Session H21: Focus Session: Microfluidic Physics I

Experimental Studies of the Effects of Mixing on Reacting Systems

Experimental studies of the effects of mixing on reacting systems are presented. The experiments can be divided into two classifications: (1) the effects of chaotic mixing on front propagation and (2) synchronization via superdiffusive mixing in an extended, fluid system. The front propagation studies are conducted in an oscillating vortex chain flow. The velocities of the propagating fronts are measured as a function of the frequency and amplitude of the external forcing. In the absence of mixing the Fisher-Kolmogorov result correctly predicts the front velocity; however these experiments show that this result is not extendable to chaotically mixed systems. Instead, the fronts are shown to mode-lock onto the external forcing, propagating an integer number of vortices in an integer number of drive periods. The flow used in the synchronization studies is an oscillating/drifting vortex chain, which may be used to produce both enhanced diffusion and superdiffusion. We show that the key to synchronization in an extended, fluid system is superdiffusive transport produced by L\'{e}vy flights, where tracers undergo rapid jumps between distant regions of the flow.   

Electrophoresis of Large DNA Molecules in Microcontractions

The ability to controllably position and stretch large DNA molecules in a microfluidic format is important for gene mapping technologies such as Direct Linear Analysis (DLA). Current technologies developed for DLA use controlled hydrodynamic flows created in a microfluidic device. The downside to this approach is that the imposition of the no-slip condition at the channel walls generates vorticity which can lead to DNA chain tumbling and incomplete stretching. We have recently shown that electric field gradients can be readily generated in a microfluidic device and the resulting field is purely elongational. We present here single molecule studies of DNA molecules driven by an electric field through a microfabricated contraction. Analogous to the hydrodynamic deformation of DNA, we can define an electrophoretic Deborah number (De) for our problem. We will discuss the effectiveness of the device to fully stretch DNA as a function of De and compare to stretching achieved in hydrodynamic flows. A detailed analysis of molecular stretching and the role of a non-homogeneous electric field will be discussed.   

Measuring Streaming Current/Potential in Microchannel Arrays

Streaming current/potential measurements have been commonly used to estimate interfacial properties. This paper explores challenges in conducting these measurements in an array of parallel microchannels, which is akin to flow through porous media. The issue that arise with these arrays is that increasing the number of channels subsequently increases the total conductance across an array. In situations with a large number of channels this array conductance can become comparable to the conductance in the bulk fluid in the reservoirs where electrodes are placed. In these cases, current drawn through an external electrical circuit connecting the two reservoirs (i.e. streaming current) become highly dependent on the location, material and surface area of the electrodes. However, with fewer channels the relative magnitude of conductances can be made such that this externally measured current is independent of these parameters and more representative of the streaming current. Streaming potential measurement, since they do not involve external current flow, also do not show these dependencies. In this study variations in the electrode materials (bright platinum, platinized platinum, silver and stain steel), size of electrodes, placement of the electrodes and electrolyte concentration ($10^{-3}$M KCL, $10^{-4}$M KCL and 0 M KCL ) were used to affect the relative conductance in the system and to highlight these characteristics.   

Electrophoretic extraction of ions from a pressure-driven flow

Coupling pressure-driven and electrokinetically driven flow streams in microfluidics is a critical issue for developing multi-dimensional separations systems. A promising method of coupling these flows is to electrokinetically extract the charged components from the pressure driven flow stream while minimizing the hydrodynamic flow in the electrokinetically driven channel. To model this extraction process we calculate the fraction f of ions in a pressure-driven microchannel that are diverted electrophoretically to a perpendicular side channel. The channel cross sections are rectangular, with aspect ratio $\gamma $. In the main channel, we use truncations of an exact series solution to describe the laminar velocity profile of the aqueous solution. The aqueous solution in the side channel is stationary; individual ions move through this channel in response to a uniform applied electric field at an electrophoretic velocity that is proportional to this field. We calculate f as a function of $\gamma $ and the ratio R between the flow rate in the main stream and that in the side stream. We find that f decreases with increasing R, as expected, and is nearly independent of $\gamma $.   

