Campuses:

<span class=strong>Reception and Poster Session</span><br><br/><br/><b>Poster submissions welcome from all participants</b><br><br/><br/><a<br/><br/>href=/visitor-folder/contents/workshop.html#poster><b>Instructions</b></a><br/><br/><br/><br/>

Monday, December 7, 2009 - 4:00pm - 6:00pm
Lind 400
  • Two-phase flow diffuse interface models for dynamic electrowetting
    Marco Fontelos (Consejo Superior de Investigaciones Científicas (CSIC))Günther Grün (Friedrich-Alexander-Universität Erlangen-Nürnberg)
    We present thermodynamically consistent models for dynamic electrowetting and other electrokinetic phenomena
    involving conductive liquids or electrolyte solutions. They combine Navier-Stokes equations, evolution equations for
    ion/charge densities and for phase field with an elliptic transmission problem forthe electrostatic potential.
    We provide numerical and theoretical argumentsi ndicating that microscopically Young's contact angle persists
    in equilibrium
    configurations. Moreover, the models allow for contact-angle hysteresis.
    In addition, 2D and 3D
    numerical simulations on electric field induced droplet
    motion are presented. Finally, rigorous mathematical
    analysis shows
    global-in-time existence of solutions to the models under consideration.
  • A diffusive interface method of modeling mutli-phase flows
    Huan Sun (The Pennsylvania State University)
    We present an diffusive interface approach to modeling
    multi-phase flows. A modified interfacial energy functional is employed
    to describe diffusive interfaces of (im)miscible phases. A fluid system
    where the Stokes equations are coupled with convection-diffusion
    equations are derived from the energy functional via the Energetic
    Variational Approaches (EVA). A particular case with slip boundary
    conditions on the interfaces were studied. In the numerical simulations
    we applied the Pressure Schur Complement (PSC) method to the
    hydrodynamical system. A Krylov subspace method with an Algebraic
    Mutligrid (AMG) preconditioner was used to solve the resulted linear
    system.
  • Traveling-wave electroosmosis and faradaic currents: the

    diffusion layer

    Antonio Ramos (University of Sevilla)
    Pumping of electrolytes in microchannels can be achieved with arrays of microelectrodes subjected to AC potentials. Here we show experiments on electrolyte flow induced by microelectrodes subjected to traveling-wave potentials. For sufficiently high voltages, Faradaic currents are present, leading to changes in the liquid properties and, in particular, changes in pH. A remarkable feature of the observations is that at voltages above
    a threshold, the direction of the fluid flow is reversed.

