# <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/>

**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 u_{t}= u_{xx}+ 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 Tsai^{1}, Christophe Pirat^{1}, Alisia M. Peters^{2}, Rob Lammertink^{2},

Matthias Wessling^{2},

Sergio Pacheco^{3}and Leon

Lefferts^{3}.

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.^{1}Physics of Fluids Group,^{2}Membrane Technology Group,^{3}Catalyst 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*(10^{5}) 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.