The effect of fluid density on the transport of particles in nanochannels

Understanding particle transport in nanochannels is crucial for the development of micro and nanofluidic devices. In this work, we investigate the effect of fluid number density on the transport of particles in nanochannels, by means of molecular dynamics simulations. Specifically, we examine the motion of a Lennard-Jones nanoparticle, under the action of a constant external force, in a Platinum nanochannel that contains a Lennard-Jones fluid. In the limiting case of a nanochannel free of fluid molecules the particle adsorbs to the surface of the nanochannel and moves at a very low velocity, due to dry friction with the wall. As the number density of the fluid increases the mobility of the nanoparticle is greatly enhanced, due to the formation of adsorbed fluid layers on the surface of both the nanochannel and the particle, which substantially reduce friction between the particle and the wall. Then, if the number density of the fluid is increased further the particle mobility drops, due to viscous drag. In fact, there is an optimal value at low fluid densities, at which the particle mobility can be significantly enhanced. We also examine the existence of a second peak at higher densities, when the fluid density is high enough to prevent the adsorption of the nanoparticles, and how these phenomena depend on the fluid-solid molecular interactions.   

Molecular Dynamics simulations of polymers in Brownian ratchets.

Brownian ratchets rely on a combination of thermal {\it noise} and an asymmetry in a system to induce directed transport of particles (e.g., pumping in ion channels). This is somewhat counter intuitive since thermal motion is often a detriment to transport mechanisms. In particular a Brownian ratchet can be used to manipulate polymers, for example in separation systems. We present a Molecular Dynamics study (with explicit hydrodynamic interactions) of short polymer chains in a fluid subjected to a periodic, asymmetric, saw-tooth potential (with zero net force) which is switched on and off for given time intervals, $\tau_{\mathrm{on}}$ and $\tau_{\mathrm{off}}$ repectively. We examine how variations of $\tau_{\mathrm{on}}$ and $\tau_{\mathrm{off}}$ affect the net migration of the polymer chains. We also examine how the width of the trapping potential and the degree of asymmetry affects the dynamics of the molecules.   

Self-Assembly of Paramagnetic Beads in Rotating Magnetic Fields

Paramagnetic beads, about 1 $\mu m$ in diameter, suspended in a liquid will aggregate to form chains when an initially random dispersion is subject to a uniform, static magnetic field. In a rotating field, the chains deform and, depending on the rotation rate, form S-shaped chains or aggregate clusters. A correct determination of the final shape requires an accurate calculation of the interparticle forces. We developed new methods to efficiently and accurately calculate the far-field and near-field magnetic interactions. Hydrodynamic interactions are resolved through the force-coupling method. We study the dynamics of single chains and suspensions of beads in rotating fields using these models and compare results from our simulations with recent experiments by Melle et. al. (Phys. Rev. E \textbf{68}, 041503). At high rotations rates, the observed particle oscillations provide information on the particle properties affecting near-contact hydrodynamic forces.   

Semiflexible magnetic filaments

Extension of the Kirchhoff model of an elastic rod by taking into account the long-range magnetic interactions allows one to describe the semiflexible filaments with body couples. Their behaviour in some aspects is similar to the flagellas of different microorganisms driven by internal torques due to molecular motors. Basing on the model different new phenomena are described - buckling instability due to the action of body torques, selfpropulsion of the filament in an ac field, a periodic regime of the magnetic filament motion under the action of the shear flow and the field and others. Taking into account the thermal noise the crossover from $t^ {3/4}$ to $t^{1/2}$ for the time dependence of the mean square displacement of the filament at magnetic field increase is predicted. The characteristics of semiflexible magnetic filaments can be studied by measuring their magnetic susceptibility in small ac magnetic field for which in the high- frequency range the scaling law $\omega ^{-3/4}$ is obtained. Application of these results for the study of the properties of magnetotactic bacteria is discussed.   