    These observations motivate the theoretical study of Faradaic currents in electrokinetics for the general case of ionic species with different mobilities. We find, using a linear analysis, that the structure of the electrical double layer (EDL) has to be extended. The EDL consists of the compact and Debye layers, as in previous models, plus a diffusion layer that arises as a consequence of Faradaic currents. For the general case of different mobilities, there is a net electrical charge associated to the diffusion layer. As a particular result of this model, we show that traveling potentials generate flow in the reverse direction for the case of a thick compact layer and facile Faradaic reactions, if the reacting ions are the more mobile. This situation is consistent with the experimental observation of changes in pH due to proton reactions at the electrodes.
  • Speed of KPP fronts with a cut-off: rigorous results
    M. Cristina Depassier (Pontificia Universidad Catolica de Chile)
    We study the reaction diffusion equation ut = uxx + f(u), with a
    cut-off ε in the reaction term. The reaction term without a
    cut-off is assumed to be of KPP type. The introduction of the cut-off on
    the reaction term has been shown to model the effect of noise and the
    finiteness of the number of diffusing particles.
    Rigorous bounds on the speed are given for arbitrary values of
    ε. For small cut-off the Brunet-Derrida value is recovered, the
    bounds from allow to determine its range of validity. In the opposite
    limit of large cut-off the speed tends to zero as the square root of
    (1-ε). The results are obtained making use of a variational
    characterization of the speed.
  • Stretch dependency of the electrophoretic mobility of DNA
    Ronald Larson (University of Michigan)
    We develop a theory on DNA electrophoresis that shows stretch-dependent electrophoretic mobility in agreement with an experiment observation. In our theory, a DNA molecule is modeled as a freely-jointed-chain, each of whose segments consists of a collinear series of charged spheres, which we call a shish-kebab segment. First, by calculating the interaction between charged spheres in an electric field, we show that the electrophoretic mobility of a shish-kebab segment is dependent on the orientation relative to the direction of the electric field. Then, the electrophoretic mobility of the whole DNA chain is evaluated by taking an ensemble average over the orientation of the shish-kebab segments in the chain. The result shows an enhancement of the magnitude of the electrophoretic mobility under the stretch of the DNA molecule.
  • Particle separation by capillary electrophoresis in nanochannels
    Paul Atzberger (University of California)
    We discuss an on-going theoretical / experimental
    effort studying particle separation through
    capillary electropohoresis in nanochannels.
    Recent experimental results in the laboratory of
    Dr. Pennathur (UCSB, Dept. ME) indicate that
    increased fidelity in separating particles by size and
    charge can be achieved when using channels with
    cross sections of nanometer dimensions
    (100nm x 1000nm) as opposed to larger
    microchannels. For short double-strands
    of DNA (10 - 100 base pairs) it is found
    that separation in free solution produces
    only one lumped peak in the fluorescence signal
    for microchannels but several clearly distinct
    peaks in nanochannels. Many effects which are
    weak in microchannels are expected to play a strong
    role in nanochannels owing to the large surface
    area to volume ratio and steric restrictions imposed
    on particle configurations. Models are presented
    for separation which investigate the role of
    the particle-particle and particle-wall steric
    interactions, the hydrodynamic flow and coupling,
    the overlap of double layers, and the translational
    and rotational diffusion of particles. This work is
    also being carried out with Dr. Gibou (UCSB, Dept. ME)
    and with the graduate student David Boy.
  • Non-monotonic energy dissipation in microfluidic cantilever

    resonators

    Thomas Burg (Max-Planck-Institut für Biophysikalische Chemie)
    Nanomechanical resonators enable a range of precision measurements in air or
    vacuum, but strong viscous damping makes applications in liquid challenging.
    Recent experiments have shown that fluid damping can be greatly reduced by
    confining the sample to a fluidic channel embedded inside the resonator while
    the outside is under vacuum. Understanding fluid damping in such systems is
    critical for future applications to problems spanning a wide range of scales
    in nanoscience and biology.
    Measurements presented here reveal that energy dissipation in cantilevers
    with embedded fluidic channels is a non-monotonic function of viscosity,
    suggesting that the quality factor may actually be enhanced through
    miniaturization. These results are found to be consistent with a first-order
    hydrodynamic model of the fluid-filled vibrating cantilever beam. In the
    regime of low-viscosity, inertia dominates the fluid motion inside the
    cantilever, resulting in thin viscous boundary layers - this leads to an
    increase in energy dissipation with increasing viscosity. In the
    high-viscosity regime, the boundary layers on all surfaces merge, leading to
    a decrease in dissipation with increasing viscosity. Effects of fluid
    compressibility also become significant in this latter regime and lead to
    rich flow behaviour. Based on these results, we anticipate that scaling of
    current devices by more than ten-fold may be possible without significant
    degradation of the quality factor due to damping induced by the fluid.
  • Hydrodynamic trap for single cells, particles and

