changing their shape. The shape and size of the polymer actuators mimics that of cilia occurring in nature. We have shown experimentally that, indeed, our artificial cilia can induce significant flow velocities of at least 75 $\mu $m/s in a fluid with a viscosity of 10 mPas. In this paper we will give an overview of our activities in developing the polymer actuators and the corresponding technology, show experimental and numerical fluid flow results, and finally assess the feasibility of applying this new and attractive micro-fluidic actuation method in functional biosensors.   

Parametric Effects on the Flow Performance of Single Disk Viscous Micropump.

The effect of design parameters such as radius ratio and channel aspect ratio on the flow performance of a newly introduced single disk viscous micropump has been investigated. A number of 3D numerical models for the single disk micropump have been built and analyzed at different boundary conditions using finite volume method. To express the effect of the pressure difference and boundary velocity on the flow performance at various design parameters, drag and pressure shape factors for the radius and aspect ratios have been defined and built numerically and compared with approximate 2D analytical solution using Navier-Stokes equation estimating for the flow rates. The effect of aspect ratio at moving and stationary walls conditions has been also analytically and numerically investigated. The obtained results showed that the error in estimating the drag shape factors at different radius and aspect ratios are less than 1.0{\%}. However, the error in estimating the pressure shape factor exceeds 10{\%} for and 3{\%} at. Numerical results were found to be best fitted with the analytical solution. Also, it has been found numerically that the flow rate varies linearly with both the pressure difference and boundary velocity for wide range of studied parameters, which supports the validity of the linear lubrication model for this problem for the full range of radius and aspect ratios studied.