### Session HM: Microfluids: General V: Particle Manipulation

Chair: Metin Muradoglu, Koc University, Turkey
Room: Long Beach Convention Center 202B

 Monday, November 22, 2010 10:30AM - 10:43AM HM.00001: Manipulating bacteria with opto-electrokinetic methods Steve Wereley , Jae-Sung Kwon , Sandeep Ravindranath , Joseph Irudayaraj Recently we developed an opto-electrokinetic method for manipulating particles and cells called Rapid Electrokinetic Patterning (REP). REP is a very fast method for manipulating thousands of particles simultaneously and controllably owing to the creation of an electrothermal vortex that transports particles rapidly and in parallel to a site determined by the focal point of a laser beam. Whether particles are trapped at the center of the vortex or not is determined by their electrical properties (conductivity and permittivity). In this talk we demonstrate that REP can be used to manipulate the bacterium Shewanella oneidensis MR-1. The bacteria are assembled into large planar arrays of organisms. The dependence of this assembly process on voltage and frequency is quantified. REP can even be used to selectively manipulate and collect live or dead bacteria. Monday, November 22, 2010 10:43AM - 10:56AM HM.00002: The Diffusiophoretic Self-Propulsion of Patchy Particles Driven By a Catalytic Reaction on the Particle Surface Nima Sharifi Mood , Charles Maldarelli , Joel Koplik , Ilona Kretzschmar , Amar Pawar The autonomous motion generated by using a catalytic reaction on part of the surface of a particle to generate a concentration gradient across the particle and an attendent diffusophoretic propulsion has received the most attention. In this presentation, we provide a theoretical description of the effect of the intermolecular forces on the hydrodynamic propulsion. The analysis is undertaken in the limit in which the fluxes generated by diffusion and the intermolecular forces are larger than the convective flux, the flow is inertialess and the catalytic reaction rate is infinite. Finite element calculations are used to determine the concentration fields as a function of the size of the cap. The velocity field and propulsion velocity are computed analytically in the low Reynolds number limit in terms of the moments of the concentration field. This propulsion velocity is compared with our own recent experimental measurements of this motor. Monday, November 22, 2010 10:56AM - 11:09AM HM.00003: Control of Biologically Inspired Robotic Microswimmers U. Kei Cheang , Jun Hee Lee , Dheeraj Roy , Min Jun Kim Flagella have been employed as nanoactuators for biomimetic microswimmers in low Reynolds number fluidic environments. The microswimmers utilize flagellar filaments isolated from Salmonella typhimurium to mimic the spiral-type propulsion mechanism of flagellated bacteria. The microswimmer included a polystyrene microbead conjugated to one or multiple magnetic nanobeads via flagellar filaments using avidin-biotin linkages. Wireless propulsion energy was supplied to magnetic bead by an AC magnetic field, which in turn rotate the bead and induce spiral-type swimming. A magnetic controller consisted of electromagnetic coils arranged in an approximate Helmholtz configuration was designed and constructed. In conjunction with a LabVIEW input interface, a DAQ controller was used as a function generator to induce AC current outputs from the power supply to the magnetic controller in order to generate an AC magnetic field. Numerical analysis was performed to characterize the magnetic controller. A high-speed camera provided real-time imaging of the microswimmer motion in a static fluidic environment. The robotic microswimmers exhibited active propulsion under an AC magnetic field, which demonstrates the possibility for future biomedical applications for drug delivery. Monday, November 22, 2010 11:09AM - 11:22AM HM.00004: Ciliar fluid propulsion in a non-Newtonian liquid Michiel Baltussen , Patric Anderson , Jaap den Toonder Natural as well as artificial cilia are used to propel fluids, or propel an animal or object through a fluid. Although the fluid is often water, other more complex fluids such as saliva and mucus are also common. These fluids show a non-constant viscosity over a range of shear rates and are hence non-Newtonian. We model a single elastic cilium in a periodic domain in both a Newtonian as well as a non-Newtonian matrix fluid. The non-Newtonian fluid model is fitted on human saliva. A body force, which is asymmetric in time, is applied to the cilium. This causes a symmetric motion of the cilium for the Newtonian case, while the motion is asymmetric for the non-Newtonian case. Due to the asymmetric motion fluid is transported in the non-Newtonian case. Monday, November 22, 2010 11:22AM - 11:35AM HM.00005: Trapping, focusing, and sorting of microparticles through bubble streaming Cheng Wang , Shreyas Jalikop , Sascha Hilgenfeldt Ultrasound-driven oscillating microbubbles can set up vigorous steady streaming flows around the bubbles. In contrast to previous work, we make use of the interaction between the bubble streaming and the streaming induced around mobile particles close to the bubble. Our experiment superimposes a unidirectional Poiseuille flow containing a well-mixed suspension of neutrally buoyant particles with the bubble streaming. The particle-size dependence of the particle-bubble interaction selects which particles are transported and which particles are trapped near the bubbles. The sizes selected for can be far smaller than any scale imposed by the device geometry, and the selection mechanism is purely passive. Changing the amplitude and frequency of ultrasound driving, we can further control focusing and sorting of the trapped particles, leading to the emergence of sharply defined monodisperse particle streams within a much wider channel. Optimizing parameters for focusing and sorting are presented. The technique is applicable in important fields like cell sorting and drug delivery. Monday, November 22, 2010 11:35AM - 