    molecules

    Charles Schroeder (University of Illinois at Urbana-Champaign)
    The ability to trap individual particles, cells and macromolecules has revolutionized many fields of science during the last two decades. Several methods of particle trapping and micromanipulation have been developed based on optical, magnetic and electric fields. In this work, we describe an alternative trapping method, the hydrodynamic trap, based on the sole action of hydrodynamic forces in a microfluidic device. A microfluidic cross slot device is fabricated consisting of two perpendicular microchannels where opposing laminar flow streams converge. In this device, a purely extensional flow field is created at the microchannel junction, thereby resulting in a semi-stable potential well at the stagnation point which enables particle trapping. We implement an automated feedback-control mechanism to adjust the location of the stagnation point which facilitates active particle trapping. Using the hydrodynamic trap, we successfully demonstrate trapping and manipulation of single particles and cells for arbitrarily long observation times. This technique offers a new venue for observation of biological materials without surface immobilization, eliminates potentially perturbative optical, magnetic and electric fields, and provides the capability to change the surrounding medium conditions of the trapped object during observation.
  • Numerical simulations of dynamic wetting
    Shahriar Afkhami (New Jersey Institute of Technology)
    With miniaturization of fluidic devices, small-scale effects such as the
    details of the flow near the contact line become important. We present a
    three-dimensional numerical model to simulate the dynamic behavior of
    moving contact line phenomena. The model consists of an adaptive mesh
    discretization of the time-dependent Navier-Stokes equations for
    incompressible two-phase flows with a volume-of-fluid technique for
    interface tracking. Equilibrium results of three-dimensional droplets with
    various contact angles are presented and compared with known solutions.
    The slip of a moving contact line on the solid surface and the dynamical
    contact angle are computationally investigated. Some numerical simulations
    of the model applied to electrowetting are presented.
  • Free energy landscaping: Nanotopographic control over

    DNA conformations and transport

    Derek Stein (Brown University)
    Nanofluidic devices with an embedded nanotopography direct the self-organization and transport of long DNA molecules by influencing the free energy landscape. We studied the pressure-driven transport of DNA in slit-like nanochannels containing linear arrays of nanopits. We imaged individual DNA molecules moving single-file down the nanopit array, undergoing sequential pit-to-pit hops using fluorescence video microscopy. Distinct transport dynamics were observed depending on whether a molecule could occupy a single pit, or was forced to subtend multiple pits. We interpret these results in terms of a scaling theory of the free energy of polymer chains in a linear array of pits. Molecules contained within a single pit are predicted to face an entropic free energy barrier, and to hop between pits stochastically by thermally activated transport. Molecules that subtend multiple pits, on the other hand, can transfer DNA contour from upstream to downstream pits in response to an applied fluid flow, which lowers the energy barrier. When the trailing pit completely empties, or when the leading pit reaches its capacity, the energy barrier is predicted to vanish, and the low-pressure, thermally activated transport regime gives way to a high-pressure, deterministic transport regime. These results contribute to our understanding of polymers in nanoconfined environments, and can guide the design of nanoscale lab-on-a-chip applications for DNA analysis.
  • Wetting transition, drop impact, and

    microfow on hydrophobic microstructures

    Detlef Lohse (Universiteit Twente)
    Joint work with
    Peichun Amy Tsai1, Christophe Pirat1, Alisia M. Peters2, Rob Lammertink2,
    Matthias Wessling2,
    Sergio Pacheco3 and Leon
    Lefferts3.

    The poster presents several different wetting phenomena on
    structured and unstructured superhydrophobic surfaces, namely
    (i) an evaporation triggered wetting transition, at which a
    drop
    on a structured surface jumps from the Cassie-Baxter state to
    the
    Wenzel state,
    (ii) a drop impact on carbon nanofiber jungles, for which
    eithers droplet
    rebound or splashes are achieved, depending on the impact
    velocity, and
    (iii) the measurement of the effective slip-length over
    micro-grooves
    through micro-PIV.

    1Physics of Fluids Group,

    2Membrane Technology Group,

    3Catalyst Materials and Process Group,
    University of Twente, the Netherlands
  • Influence of ion sterics and hydrodynamic slip on

    electrophoresis of a colloidal particle

    Aditya Khair (University of California)
    The classical theory of a spherical colloids' electrophoretic mobility is founded on the Poisson-Nernst-Planck (PNP) equations and assumes the standard hydrodynamic no-slip boundary condition at the fluid/solid interface. In the (common) limit of thin double-layers, the mobility has long been known to exhibit a maximum at some zeta potential, then decrease and asymptote to a constant value. Dukhin, O'Brien, White and others showed this to result from the importance of excess ionic surface conductivity within the double-layer. The fundamental assumptions that underpin this result are, however, subject to challenge: in recent years, a finite liquid/solid slip has been measured over a variety of surfaces, and the PNP equations predict physically impossible ion concentrations precisely at the high zeta potentials where the mobility maximum occurs. Here, we discuss the dramatic effect that hydrodynamic slip and finite-ion-size steric effects in double-layers have upon the electrophoretic mobility of spherical colloids, and therefore upon the interpretation of electrophoretic mobility measurements.
  • Micro and nanoscale transport of biomolecules through