Using nanowires to perform in-vivo measurements of elastic and viscous properties of an anisotropic fluid

The immersion of a small wire within an anisotropic fluid is studied using a lattice Boltzmann algorithm. A magnetic field is used to manipulate and rotate the wire. The field and the anisotropic fluid each impose a torque on the wire. Our simulations agree well with experiments on the dynamics of high aspect ratio wires within a liquid crystal. In addition, our simulations are able to extend the range of predictive measurements to low aspect ratio wires, more suitable for use in biological environments. We are able to predict elastic and viscous properties of the anisotropic fluid environment based on the torque response of the rotating wire.   

Experimental and theoretical study of mixing and transport due to the motion of a slender body sweeping out a cone.

We have used singularity theory to construct an exact solution for the fluid motion induced by a spheriod spinning about its center sweeping out a double cone in a low Reynolds number flow. We have additionally used slender body theory to construct an asymptotic solution for a slender cylinder attached to a no-slip plane spinning about its base sweeping out an upright cone. These time-varying, three- dimensional hydrodynamic solutions have been used to benchmark micro-fluidic experiments which have immediate consequences to understanding transport and mixing in ciliated tissues. A similar macro-scale experiment that is absent of thermal flucutations has been designed which validates the theory.   

Field Effect Modulation of Ion Transport in Single Nanotubes

Field effect control in metal-oxide semiconductor field effect transistors (MOSFETs) has revolutionized how information is processed and stored, and created the modern digital age. Introducing field effect in fluidic systems would enable the manipulation of ionic and molecular species at a similar level and even logic operation. Due to strong Debye screening, field effect control in ionic solutions has to be occurring in nanoscale. Here we present the integration of chemically synthesized inorganic nanotubes into metal-oxide-solution field effect transistors (MOSolFETs), and demonstrated a rapid field effect modulation of ionic conductance. Surface modification, functioning as doping in semiconductors, alters the nanofluidic transistors from p-type field effect transistors, to ambipolar FETs, and n-type field effect transistors. Ambipolar behavior is of special interests in this gapless transport system. Possion-Boltzmann model has been employed to extract two key physical parameters -- zeta potential and surface charge density. Furthermore, transient study was conducted, leading to the first kinetic model of field effect in ionic solutions. Nanofluidic FETs would be the key elements in sub-femtoliter analytical techniques and the integration of large-scale nanofluidic circuits.   

Performance based applications of the Ultrasound Contrast Agents in the bio-medical field

Ultrasound Contrast Agents are micron size bubbles encapsulated by nanometer-thick layer of surface active materials such as proteins and lipids. They are injected to patients to improve the quality of ultrasound images. They are also being used for drug delivery and arteriogenesis. We present results of \textit{in-vitro }ultrasound investigation on two such contrast agents, Definity and Optison. We measure attenuation and scattering of ultrasound through emulsion of these agents. We investigate destruction of contrast agents and measure sub- and super-harmonic contents in their scattered response. Optison has a much lower threshold excitation level compared to Definity. Definity has a persistent sub-harmonic generation compared to Optison, for which the sub-harmonics go down above a certain pressure level. Both agents experiences transient bubble growth at lower excitation pressures due to increased permeability of the membrane to dissolved air outside. The results along with their implications on the applications such as drug delivery and imaging will be discussed.   

Tuning the orbital angular momentum in optical vortices

Optically-driven micromachines rely upon the precise definition of the intensity distribution and the angular momentum content of the controlling light fields. One such manipulation tool is the optical vortex (OV), which employs orbital angular momentum to spin particles around a ring of light. The orbital angular momentum of an OV is tuned by changing the helicity of its electric field's wavefronts or by tuning the input power. However, changing wavefront helicity has the undesirable effect of altering the vortex diameter. Thus, making complex patterns of OVs with fixed sizes but adjustable rotational frequencies is difficult. We introduce a new class of OVs with an additional independent tuning parameter to overcome these limitations. With these OVs, it is possible to smoothly increase particles' rotational frequency without changing the radius or power. We show that this tunability can be extended to groups of OVs with similar or different radii, allowing for complete flexibility to construct optical micromachines, or large arrays of OVs for parallel assays of biomolecules and cells.