11:48AM HM.00006: Controlling particle trajectories using oscillating microbubbles Shreyas Jalikop , Cheng Wang , Sascha Hilgenfeldt In many applications of microfluidics and biotechnology, such as cytometry and drug delivery, it is vital to manipulate the trajectories of microparticles such as vesicles or cells. On this small scale, inertial or gravitational effects are often too weak to exploit. We propose a mechanism to selectively trap and direct particles based on their size in creeping transport flows ($Re\ll 1$). We employ Rayleigh-Nyborg-Westervelt (RNW) streaming generated by an oscillating microbubble, which in turn generates a streaming flow component around the mobile particles. The result is an attractive interaction that draws the particle closer to the bubble. The impenetrability of the bubble interface destroys time-reversal symmetry and forces the particles onto either narrow trajectory bundles or well-defined closed trajectories, where they are trapped. The effect is dependent on particle size and thus allows for the passive focusing and sorting of selected sizes, on scales much smaller than the geometry of the microfluidic device. The device could eliminate the need for complicated microchannel designs with external magnetic or electric fields in applications such as particle focusing and size-based sorting. Monday, November 22, 2010 11:48AM - 12:01PM HM.00007: Capillary threads and droplet-decorated streams in microchannels Samira Darvishi , Thomas Cubaud We investigate the evolution of high-viscosity fluid threads flowing in a sheath of immiscible liquid in a diverging/plane microchannel. A steady viscous-core annular flow is produced upstream in a square microchannel. Downstream, the fluids enter a diverging channel that causes the thread to bend and deform into a complex microstructure having a large interfacial area. Using a variety of fluids, we study the effect of interfacial tension, viscosities, and flow rates on the flow morphology and we characterize the evolution of the thread thickness, arc length, fold wavelength, and envelope amplitude in the plane and straight microchannel downstream from the divergence. In particular, we focus on the coalescence mechanism between adjacent folds that can produce small droplets embedded into a highly viscous matrix (i.e., droplet-decorated'' streams). Other original phenomena, such as capillary breakup by folding at high capillary numbers and secondary folding, are also examined. Monday, November 22, 2010 12:01PM - 12:14PM HM.00008: Inertially Stabilized Microfluidic Crystals: Self-assembly and Spatial Frequency Tuning Wonhee Lee , Hamed Amini , Howard Stone , Dino Di Carlo Dynamic self-assembly can be found over a wide range of scales originating from different fundamental opposing forces. By taking advantage of inertial effects we demonstrate controllable self-assembling particle systems at the microscale. Inertially focused particles in confined high-speed microfluidic channel flows dynamically self-assemble into uniformly spaced lattices through purely hydrodynamic interactions with no external force fields. Focusing on the dynamics of the particle-particle interactions reveals a mechanism for the dynamic self-assembly process; inertial lift forces and a parabolic flow field act together to stabilize interparticle spacings that otherwise would diverge to infinity due to viscous wakes. The interplay of the repulsive viscous interaction and inertial lift also allow us to design and implement microfluidic structures that irreversibly change interparticle spacing, similar to a low-pass filter. Although often not considered at the microscale, nonlinearity due to inertia can provide a platform for high-throughput passive control of particle positions in all directions, which will be useful for applications in flow cytometry, tissue engineering, and synthesis of metamaterials. Monday, November 22, 2010 12:14PM - 12:27PM HM.00009: Modification of inertial focusing position by the restriction effect Elodie Sollier , Hamed Amini , Jean-Luc Achard , Dino Di Carlo A recent study from Faivre et al. has demonstrated that a rapid decrease in a channel cross-section leads to a lateral deviation in deformable particles' positions downstream of the constriction. This hydrodynamic drift, or restriction effect'', has been experimentally shown for low Reynolds numbers and only deformable cells. In parallel, Di Carlo and others have shown the usefulness of inertial forces in a confined flow, especially for ordering randomly distributed bioparticles. To further study this restriction effect but under these inertial conditions, we tracked the equilibrium positions of different bioparticles and measured their deviation. Just after the restriction, a deviation is observed for red cells and unexpectedly for spherical rigid beads. 1 cm further, beads go back to their initial position but red cells remained deviated. These results are interpreted as originating in differences in particle deformability and longer time necessary for a stressed cell to recover its initial equilibrium position. These results may provide a novel and simple technique for separation of particles with similar inertial equilibrium positions but different deformability, with specific applications for high-throughput flow cytometry. Monday, November 22, 2010 12:27PM - 12:40PM HM.00010: Droplets and stratifications in square microchannels Thomas Cubaud , Ruopeng Sun We experimentally investigate the interactions between a microfluidic train of droplets and stratifications formed by side-flow injections into a square microchannel. A system composed of a two-step hydrodynamic focusing junction is used to produce droplets upstream and to introduce a third liquid downstream. The additional liquid is selected to form miscible stratifications with the continuous phase. Using different liquids, we focus on the effect of viscosity stratifications and interfacial tension stratifications on droplet dynamics, including deformation, relaxation, and breakup.