    pores

    A. Terrence Conlisk (The Ohio State University)
    Computational and theoretical models are developed for the transport of
    biomolecules and electrostatic and electrokinetic phenomena in nanopore membranes.
    For the application of nanopore sequencing, the electrophoretic transport of
    double stranded DNA molecules through a converging nanopore is investigated.
    The forces that affect the DNA translocation are analyzed and the DNA translocation
    velocity is predicted. The computational model is validated by good agreement
    between the computational results and the experimental data.

    Motivated by the design requirements for a hemofilter in an implantable artificial kidney,
    the hindered transport of biomolecules through a nanopore membrane is studied,
    particularly for the selectivity of the charged membrane to charged biomolecules of
    biological interest, particularly human serum albumin. The developed theory is
    applied to the problem of choosing a hemofilter pore size that provides adequate
    retention/clearance of desirable/undesirable solutes from blood.
  • Locomotion of synthetic nanomotors
    Jonathan Posner (Arizona State University)
    At ASU, we are investigating locomotion of bimetallic synthetic nanomotors that, analogous to their biological counterparts, harvest chemical energy from their local environment and convert it to useful work. Bimetallic nanorods can autonomously propel themselves at a hundred body lengths per second through aqueous solutions by using hydrogen peroxide as a fuel. Magnetic fields and electrochemically induced chemical species are used to control the motion of Pt-Ni-Au nanorods. We use the magnetic properties of nickel-loaded nanomotors to control their motion through micron-scale structures as well as the loading, transport, and release of spherical cargo that have volumes two orders of magnitude larger than the nanomotors itself. Nanomotor locomotion forces are determined by measuring their velocity while towing spherical cargo that have Stokes drag eight times the nanomotors themselves.

    Several physical arguments have been proposed to describe the physics underlying chemically-powered locomotion, but there is no detailed theory on the propulsion mechanism. We are simulating the physics of rod-shaped nanoparticles with asymmetric surface fluxes. Our models show that locomotion is driven by electric body forces in the fluid that arise due to finite space charge and internally generated electric fields surrounding the rod. The electric fields and charge density are generated by dipolar cation fluxes, such as those generated by heterogeneous electrochemical reactions with broken symmetry. The scaling analysis and detailed simulations predict that the nanomotor velocity depends on the reaction flux, nanorod electrical surface potential, solvent viscosity, and rod geometry.



  • Surface charge measurement and control by gate voltage in

    electroosmotic flow

    Frieder Mugele (Universiteit Twente)
    We present a simple analytical model that allows for determining the
    surface charge in electro-osmotic flow channels using the so-called
    solution displacement method. In contrast to earlier techniques, which
    have either been limited to small ratios of salt concentration or
    required a numerical solution of the convection-diffusion equation, our
    method provide a simple functional form with merely two fit parameters
    and thus allow for more accurate measurements of surface charge.
    Moreover, we demonstrate flow reversal inside our microfluidic channels
    controlled by gate electrodes underneath insulating layers that allow
    for external tuning of the surface charges. We discuss possible
    applications as a rheometer for applying shear forces to ultrasoft
    complex fluids.



  • Dielectrophoretic deflection and rebound of continuous droplet

    streams

    Thomas Jones (University of Rochester)
    Joint work with Paul Chiarot (Department of Electrical and Computer Engineering, University of Rochester).

