Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L39: Microscale Flows: Particles, Orientation, Active Matter and Self Assembly |
Hide Abstracts |
Chair: Enkeleida Lushi, Brown University Room: Sheraton Back Bay C |
Monday, November 23, 2015 4:05PM - 4:18PM |
L39.00001: Transition to collective motion and mixing in suspensions of micro-rotors Petia Vlahovska, Enkeleida Lushi Self-organization of active objects has attracted considerable attention recently, in particular in the context of living matter. Hydrodynamic interactions play a crucial role in the emerging behavior when the objects are immersed in fluid, especially in the low Reynolds number regime. While self-propelled active objects have been extensively investigated, the collective behavior of rotating active particle has received limited attention.To elucidate the transition to collective behavior and especially the role of multi-body hydrodynamic interactions, we numerically study systems of co- and counter-rotating spheres by varying the mixture ratio as well as the total volume fraction. We show that while macroscopic patterns emerge with increasing volume fraction in all the mixtures, the organization of the 100-0 and 50-50 mixtures are different in nature. The 50-50 rotor mixtures generate macroscopic fluid flows that are larger in magnitude and more chaotic, due to the propensity of rotors of opposite spins to pair up and co-swim. The properties of these generated fluid flows are investigated, and in particular we show that the mixing of a passive dye field is more efficiently done by 50-50 rotor mixtures. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L39.00002: Phase transition of active rotors due to passive particles Kyongmin Yeo, Enkeleida Lushi, Petia Vlahovska We study the emergent collective dynamics of active suspensions of micro-rotors due to passive, tracer particles. The active suspensions consist of the 50:50 mixture of the opposite-spin rotors together with the passive particles. In the active suspensions, the rotational kinetic energy of the active rotors is converted to the translational kinetic energy (TKE) of the suspended particles. At low rotor densities, TKE of the passive particles is smaller than TKE of the active rotors. However, at higher rotor densities ($\phi_R \ge 0.16$), TKE of the passive particles becomes larger than the active rotors. Depending on the densities of the active rotors and the passive particles, the microstructures of the active rotors change from the doublet of opposite-spin rotors to the phase-separated flows. When both $\phi_R$ and $\phi_P$ are high, two large counter-rotating vortices emerge, while the passive particles move along the boundaries of the vortices. It is found that the mean-square displacement of the passive particles become larger than that of the active rotors in the phase-separated fluid regime. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L39.00003: Electro-Orientation of Boron-Nitride Nanotubes in Aqueous Solution Semih Cetindag, Sangil Kim, Bishnu Tiwari, Shiva Bhandari, Dongyan Zhang, Yoke Khin Yap, Jerry Shan Boron-nitride nanotubes (BNNTs), which have similar structure to carbon nanotubes but very different electronic properties, are of interest for a variety of applications, including hydrogen storage, water desalination, mechanical reinforcement and improving the thermal conductivity of composites. Many potential applications would benefit from alignment of BNNTs. We demonstrate, for the first time, the ability to align BNNTs, which are insulating at room temperature, with spatially uniform AC fields in aqueous solution. Electro-orientation rates are experimentally found to decline as the frequency of the AC field is increased. The behavior of the cross-over frequency with varying solution conductivity is suggestive of induced-charge-electro-osmotic (ICEO) alignment, despite the extremely low electrical conductivity of BNNTs. We further discuss electro-orientation with DNA-wrapped BNNTs and compare with pristine nanotubes. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L39.00004: Magnetic self-assembly of microparticle clusters in an aqueous two-phase microfluidic cross-flow Niki Abbasi, Steven G. Jones, Byeong-Ui Moon, Scott S.H. Tsai We present a technique that self-assembles paramagnetic microparticles on the interface of aqueous two-phase system (ATPS) fluids in a microfluidic cross-flow. A co-flow of the ATPS is formed in the microfluidic cross channel as the flows of a dilute dextran (DEX) phase, along with a flow-focused particle suspension, converges with a dilute polyethylene glycol (PEG) phase. The microparticles arrive at the liquid-liquid interface and self-assemble into particle clusters due to forces on the particles from an applied external magnetic field gradient, and the interfacial tension of the ATPS. The microparticles form clusters at the interface, and once the cluster size grows to a critical value, the cluster passes through the interface. We control the size of the self-assembled clusters, as they pass through the interface, by varying the strength of the applied magnetic field gradient and the ATPS interfacial tension. We observe rich assembly dynamics, from the formation of Pickering emulsions to clusters that are completely encapsulated inside DEX phase droplets. We anticipate that this microparticle self-assembly method may have important biotechnological applications that require the controlled assembly of cells into clusters. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L39.00005: Quantum dots deposition in a capillary tube Yong Lin Kong, Francois Boulogne, Hyoungsoo Kim, Janine Nunes, Jie Feng, Howard Stone The ability to assemble nanomaterials, such as quantum dots, enables the creation of functional devices that present unique optical and electronic properties. For instance, light-emitting diodes with exceptional color purity can be printed via the evaporative-driven assembly of quantum dots. Nevertheless, current studies of the colloidal deposition of quantum dots have been limited to the surfaces of a planar substrate. Here, we investigate the evaporation-driven assembly of quantum dots inside a confined cylindrical geometry. Specifically, we observe distinct deposition or coating patterns of quantum dots at different positions along the length of a capillary tube. Such changes of coating behavior could be influenced by the evaporation speed as well as the concentration of quantum dots. Understanding the factors governing the coating process can provide a means to control the assembly of quantum dots inside a capillary tube, ultimately enabling the creation of novel photonic devices. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L39.00006: Oscillatory flow and induced steady streaming flow around two spheres David Fabre, Javeria Jalal, Justin Leontini, Richard Manasseh We investigate the flow around two fixed spheres of identical radius, subjected to a oscillating flow at frequency $\omega$ and weak amplitude $u_a$. Expanding the flow in series of $u_a$, the leading order corresponds to an oscillating flow with zero mean, while the second-order correction contains a steady streaming component. Thanks to a modal decomposition in the azimuthal direction, we are able to reduce the problem to a few linear problems in a 2D domain corresponding to the meridional ($r,z$) plane. Investigation of the streamlines of the steady component of the flow shows intricate patterns due to the interaction between the streaming flows induced by both spheres. The analysis also allows to compute the mean forces felt by both spheres. If the spheres are aligned obliquely with respect to the oscillating flow, they experience a lateral force which tend to realign them in a transverse configuration. In this transverse configuration, they experience an axial force which can be either attractive or repulsive. At high frequencies the force is always attractive. At low frequencies, it is repulsive. At intermediate frequencies, the force is attractive at large distances and repulsive at small distances, leading to the existence of an equilibrium configuration. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L39.00007: Particle trajectory entanglement in microfluidic channels Alvaro Marin, Massimiliano Rossi, Christian K\"ahler Suspensions in motion can show very complex and counterintuitive behavior, particularly at high concentrations. In this talk we show an overlooked phenomenon occurring when a dilute particle solution is forced to travel in a narrow channel (only a few times the particle size). At critical interparticle distances, particles tend to interlace their trajectories forming a sort of {\it hydroclusters} only bonded by hydrodynamic interactions. While classical studies on non-Brownian self-diffusivity report average particle displacements of fractions of the particle diameter, the trajectories observed in our system show displacements of several particle diameters. Indeed, such a behavior resemble the deterministic trajectories found by Uspal {\it et al.} (Nat. Comm. 4, 2013) with engineered particle doublets. Trajectory statistics are obtained for different shear rates and particle sizes. The results are compared with particle dynamics simulations and analyzed under the light of recent studies on the irreversibility of non-Brownian suspensions (Metzger {\it et al.}, Phys. Rev. E, 2013) to elucidate the nature of the hydrodynamic interactions entering into play. The reported phenomenon could be applied to promote advective mixing in micro-channels or particle/droplet self-assembly. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L39.00008: Dynamic self-assembly of microscale rotors and swimmers Megan Davies Wykes, Jeremie Palacci, Takuji Adachi, Leif Ristroph, Yanpeng Liu, Xiao Zhong, Jun Zhang, Michael Ward, Michael Shelley Self-assembly is a process found throughout nature and is often dynamic, requiring fuel to occur. Artificial examples are valuable both as aids to understanding biological systems and for developing manufacturing techniques for micron-scale machines. We will describe the behaviour of micron-scale rods, constructed of three equal length segments of gold, platinum and gold (Au-Pt-Au). When placed in a solution of hydrogen peroxide fuel, these are expected to create an extensile-like flow in the surrounding fluid. These immotile rods self-assemble into structures that exhibit the two fundamental types of motion: rotation and translation, in the form of steadily rotating stacks and T-shaped swimmers. This is a rare example of an artificial system where dynamic and reversible self-assembly results in ordered structures which exhibit emergent motility. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L39.00009: Forming particle chains in inertial microfluidic devices Kaitlyn Hood, Lawrence Liu, Marcus Roper Particles in microfluidic devices at finite Reynolds number self-assemble into evenly-spaced chains, which can be exploited in inertial microfluidic devices for flow cytometry, high speed imaging, and entrapment. While the location and number of chains can be manipulated by changing the channel geometry, the particle interactions are not understood well enough to manipulate the spacing between particles. We present a mathematical model of particle interactions and the formation of particle chains. We will address the following questions: Is there a preferred particle spacing? What are the conditions needed for chain formation? [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L39.00010: Quantifying colloidal particle bands and their formation in combined electroosmotic and Poiseuille flow Andrew Yee, Necmettin Cevheri, Minami Yoda Recently, we have shown that suspended radii $a = 245~$nm particles flowing through a microchannel driven by the combination of a dc electric field and pressure gradient (where the resulting electroosmotic and shear flows are in opposite directions) are attracted to the wall at low electric field magnitude $|E|$, then assemble into concentrated bands that only exist within a few $\mu$m of the wall above a threshold value of $|E|$, $|E_{\rm cr}|$. The $\sim 6~\mu$m wide bands are aligned with the flow direction and are roughly periodic along the cross-stream direction. This talk focuses on quantitative characterization of these bands, for example how $|E_{\rm cr}|$, the time required for bands to form after applying the electric field $T_{\rm o}$, and the number of bands depend upon parameters such as particle volume fraction $\varphi$, shear rate $\dot{\gamma}$, $|E|$, and $a$. The dynamics of the particles within the bands are visualized by imaging a mixture of particles with different fluorescent labels. The visualizations show that the particles are in a liquid state within these bands, and suggest that the particles nearest the wall move in the direction of the electroosmotic flow, while those farther from the wall move in the direction of the shear flow. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L39.00011: Hydrodynamic alignment and assembly of nano-fibrillated cellulose in the laminar extensional flow: Effects of solidifying agents Nitesh Mittal, Fredrik Lundell, Daniel Soderberg There are several fiber production technologies that are based on wet-spinning processes. Many such processes rely on the transformation of a liquid solution into a solid filament. The kinetics of solidification depends largely on the diffusion of the solvents, additives and polymer molecules, which make such systems quite complex and differ from a system to another as a function of the specific chemical, physical and structural features of the used material components. Moreover, tuning the orientation of the polymers in the liquid suspensions makes it further possible to control their structure, which in turn can lead to materials having improved properties. By keeping in mind the facts mentioned above, the aim of the current study is to utilize benefits of a flow focusing approach to align carboxymethylated cellulose nanofibrils (CNF), as a colloidal dispersion, with the help of a laminar elongational flow-field followed by the solidification using different solidifying agents or molecules (with dissimilar diffusion behavior based on their size and charges) to synthesize fibers with enhanced mechanical properties. CNF are charged elongated particles obtained from woods with diameter of 4-10 nm and length of 1-1.5 $\mu$m, and they are completely biodegradable. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L39.00012: Effect of flow on Janus rods organization in polymer blends Shaghayegh Khani, Safa Jamali, Arman Boromand, Joao Maia In the past decade, Janus particles have attracted a lot of attention due to their amphiphilic nature. Directed assembly of these particles in polymer matrices can provide a tool for fabricating new functional materials. For example, the strong affinity of Janus particles to interfaces, could allow the control of the interface of phase separating polymer blends by controlling the Janus particles assembly. In this work, using mesoscale computational methods, we show that the spatial organization of Janus rods can be exploited for tuning mixtures of immiscible polymer blends. In particular, we explore the effect of different parameters that influence the rods alignment and orientation at the interface under equilibrium condition. Flow can dramatically alter the localization of these particles within the polymer blend. Therefore, we not only monitor the microstructures formed by these systems at rest, but we also do so under flow conditions and upon relaxation after flow cessation. The results of this study can be used for designing new approaches for directing nano-particles into desired morphologies, which will subsequently tune the final characteristics and properties of nano-composites. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700