Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L10: Microscale Flows: Particles |
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Chair: Haim Bau, University of Pennsylvania Room: 3005 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L10.00001: Towards mico-ThFFF for polymer analysis: Lattice-Boltzmann based simulations Michael Antonelli, Jennifer Kreft Pearce Thermophoresis describes a behavior, observed at micro-scales, in which particles migrate due to a temperature gradient. The purpose of this project is to study the parameters that have the greatest effect on thermophoresis and to use these properties to design a device for separating biological macromolecules using extremely small samples. A Lattice-Boltzmann based computer simulation of a microfluidic cell was used to determine the conditions under which DNA molecules, in a buffered salt solution, will exhibit this phenomenon. The simulation monitored particle positions within the cell, beginning from random initial conditions. Particle-solvent and particle-particle interactions were examined. Particle-particle interactions were modeled using the Lennard-Jones potential. By modifying the distance at which potential is minimized as well as the magnitude of the potential, conditions that increase the response of the molecule to the temperature gradient were observed. Once satisfactory conditions had been determined, separation of particles in a theoretical microfluidic device was simulated. The periodic boundary conditions were changed and a more dynamic channel was modeled. Unidirectional flow fields as well as particles with differing thermophoretic properties were simulated in the micro-channel and their concentrations across the channel measured. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L10.00002: Glancing, reversing, tumbling, and sliding: sedimentation near walls in viscous fluids William Mitchell, Saverio Spagnolie The sedimentation of ellipsoidal particles near a wall in a viscous fluid has been studied from a numerical perspective by a number of authors, but analytical solutions have been given only in special cases, such as for spherical particles. As an application of the method of images, the dynamics of ellipsoids of arbitrary aspect ratio in a wall-bounded Stokes flow may be reduced to a system of ordinary differential equations. In many cases the system leads to analytical descriptions of the particle motion which agree very well with full numerical simulations. As an application, we investigate the conditions under which the ``glancing'' and ``reversing'' trajectories first observed by Russel et al. prevail, and we identify two new possibilities: a periodic ``tumbling'' trajectory for nearly spherical bodies and a ``sliding'' trajectory which occurs when the wall is inclined at a small angle from the vertical. The sliding trajectory is an attracting fixed point for the dynamics, and thus may have applications in sorting processes for heterogeneous dilute suspensions. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L10.00003: Model colloid system for interfacial sorption kinetics Paul Salipante, Steven Hudson Adsorption kinetics of nanometer scale molecules, such as proteins at interfaces, is usually determined through measurements of surface coverage. Their small size limits the ability to directly observe individual molecule behavior. To better understand the behavior of nanometer size molecules and the effect on interfacial kinetics, we use micron size colloids with a weak interfacial interaction potential as a model system. Thus, the interaction strength is comparable to many nanoscale systems (less than 10 k$_{\mathrm{B}}$T). The colloid-interface interaction potential is tuned using a combination of depletion, electrostatic, and gravitational forces. The colloids transition between an entropically trapped adsorbed state and a desorbed state through Brownian motion. Observations are made using an LED-based Total Internal Reflection Microscopy (TIRM) setup. The observed adsorption and desorption rates are compared theoretical predictions based on the measured interaction potential and near wall particle diffusivity. This experimental system also allows for the study of more complex dynamics such as nonspherical colloids and collective effects at higher concentrations. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L10.00004: Theory and simulation of acoustic interaction forces between small particles in an ideal fluid Henrik Bruus, Glauber T. Silva We present a theoretical expression for the acoustic interaction force between small spherical particles suspended in an ideal fluid exposed to an external acoustic wave as used in, say, microchannel acoustophoresis. The acoustic interaction force is the part of the acoustic radiation force on one given particle involving the scattered waves from the other particles. The particles, either compressible liquid droplets or elastic microspheres, are considered to be much smaller than the acoustic wavelength. In this so-called Rayleigh limit, the acoustic interaction forces between the particles are well approximated by gradients of pair-interaction potentials with no restriction on the inter-particle distance. The theory is applied to studies of the acoustic interaction force on a particle suspension in either standing or traveling plane waves. The results show aggregation regions along the wave propagation direction, while particles may attract or repel each other in the transverse direction. In addition, a mean-field approximation is developed to describe the acoustic interaction force in an oil-in-water emulsion. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L10.00005: Clogging in a microfluidic hourglass Alvaro Marin, Massimiliano Rossi, Christian J. K\"ahler One of the main disadvantages of microfluidic devices is their tendency to clog when a high density of particles or droplets is forced through them. The same problem is often encountered in classical granular flows in silos and hourglasses. It is well-known that hourglasses work optimally when the particle-to-neck ratio is within certain ratio without interruption (Zuriguel et al., Phys. Rev. E, 2003), while arching occurs for particle-to-neck ratios above $d/D \approx 2$. Microfluidic devices normally work in geometries in which $d/D>10$, in which the arching probability is negligible. Clogging is nonetheless possible, but mainly due to the accumulation of particles at the walls (Wyss et al, Phys. Rev. E, 2006). On the other hand, clogging by arching in systems with $d/D \sim O(1)$ are expected to have radically different physics and statistics, due to collective behavior and hydrodynamic interactions. To study these regimes, we study microfluidic devices with a bottleneck of squared crossed section and side length D through which we force polystyrene particles with diameters from $d/D \approx 1$ to $0.25$ at packing fractions ranging from 10\% up to 50\%. Our results show that clogging of such systems have more in common with granular flows in hourglasses than expected. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L10.00006: Dynamics of flexible fibers in shear flow Agnieszka Slowicka, Eligiusz Wajnryb, Maria Ekiel-Jezewska We consider dynamics and shape evolution of a flexible non-Brownian fiber in steady shear flow under low-Reynolds-number. Fibers are described by the bead-spring model. Their evolution is determined by solving the Stokes equations with the use of the multipole method, corrected for lubrication within the accurate numerical code HYDROMULTIPOLE. The fibers are initially aligned with the ambient flow. Owing to symmetry, their motion takes place in the plane perpendicular to vorticity direction. We investigate migration of fibers across the flow and quantify their shape evolution. Depending on the ratio of the fiber bending energy to its hydrodynamic energy, we find out different modes of the dynamics. Distinction between these modes is based on values of the fiber migration velocity, its tumbling frequency, curvature and length of the end-to-end distance. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L10.00007: Nanohybrid particle-particle interaction in Dissipative Particle Dynamics (DPD) simulations Minh Vo, Dimitrios Papavassiliou Carbon nanotube (CNT) hybrid particles have recently received attention in hydrocarbon reservoir technology due to their ability to stabilize water/oil interface. CNTs tend to agglomerate in solution, so polymers are used to prevent this phenomenon forming nanohybrid (NH) particles (i.e., CNT-polymer particles). In the presence of PVP polymer, CNTs can be dispersed and stabilized successfully. In this work, the coarse graining DPD method is utilized to explore NH particle interactions in water. The NH particles are created after the equilibrium of the system with cylindrical CNTs and polymers is reached. To compute the interaction force, one NH particle is stationary and another is moving around it. Then, the effect of distance and angle between the two main axes of the particles on the interaction force is determined. Based on these data, a general equation to describe this interaction is obtained. Besides, different sizes of particles are considered in order to find out the effect of the CNT aspect ratio on the interaction force. Additionally, the steric effect of polymer on particle-particle interaction is studied. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L10.00008: Hydrodynamic interactions for complex-shaped nanocarriers in targeted drug delivery Yaohong Wang, David Eckmann, Ravi Radhakrishnan, Portonovo Ayyaswamy Nanocarrier motion in a blood vessel involves hydrodynamic and Brownian interactions, which collectively dictate the efficacy in targeted drug delivery. The shape of nanocarriers plays a crucial role in drug delivery. In order to quantify the flow and association properties of elliptical nanoparticles, we have developed an arbitrary Lagrangian-Eulerian framework with capabilities to simulate the hydrodynamic motion of nanoparticles of arbitrary shapes. We introduce the quaternions for rotational motion, and two collision models, namely, (a) an impulse-based model for wall--particle collision, and (b) the short-range repulsive Gay-Berne potential for particle-particle collision. We also study the red blood cell and nanocarrier (such as ellipsoid) interactions. We compare our results with those obtained for a hard sphere model for both RBCs and nanocarriers. Supported by NIH through grant U01-EB016027. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L10.00009: Analysis of angle effect on particle flocculation in branch flow Karthik Prasad, Kathryn Fink, Dorian Liepmann Hollow point microneedle drug delivery systems are known to be highly susceptible to blockage, owing to their very small structures. This problem has been especially noted when delivering suspended particle solutions, such as vaccines. Attempts to reduce particle flocculation in such devices through surface treatments of the particles have been largely unsuccessful. Furthermore, the particle clog only forms at the mouths of the microneedle structures, leaving the downstream walls clear. This implies that the sudden change in length scales alter the hydrodynamic interactions, creating the conditions for particle flocculation. However, while it is known that particle flocculation occurs, the physics behind the event are obscure. We utilize micro-PIV to observe how the occurrence and formation of particle flocculation changes in relation to the angle encountered by particle laden flow into microfluidic branch structures. The results offer the ability to optimize particle flocculation in MEMS devices, increasing device efficacy and longevity. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L10.00010: Capillary Interactions of Micro-particles on Curved Interfaces Nima Sharifi-Mood, Lu Yao, Iris Liu, Kathleen Stebe Microparticles trapped at fluid interfaces interact by capillarity to migrate, form structures and find preferred locations. These phenomena are exploited to organize colloids at fluid interfaces, impacting emulsion stabilization and forming the basis for advanced materials which exploit, e.g. the mechanics or optical properties of the structures which form. Interface curvature plays a strong role in microparticle behavior by acting as a field which directs microparticle migration. Here, we discuss the behavior of microparticles with pinned contact lines at the oil-water interface on interfaces with well-defined curvature fields. Once the particles attach to the interface, they migrate in deterministic paths towards sites of high curvature. These experiments are well described by our analysis. We theoretically determine the disturbance field imposed by particles via an asymptotic analysis, and have quantified the associated capillary energy and the capillary force. Forces can be understood simply in terms of slope variation of the disturbed interface along the contact line. Capillary energies are expressed in closed form as a function of mean and deviatoric curvatures of the interface prior to the particles deposition. Pair interactions between particles will be also discussed. [Preview Abstract] |
(Author Not Attending)
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L10.00011: Controlling Lateral Inertial Migration Rate of Particles in Microchannels Armin Karimi, Samuel Bray, Dino Di Carlo It was previously demonstrated that particles in confined channels can migrate across streams due to the net inertial lift force acting on them. The initial location of particles within the channel cross-section is shown to effect the migration time as particles starting at different locations experience a different history of lift forces. This initial variation in distribution of focusing positions of particles upstream was a limiting factor in achieving precise control over the migration time in previous studies. In order to improve uniformity of the focusing position, a set of sequential cylindrical pillars is integrated to one side of the channel which is shown to aid particles in achieving a single stable equilibrium position, by inducing a net helical flow. The modified focusing positions are characterized as a function of pillar diameter and spacing for various channel Reynolds numbers. Using this initial focusing channel, a comprehensive numerical and experimental study is performed to characterize the range of lateral migration rate for particles as a function of particle position, and flow rates of each stream for a given finite Reynolds number and channel geometry. The tool developed in this study can be used to achieve precise migration characteristics for the microparticles crossing fluid streams in microchannels. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L10.00012: Tuning particle focusing in inertial microfluidic devices Kaitlyn Hood, Soroush Kahkeshani, Dino di Carlo, Marcus Roper Particles in microfluidic devices at finite Reynolds number are subject to two forces: (i) inertial focusing and (ii) particle-particle interactions. Although microfluidic chips exploit these forces to manipulate particles for particle/cell sorting and high throughput flow cytometry, the forces are not understood well enough to allow rational design of devices that can tune and attenuate particle focusing. We present a mathematical model addressing both inertial focusing and particle interactions, and we apply our model to various channel geometries to determine the balance of forces. In addition, we present experimental data that illustrate the accuracy of our model. We will address the following questions: Why do high aspect ratio channels favor two equilibrium positions? Why do particle chains form? [Preview Abstract] |
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