    In continuous ink jet systems, streams of ~10 picoliter liquid droplets (diameter ~30 microns) are ejected from an array of orifices at rates of up to 350,000 per second and velocities in excess of 20 m/s. Applications as diverse as printing, microfabrication, and microarraying benefit from this technology; however, reliable manipulation of the jet, including basic on/off control and steering of droplet streams and individual liquid droplets, remains difficult to achieve. We have developed a novel deflection scheme to manipulate the trajectories of droplets rebounding at shallow angles from a solid substrate based on the dielectrophoretic force exerted by patterned electrodes. Droplet rebound, key to the performance of this scheme, has been investigated for both fluorocarbon (Teflon) and superhydrophobic surface coatings. Our experiments reveal interesting droplet behavior, and at least two regimes of operation, that are dependent on the Weber number and on the properties of the solid surface with which the droplets collide and rebound.

    This work was supported by a grant from Eastman Kodak Co. in Rochester, NY (USA).
  • High order quadratures for the evaluation of

    interfacial velocities in axi-symmetric Stokes flows

    Monika Nitsche (The Ohio State University)
    Boundary integral methods are computationally efficient
    in computing the evolution of interfaces in Stokes flow.
    For axisymmetric interfaces, they reduce to evaluating a
    1d integral at each time step. We have performed a detailed
    analysis of the structure of the integrands and show that
    standard methods of integration present two difficulties.
    One arises from loss of precision due to cancellation, the
    other from singular behaviour of the integrands near the
    axis of symmetry. As a result, high order quadrature
    proposed previously for these types of integrals are not
    uniformly high order. Instead, the maximal errors are
    always of second order. We propose a remedy to both
    difficulties and present a uniformly accurate 5th order
    approximation. This new quadrature is implemented to evolve
    (1) an initially bar-belled bubble that pinches at a
    point in finite time, and (2) a sphere in a strain flow
    that approaches a steady state. We compare the results
    with commonly used second order approximations and show
    that significant improvement is obtained using 5th order
    rules. The examples also illustrate when the corrections
    needed for uniformity have an impact in practice.
  • Understanding electrokinetics at the nanoscale: Beyond

    the limiting current

    Gilad Yossifon (Technion-Israel Institute of Technology)
    We examined the important over-limiting ionic current phenomenon, occurring at
    ion-permselective nanoporous membrane or nanochannel, and suggested a modified
    theoretical description of the entire nonlinear current-voltage curve based on
    the instability selected concentration-polarization layer thickness. In the
    process we discovered several curious and non-intuitive behaviors: 1) a
    nanoslot array with a uniform surface charge and height but with asymmetric
    slot entrances is shown to exhibit strong rectification, gating type
    current-voltage characteristics and a total current higher than the sum of
    isolated slots at a large voltage; 2) the vanishing of the limiting resistance
    voltage window with increased geometrical field focusing effect obtained by
    varying the nanoslot width. Hence, suggesting that an optimal pore
    radius/separation ratio exists for maximum current density across a membrane;
    3) strong nanocolloid-nanoslot interaction that leads to an additional
    transition region (or critical voltage) prior to the overlimiting region.
  • Capillary-driven thin-film flows on stationary and

