Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H22: Microscale Flows: Particles-Orientation and Self-assembly |
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Chair: Minami Yoda, Georgia Institute of Technology Room: E141/142 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H22.00001: Effect of flow parameters on assembly of colloidal particle bands in Poiseuille and electroosmotic flow Andrew Yee, Minami Yoda Recent evanescent-wave visualizations (that only image the first $\sim 1~ \mu$m next to the wall) have shown that dielectric colloidal particles in combined Poiseuille and electroosmotic flow of dilute suspensions through fused-silica channels (with a depth of 34 $\mu$m) assemble into streamwise bands. These bands have cross-sectional dimensions of a few $\mu$m and length comparable to that of the channel (\emph{i.e.}, a few cm). They are roughly periodic along the cross-stream direction, even though there are no external forces in this direction. For moderate electric fields $|E|<$ 120 V/cm, the time scales for band formation at a given channel location appear to scale with the inverse of the shear rate (determined by Poiseuille flow), or $\dot{\gamma}^{-1}$. The results also suggest that the average number of bands $N$ in steady-state (over a field of view of 200 $\mu$m square) decreases linearly with increasing $|E|$. These trends are not observed at higher $|E|$ and lower $\dot{\gamma}$, corresponding to cases where $N<5$. In some cases, a large number of bands appear within a few seconds, then completely ``disappear'' from the near-wall region, and a much smaller number of bands then re-appear after several seconds. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H22.00002: How particle properties affect the assembly and characteristics of colloidal particle bands Minami Yoda, Andrew Yee The interaction of suspended particles with a planar wall is a classic problem of colloid science. Particle-wall interactions in a flowing suspension are a newer area of interest, motivated by applications in microfluidics. Recent studies show that radius $a =$ 245 nm particles in a dilute (volume fraction $\varphi =$ 0.17\%) suspension are attracted to the wall, form 1D ``pearl chains,'' then assemble into concentrated streamwise bands with a roughly constant cross-stream spacing in combined Poiseuille and electroosmotic flow through fused-silica microchannels. The bands only exist within a few $\mu$m of the wall, and occur above a minimum shear rate $\dot{\gamma}$ and electric field magnitude $|E|$. Attracting (\emph{i.e.}, concentrating) the particles to (near) the wall is a prerequisite for band formation; however, bands are not observed in all cases when particles are attracted to the wall. Particle properties appear to have a significant effect on these phenomena: decreasing $\varphi$, for example, appears to increase both the minimum $\dot{\gamma}$ and $|E|$ for band formation. Results are presented on how the assembly and characteristics of the bands are affected by properties such as $\varphi$, $a$ (where $a < 1~\mu$m), and zeta-potential $\zeta_{\rm p}$. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H22.00003: Mind the gap: a flow instability controlled by particle-surface distance Michelle Driscoll, Blaise Delmotte, Mena Youssef, Stefano Sacanna, Aleksandar Donev, Paul Chaikin Does a rotating particle always spin in place? Not if that particle is near a surface: rolling leads to translational motion, as well as very strong flows around the particle, even quite far away. These large advective flows strongly couple the motion of neighboring particles, giving rise to strong collective effects in groups of rolling particles. Using a model experimental system, weakly magnetic colloids driven by a rotating magnetic field, we observe that driving a compact group of microrollers leads to a new kind of flow instability. First, an initially uniformly-distributed strip of particles evolves into a shock structure, and then it becomes unstable, emitting fingers with a well-defined wavelength. Using 3D large-scale simulations in tandem with our experiments, we find that the instability wavelength is controlled not by the driving torque or the fluid viscosity, but a geometric parameter: the microroller’s distance above the container floor. Furthermore, we find that the instability dynamics can be reproduced using only one ingredient: hydrodynamic interactions near a no-slip boundary. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H22.00004: Long-lived "critters" formed by hydrodynamic clustering Blaise Delmotte, Michelle Driscoll, Mena Youssef, Stefano Sacanna, Aleksandar Donev, Paul Chaikin Self-assembly in colloidal systems often requires finely tuning the interactions between particles. When colloids are active, or moving due to an external drive, the assembly is even harder to achieve. Here we show that long-lived compact motile structures, called ``critters'', can be formed just with hydrodynamic interactions. They naturally emerge from a recently discovered fingering instability in a system of microrollers near a floor. Our 3D large-scale simulations show that these critters are a stable state of the system, move much faster than individual rollers, and quickly respond to a changing drive. The formation of critters is robust to any initial condition and our experiments suggest that similar structures are formed even in a thermal colloidal system. We believe the critters are a promising tool for microscopic transport, flow, aggregation and mixing. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H22.00005: Numerical simulations of electric field driven hierarchical self-assembly of monolayers of binary mixtures of particles Edison Amah, Naga Musunuri, Shahadat Hossain, Ian Fischer, Pushpendra Singh We numerically study the process of self-assembly of particle mixtures on fluid-liquid interfaces when an electric field is applied in the direction normal to the interface. Lateral forces cause particles to self-assemble into molecular-like hierarchical arrangements consisting of composite particles arranged in a pattern. As in experiments, if the particles sizes differ by a factor of two or more, the composite particle has a larger particle at its core with several smaller particles forming a ring around it. Approximately same sized particles form chains (analogous to polymeric molecules) in which positively and negatively polarized particles alternate when their concentrations are approximately equal, but when their concentrations differ substantially the particles whose concentration is larger form rings around the particles whose concentration is smaller. In some instances, particle chains with a positively polarized particle at one end and a negatively particle at the other folded to form circular chains. For submicron particles, only when the electric field intensity is larger than a critical value required for overcoming Brownian forces, a hierarchical pattern consisting of composite particles will form. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H22.00006: Light-structured colloidal assemblies Antoine Aubret, Youssef Mena, Sophie Ramananarivo, Stefano Sacanna, Jeremie Palacci Self-propelled particles (SPP) are a key tool since they are of relative simplicity as compared to biological micro-entities and provide a higher level of control. They can convert an energy source into motion and work, and exhibit surprising non-equilibrium behavior. In our work, we focus on the manipulation of colloids using light. We exploit osmotic and phoretic effects to act on single and ensemble of colloids. The key mechanism relies on the photocatalytic decomposition of hydrogen peroxide using hematite, which triggers the motion of colloids around it when illuminated. We use hematite particles and particles with photocatalytic inclusions (i.e. SPP). We first show that the interactions between hematite and colloidal tracers can be tuned by adjusting the chemical environment. Furthermore, we report a phototaxic behavior (migration in light gradient) of the particles. From this, we explore the effect of spatio-temporal modulation of the light to control the motion of colloids at the single particle level, and to generate self-assembled colloidal structures through time and space. The so-formed structures are maintained by phoretic and hydrodynamic forces resulting from the motion of each particles. Ultimately, a dynamic light modulation may be a route for the creation of active colloidal motion on a collective scale through the synchronization of the individual motions of SPP. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H22.00007: Five Degree of Freedom Fluorescence Localization of Ellipsoidal Particles Craig Snoeyink, Md. Anisul Islam, Gordon Christopher Symmetry breaking non-spherical particles can exhibit unique behavior when self-assembling due to increased degrees of freedom. For example, ellipsoidal particles on a fluid interface exhibit mesostructures that are dependent upon the both the contact angle of the ellipsoidal particle as well as the orientation. However, measuring the three dimensional position and orientation of these particles can be challenging. Here we present preliminary results on five degree of freedom fluorescence measurements of ellipsoidal particles on a fluid interface. Using the Bessel Beam Microscopy system and a novel compressed sensing based image analysis algorithm we will demonstrate 3D localization of ellipsoidal particles with 50 nm accuracy as well as pitch and yaw measurements with a resolution of 10 and 1 degrees respectively. We will discuss the technique as well as its implications for our understanding of non-spherical particle interactions and assembly at interfaces. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H22.00008: Flow-driven Assembly of Microcapsule Towers Henry Shum, Anna Balazs Large populations of the slime mold, \textit{Dictyostelium discoideum}, are able to aggregate over a surface and collectively form a long, vertical stalk. Inspired by this biological behavior, we develop a synthetic mechanism for assembling tower-like structures using microcapsules as the building blocks. We accomplish this in simulations by generating a fluid flow field that draws microcapsules together along a surface and lifts them up at a central point. We considered a fluid flow generated by the local release of a chemical species from a patch on the surface. The concentration gradient of the diffusing chemical species causes radial diffusioosmotic flow along the solid surface toward the patch. Adhesive interactions keep the microcapsules attached to the surface as they are drawn together above the patch. To build a tower-like structure, some of the microcapsules must detach from the surface but remain attached to the rest of the cluster. The upward directed fluid flow above the patch then draws out the cluster into a tower shape. The final morphology of the aggregate structure depends on the flow field, the adhesive capsule-capsule and capsule-surface interaction strengths, and the sedimentation force on the capsules. Tuning these factors changes the structures that are produced. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H22.00009: Interactions of inertially focused particles Kaitlyn Hood, Marcus Roper In inertial microfluidic devices, fluid inertia aligns submerged particles to a finite number of streamlines. Once particles are aligned on a streamline, particle interactions produce regularly spaced chains of particles. We demonstrate that viscous particle-particle and particle-wall interactions, combined with inertial focusing, give rise to a set spacing length for two particles. This model shows how the spacing length scale depends on particle size and Reynolds number. We also show that for two particles of different sizes, a range of spacing lengths can be achieved by tuning the Reynolds number. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H22.00010: Elongational Flow Assists with the Assembly of Protein Nanofibrils Nitesh Mittal, Ayaka Kamada, Christofer Lendel, Fredrik Lundell, Daniel Soderberg Controlling the aggregation process of protein-based macromolecular structures in a confined environment using small-scale flow devices and understanding their assembly mechanisms is essential to develop bio-based materials. Whey protein, a protein mixture with $\beta $-lactoglobulin as main component, is able to self-assemble into amyloid-like protein nanofibers which are stabilized by hydrogen bonds. The conditions at which the fibrillation process occurs can affect the properties and morphology of the fibrils. Here, we show that the morphology of protein nanofibers greatly affects their assembly. We used elongational flow based double flow-focusing device for this study. In-situ behavior of the straight and flexible fibrils in the flow channel is determined using small-angle X-ray scattering (SAXS) technique. Our process combines hydrodynamic alignment with dispersion to gel-transition that produces homogeneous and smooth fibers. Moreover, successful alignment before gelation demands a proper separation of the time-scales involved, which we tried to identify in the current study. The presented approach combining small scale flow devices with in-situ synchrotron X-ray studies and protein engineering is a promising route to design high performance protein-based materials with controlled physical and chemical properties. [Preview Abstract] |
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