Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L30: Micro and Nano Fluidics |
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Sponsoring Units: DFD Chair: Patrick Tabeling, CNRS, France Room: Colorado Convention Center 304 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L30.00001: Microfluidic droplets and electric fields Patrick Tabeling, Laure Menetrier, Alice McDonald, Herve Willaime, Dan Angelescu Manipulating droplets through mazes of microchannels is a challenge faced by digital microfluidics (i.e microfluidics based on droplets). In this domain, using electric fields is an option. This option is justified by the fact that producing large electric fields in miniaturized systems is feasible, and dielectric contrasts between dispersed and continuous phases are typically large. Examples of devices reported in the literature are droplet guides, droplet mergers. In the present paper, we extend this approach by reporting two novel examples of droplet manipulations that exploit the action on an electric field in a microfluidic system: one is the control of droplet emission frequencies and the other is the inhibition of droplet breakup. Throughout the work, we analyze in some detail the various aspects of the action of the electric field. The experiments are performed in PDMS microfluidic systems using hexadecane and water for the continuous and dispersed phases respectively. [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L30.00002: Chemotaxis in Microfluidic Devices: What does a cell see? Carsten Beta, Toni Froehlich, Gabriel Amselem, Eberhard Bodenschatz The use of microfluidic devices is increasingly popular in the study of chemotaxis due to the exceptional control of the flow field and the concentration profiles on the length scale of individual cells. One aspect often forgotten is that the cells are attached to the inner surfaces of the microfluidic channel. The flow field is perturbed and distorted as the fluid is flowing around/over the cells. Depending on the flow speed and dynamics (steady flow - increasing flow - decreasing flow) the cell membrane is not exposed to the ``nominal'' concentration profiles, but may see a very different signal. The underlying physics will be discussed and ``optimal'' flow conditions will be identified. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L30.00003: Active microfluidic mixing based on transverse electro-osmotic flows Nicholas S. Lynn Jr., Charles S. Henry, David S. Dandy As with their macroscale counterparts, laminar fluid mixing becomes a very important, albeit inherently difficult step at the microscale. Micromixers based on electro-osmotic flow (EOF) rely on either a modification of microchannel geometries or a modification of the $\zeta $-potential of the microchannel surfaces to enhance fluid mixing. Here we present a new method of achieving chaotic advection in microchannels by applying an electric field perpendicular to the mean flow direction driven by a pressure gradient in a planar rectangular microchannel. EOF on microchannel surfaces in a direction orthogonal to the main channel axis is generated via an electric field produced by integrated electrodes at the corners of a microchannel. By using serial combinations of different mixing cycles, we show that complete mixing can occur in straight microchannels of length scales on the order of a millimeter. Computational fluid dynamics (CFD) is used to characterize and optimize the mixing efficiency of the system and to compare with experimental measurements. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L30.00004: Microfluidic bubble logic and applications Manu Prakash, Neil Gershenfeld We present a novel all-fluidic logic family operating at low Reynolds numbers in newtonian fluids. A bubble in a microfluidic channel represnets a bit. Nonlinearities are introduced in an otherwise linear, reversible flow by bubble-bubble interactions. This allows us to simultaneously perform chemistry and process control without external control elements. A toggle flip-flop, AND/OR/NOT gates, ring oscillator and an electro-bubble modulator will be presented. Applications in high-throughput screening and combinatorial chemistry will be highlighted. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L30.00005: Using patterned surfaces to sort elastic microcapsules Alexander Alexeev, Rolf Verberg, Anna C. Balazs For both biological cells and synthetic microcapsules, mechanical stiffness is a key parameter since it can reveal the presence of disease in the former case and the quality of the fabricated product in the latter case. To date, however, assessing the mechanical properties of such micron scale particles in an efficient, cost-effective means remains a critical challenge. By developing a three-dimensional computational model of fluid-filled, elastic spheres rolling on substrates patterned with diagonal stripes, we demonstrate a useful method for separating cells or microcapules by their compliance. In particular, we examine the fluid-driven motion of these capsules over a hard adhesive surface that contains soft stripes or a weakly adhesive surface that contains ``sticky'' stripes. As a result of their inherently different interactions with the heterogeneous substrate, particles with dissimilar stiffness are dispersed to distinct lateral locations on the surface. Since mechanically and chemically patterned surfaces can be readily fabricated through soft lithography and can easily be incorporated into microfluidic devices, our results point to a facile method for carrying out continuous ``on the fly'' separation processes. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L30.00006: Lab-on-a-chip Single Particle Dielectrophoretic Traps Weina Wang, Hua Shao, Kevin Lear Cell-patterning and cell-manipulation in micro-environments are fundamental to biological and biomedical applications, for example, spectroscopic cytology based cancer detection. Dielectrophoresis (DEP) traps with transparent centers for stabilized cell and particle optofluidic intracavity spectroscopy (OFIS) were fabricated by patterning 10 $\mu $m wide, planar gold electrodes on glass substrates. The capturing strength of DEP traps was quantified based on the minimum AC voltage required to capture and hold varying diameter polystyrene or was it some other material, e.g. silica or PMMA microspheres in water as a function of frequency required under a constant flowrate of 20 $\mu$m/s. The maximum required trapping voltage in the negative DEP regime of $f$ = 1 kHz to 40 MHz was 5.0 VAC. The use of AC fields effectively suppressed hydrolysis. New geometries of DEP traps are being explored on the basis of 3-D electrostatic field simulations. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L30.00007: Hybrid CMOS / Microfluidic Systems for Cell Manipulation with Dielectrophoresis Tom Hunt, David Issadore, Robert M. Westervelt A hybrid CMOS/microfluidic chip combines the biocompatibility of microfluidics with the built-in logic, programmability, and sensitivity of CMOS integrated circuits (ICs)$^{1}$ We have designed a CMOS IC for moving individual cells using dielectrophoresis (DEP). The IC was built in a commercial foundry and we subsequently fabricated a microfluidic chamber on the top surface. The chip consists of a 1.4 by 2.8mm array of over 32,000 individually addressable 11x11 micron pixels. An RF voltage of 5V at 10MHz can be applied to each pixel with respect to the conductive lid of the microfluidic chamber, producing a localized electric field that can trap a cell. By shifting the location of energized pixels, the array can trap and move cells along programmable paths through the microfluidic chamber. We show the design, fabrication, and testing of the hybrid chip. Bringing together the biocompatibility of microfluidics and the power of CMOS chips, hybrid CMOS / microfluidic systems are an exciting technology for biomedical research. Thanks to NSEC NSF grant PHY-0117795 and the NCI MIT-Harvard CCNE. [1] H Lee, Y Liu, RM Westervelt, D Ham, IEEE JSSC 41, 6, pp. 1471-1480, 2006 [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L30.00008: Optical Chromatography of Bacterial Spores Steven Sundbeck, Alex Terray, Jonathan Arnold, Tomasz Leski, Sean Hart The technique of optical chromatography uses a laser mildly focused against fluid flow in a microfluidic channel to trap microscopic particles. Particles in the channel near the focal point of the laser are drawn toward the beam axis and then accelerated via optical pressure against the fluid flow, reaching an equilibrium point when the optical and fluidic forces on the particle are balanced. This equilibrium point may occur at differing distances from the focal point for microscopic particles with differing properties, such as size, shape, morphology, and refractive index. Thus, identification and separation of particles may be achieved in the system. Optical chromatography may be used as a detection technique for biological particles of interest, either directly or as a means of concentrating and filtering a sample. Of particular interest would be reliable methods for detection of {\em Bacillus anthracis}, a common weaponized biological agent. In this work we present optical chromatography experiments on bacterial spores which may be environmentally present with {\em B. anthracis} spores and interfere with detection. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L30.00009: ``Nanonails'' -- a Simple Geometrical Approach to ``Superlyophobic'' Surfaces Tom Krupenkin, Amir Ahuja, Ashley Taylor, Alex Sidorenko, Todd Salamon, Edgar Labaton Modern nanofabrication techniques allow creation of a wide range of sophisticated surface topographies that strongly enhance wetting properties of solids. Such surfaces serve as a basis for so-called superhydrophilic and superhydrophobic materials that demonstrate a range of remarkable properties. In both of these cases the topography acts to ``amplify'' the type of wetting behavior, which is already determined by the surface energies of the liquids and solids involved. In this work we propose and experimentally demonstrate a unique three-dimensional nano-scale geometry that dramatically extends the influence of topography on the wetting properties of the substrate. Using this approach we are able to transform ordinary Teflon-like fluoropolymer surfaces, which are readily wetted by the majority of common low-surface tension liquids into nanostructured substrates with profound superlyophobic behavior. The resulting surfaces are essentially non-wetting and support highly mobile liquid droplets with contact angles close to 150\r{ } for a wide variety of liquids with surface tensions ranging from 72.0 mN/m (water) to 21.8 mN/m (ethanol). The proposed approach provides a simple, material-independent method for creating practically useful superlyophobic surfaces. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L30.00010: Double Emulsions through Wettability Control in PDMS Microfluidic Devices Christian Holtze, Elisa Mele, David Weitz Hydrodynamic Flow Focusing allows for the well-controlled production of monodisperse double and multiple emulsions. While this method of emulsification is well described for glass capillary devices, it has not yet been developed for PDMS devices that are readily accessible using soft-lithography. The reason is the difficulty of spatially controlling the wetting behavior of PDMS microchannels. We will present a novel technique of photopatterning that allows for the production of double emulsions in PDMS devices. Moreover, owing to an optimized setup, smaller droplets may be made down to a size range that was not accessible using the conventional approaches. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L30.00011: Atomistic simulation study of charge inversion in silica nanochannels Christian D. Lorenz, Alex Travesset Recent experiments report charge inversion, i.e. interfacial charges attracting couterions in excess of their own nominal charge, in divalent ionic solutions near charged silicon oxide interfaces. We have conducted a series of atomistic molecular dynamics simulations in order to investigate the mechanism of charge inversion in these systems. We studied both CaCl$_2$ and MgCl$_2$ solutions near an amorphous silica substrate which had a charge density of $\sim 1/50${\AA}$^2$. Our simulation results give a detailed description of the structure of the ions and water near the silica interface. Finally, we show that our simulations are in remarkable agreement with the experimental results. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L30.00012: Ion solvation in confined water: A first-principles molecular dynamics investigation Eric Schwegler, Giancarlo Cicero, Jeffrey Grossman, Francois Gygi, Giulia Galli The importance of water in many areas of science has motivated an enormous number of experimental and theoretical investigations. While the properties of bulk liquid water have been relatively well characterized, much less is known about the properties of water when it is confined in a nanoscale environment. We have carried out a series of first-principles molecular dynamics simulations in order to examine how the solvation properties of simple ions are modified upon nanoscale confinement. These simulations include the aqueous solvation of cations contained within a carbon nanotube. By comparing to the properties of ions in bulk liquid water, the dynamical and structural characteristics of confined ion solvation will be discussed in detail. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L30.00013: Measurement of the growth rate of the breakup instability of a propane nanobridge using molecular dynamics simulations Wei Kang, Uzi Landman Understanding the instability of a nanojet or a nanobridge is of importance for the design of nanoscale fluid devices. Examination and determination of the whole growth rate curve of the instability in these nanostructures is a theoretical challenge. Using large scale molecular dynamics (MD) simulations at 185K we determined the growth rate curve of a nanoscale liquid propane nanobridge of a 0.3 micron length and a 6-8 nm diamater; the system consistes of ~340,000 particles. We analyzed, using a discrete spatial Fourier transform, the time evolution of small sinusoidal perturbations of various wavelengths applied to the fluid nanobridge. The large length-to-diameter ratio of our systems allows us to achieve suffcient wavelength resolution. The results of 100 independent simulations were averaged to reduce fluctuation noise. The results were compared with both Rayleigh's and Chandrasekhar's theories and we conclude that the latter is a better fit to our data. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L30.00014: Instability in extensional microflow of aqueous gel Robert Bryce, Mark Freeman Microfluidic devices are typically characterized by laminar flows, often leading to diffusion limited mixing. Recently it has been demonstrated that the addition of polymer to fluids can lead to elastic instabilities and, under some conditions, turbulence at arbitrarily low Reynolds numbers in mechanically driven flows [1]. We investigated electroosmotic driven extensional flow of an aqueous polymer gel. Microchannels with 100 micron width and 20 micron depth with the characteristic ``D'' chemical etch cross section were formed in glass. A Y-channel geometry with two input channels and a single output created extensional flow at the channel intersection. Instabilities where observed in the extensional region by fluorescently tagging one input stream. Instabilities were characterized by 1/f spectra in laser induced fluorescent brightness profiles. Due to the simple geometry of extensional flow and the importance of electroosmotic flows for integrated applications and in scaling, this is of interest for device applications. [1] A. Groisman and V. Steinberg, Nature 405, 53-55, 2000. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L30.00015: Interactions between micro droplets and a flowing soap film Ildoo Kim, Xiao-Lun Wu When a jet of micron sized water droplets impact on a thin freely suspended soap film, craters of various sizes are created in the film. Depending on the velocity of the jet and the thickness of the film, a fraction of the particles is able to penetrate through the film without breaking it while others merge with the film. The statistical nature of penetration suggests that the energy barrier for passage is a fluctuating quantity but the cause of such fluctuation is not understood. Using a high-speed video camera, the interaction between the droplet and the film is investigated for various conditions. Aside from its fundamental interest, the technique is potentially useful for generating predetermined number of vortices in the fluid and for depositing precisely passive scalar quantities, such as dyes, into two-dimensional turbulence in the flowing film. [Preview Abstract] |
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