    periodically-stretched substrates having isolated topographic features

    Gregory Chini (University of New Hampshire)
    The capillary-driven readjustment of thin liquid films subject to sudden,
    localized changes in shape or to periodic stretching of adjacent solid
    surfaces is important in a variety of industrial and physiological flow
    configurations. To investigate this process, we perform a combination of
    finite-difference numerical simulations and matched and multiple-scale
    asymptotic analyses of several related, simplified models. Thin films
    readjusting near isolated interior corners or large humps generically
    attain an intermediate-asymptotic state consisting of a corner puddle,
    a Jones--Wilson (or Hammond) draining region, through which fluid
    slowly drains into the puddle, and a far-field, propagating capillary wave.
    (For thin-film flows near small humps, the capillary wave attaches
    directly to the hump.) In the presence of distant lateral no-flux
    boundaries, the thin film ultimately reaches a quasi-steady configuration
    consisting of a droplet, a Jones--Wilson draining region, and a corner
    puddle, as has long been known. This quasi-steady film distribution is
    dramatically altered by the introduction of prescribed substrate stretching.
    At low frequencies, the pressure distribution becomes non-monotonic and the
    drainage region is rendered passive. A Bretherton region, which connects
    the corner puddle to a wedge-like region emerges, and drag-out and drag-in
    profiles are asymmetric. At high frequencies, the effects of the pressure
    oscillation are screened in a small neighborhood of the corner. This work
    is motivated by applications in pulmonary alveolar mechanics.
  • Electric field gradient focusing in microchannels with embedded bipolar electrode
    Ulrich Tallarek (Philipps-Universität Marburg)
    The complex interplay of electrophoretic, electroosmotic, bulk convective, and diffusive mass/charge transport in a hybrid poly(dimethylsiloxane) (PDMS)/glass microchannel with embedded floating electrode is analyzed. The thin floating electrode attached locally to the wall of the straight microchannel results in a redistribution of local field strength after the application of an external electric field. Together with faradaic reactions taking place at the bipolar electrode and buffer reactions, as well as bulk convection based on cathodic electroosmotic flow, an extended field gradient is formed in the anodic microchannel segment. It imparts a spatially dependent electrophoretic force on charged analytes and, in combination with the bulk convection, results in an electric field gradient focusing at analyte-specific positions. Analyte concentration in the enriched zone approaches a maximum value which is independent of its concentration in the supplying reservoirs. A simple approach is shown to unify the temporal behavior of the concentration factors under general conditions.
  • Strongly nonlinear dynamics of electrolytes in large ac

    voltages

    Henrik Bruus (Technical University of Denmark)
    Preprint (ArXiv)

    We study the response of a model micro-electrochemical cell to a large
    ac voltage of frequency comparable to the inverse cell relaxation time.
    To bring out the basic physics, we consider the simplest possible model
    of a symmetric binary electrolyte confined between parallel-plate
    blocking electrodes, ignoring any transverse instability or fluid flow.

    We analyze the resulting one-dimensional problem by matched
    asymptotic expansions in the limit of thin double layers and extend
    previous work into the strongly nonlinear regime, which is characterized
    by two novel features (1) significant salt depletion in the electrolyte
    near the electrodes and (2), at very large voltage, the breakdown of the
    quasi-equilibrium structure of the double layers. The former leads to
    the prediction of ac capacitive desalination, since there is a
    time-averaged transfer of salt from the bulk to the double layers, via
    oscillating diffusion layers. The latter is associated with transient
    diffusion limitation, which drives the formation and collapse of
    space-charge layers, even in the absence of any net Faradaic current
    through the cell.

    We also predict that steric effects of finite ion sizes (going
    beyond dilute solution theory) act to suppress the strongly nonlinear
    regime in the limit of concentrated electrolytes, ionic liquids and
    molten salts. Beyond the model problem, our reduced equations for thin
    double layers, based on uniformly valid matched asymptotic expansions,
    provide a useful mathematical framework to describe additional nonlinear
    responses to large ac voltages, such as Faradaic reactions,
    electro-osmotic instabilities, and induced-charge electrokinetic
    phenomena.

  • Dynamics of drops and vesicles in electric fields
    Petia Vlahovska (Dartmouth College)
    Drop deformation in uniform electric fields is a classic problem. The pioneering work of G.I.Taylor demonstrated that for weakly conducting media, the drop fluid undergoes a toroidal flow and the drop adopts a prolate or oblate spheroidal shape, the flow and shape being axisymmetrically aligned with the applied field. However, recent studies have revealed a nonaxisymmetric rotational mode for drops of lower conductivity than the surrounding medium, similar to the rotation of solid dielectric spheres observed by Quincke in the 19th century.

    I will present an experimental and theoretical study of this phenomenon in DC fields. The critical electric field, drop inclination angle, and rate of rotation are measured. For small, high viscosity drops, the threshold field strength is well approximated by the Quincke rotation criterion. Reducing the viscosity ratio shifts the onset for rotation to stronger fields. The drop inclination angle increases with field strength. The rotation rate is approximately given by the inverse Maxwell-Wagner polarization time. We also observe a hysteresis in the tilt angle for low-viscosity drops.

    I will also discuss our work on drops encapsulated by complex interfaces such as lipid bilayer membranes. A comparison between the behavior of drops and giant vesicles (cell-size lipid membrane sacs) highlights new features due to the membrane electromechanics.

    This work is in collaboration with Paul Salipante (Dartmouth) and Dr. Rumiana Dimova’s group (Max Planck Institute of Colloids and Interfaces).
  • Interfacial dynamics of colloidal particles in

    electrokinetically driven flows measured by multilayer

    nano-particle image velocimetry (MnPIV)

    Yutaka Kazoe (Georgia Institute of Technology)
    The transport and dynamics of colloidal particles
    near a solid-liquid interface (i.e., the wall) is important in
    many microfluidic applications, including microscale
    particle-image velocimetry (PIV). Experimental studies using
    total internal reflection microscopy to study near-wall
    colloidal particle dynamics have for the most part only
    considered a single particle in a quiescent fluid. In contrast,
    our group has developed an evanescent wave-based technique that
    analyzes the dynamics of ensembles of up to
    O(105) near-wall
    colloidal tracers, multilayer nano-particle image velocimetry
    (MnPIV). The technique exploits the exponentially decaying
    intensity of evanescent-wave illumination, to extracts
    near-wall particle distributions and flow velocities at
    different distances from the wall, all within about 500 nm of
    the wall. The technique has already been validated for steady
    and creeping Poiseuille flow, where the shear rates were found
    to be within about 5% of analytical predictions. In this
    study, we use MnPIV to investigate electrokinetically driven
    flows through fused-silica microchannels about 40 microns deep.
    The results for 100 nm to 500 nm diameter tracers show that the
    flows are uniform with constant electroosmotic mobility, and
    that the Brownian diffusion coefficients for tangential
    fluctuations are within 7% of the Faxén relation. The particle
    distributions near the wall are, however, in all cases, highly
    nonuniform, with very few particles within 100 nm of the wall
    due to electrostatic and van der Waals effects. Finally, the
    near-wall distribution of the 500 nm tracers are shown to vary
    with applied electric field, due presumably to
    dielectrophoresis and perhaps induced-charge electroosmosis.
  • Multi-physics computational models for neuro-chip

    simulation

    Riccardo Sacco (Politecnico di Milano)
    Neuro-chips (NCs) are bio-hybrid devices in which living brain cells
    and silicon circuits are coupled together. NCs are presently being
    used as a non-invasive technique to record cellular response to drugs,
    and are expected to be used in the cure of neurological disorders
    through the creation of sophisticated neural prostheses.
    The main technological challenge in the design of NCs is the
    efficient transduction of the input biological signal (ion current of
    the order of nA) into an output signal (electrical current) which is
    modulated by the effective driving voltage of the open Gate of
    the silicon device (of the order of mV). In order to devise a
    sound simulation tool of the I/O behavior of a NC device, we
    propose a multi-physics computational model including:


    1) the Poisson-Nernst-Planck system, to account for intracellular and
    extracellular electrochemical ion transport;

    2) the Hodgkin-Huxley system, to describe ion transport across
    membrane channels;

    3) a nonlinear MOS capacitor approximation, to account for
    cell-to-chip coupling.


    The nonlinear system arising from the coupled solution of
    1)-3) is successively solved by a functional iteration procedure,
    and for each time level of the simulation, each obtained
    sub-problem is numerically solved using a stabilized mixed-hybridized
    finite element discretization scheme.
    In order to provide a successful validation of the computational
    procedure, we discuss preliminary results on two cases of physiological
    interest, namely, the Hodgkin-Huxley axon and the response of a
    field-effect transistor with metal-free gate oxide under an
    intracellular voltage depolarization stimulating impulse.
  • A fluid mechanical origin of sheet ejection during droplet impacting a dry surface
    Shreyas Mandre (Harvard University)
    No abstract
  • Enhancement of charged macromolecule capture by nanopores in a salt gradient
    Tom Chou (University of California, Los Angeles)
    An theoretical analysis is performed to explain recently observations that salt gradients across a nanopore can increase charged analyte capture rates.