Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R61: Steerable Colloids IFocus
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Sponsoring Units: GSOFT DFD Chair: Naomi Oppenheimer, Simons Foundation Room: BCEC 258B |
Thursday, March 7, 2019 8:00AM - 8:36AM |
R61.00001: Chiral electrophoresis: a new steering wheel Invited Speaker: Thomas A Witten Even rigid colloidal objects in conventional fluids in general respond chirally to external gradients such as electric fields. They twist around the gradient direction in a sense dictated by the object. Do real objects have enough chiral response to make a difference? Any chiral response requires either a chiral (non-inversion-symmetric) shape or a chiral charge distribution. But no shape can give chiral electrophoresis if it is uniformly charged. Nor can a sphere respond chirally even with the most chiral charge distribution. Here we demonstrate a new approach for calculating chiral responses of asymmetric objects. It represents the sheath of electrically-driven flow over the object as a superposition of stokeslet sources. Insights from this approach show how symmetric shapes can show strongly chiral response. We illustrate using a cube-shaped object. Chiral response is a handle by which a dispersion can be manipulated in powerful ways using time-varying electric fields. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R61.00002: Orientation of spheroidal colloids near a charged surface Yoav Tsori We look at an uncharged spheroidal colloid in a water near a charged flat surface. |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R61.00003: The Electrohydrodynamic Magnus Effect: Coupling between Electrophoresis and Quincke Rotation Propels Colloids Orthogonal to a Driving Electric Field Zachary Sherman, James W Swan Colloids dispersed in electrolytes polarize the surrounding ion cloud when exposed to an electric field. For sufficiently strong fields, an instability occurs that causes spherical colloids to break symmetry and spontaneously rotate about an axis orthogonal to the applied field, a phenomenon named Quincke rotation. If the colloids also have a net charge, the electrophoretic motion couples to Quincke rotation and propels particles in a direction orthogonal to both the driving field and the axis of rotation, an electrohydrodynamic analogue to the Magnus effect. The orthogonal Magnus velocity is of a comparable magnitude to the electrophoretic velocity. Typically, motion orthogonal to the field requires anisotropy in particle shape, dielectric properties, or geometry of boundaries. Here, the electrohydrodynamic Magnus effect occurs for bulk, isotropic spherical particles, with the Quincke rotation instability providing broken symmetry driving orthogonal motion. The direction of the Magnus velocity is not changed by flipping the sign of the field, so net orthogonal motion persists in AC electric fields. This orthogonal motion acts as a type of “self-propulsion”, and colloids with an electrohydrodynamic Magnus velocity can be used to create a new type of active matter. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R61.00004: Active Boomerangs Driven by Electrohydrodynamic Flows Qi-Huo Wei, Kelley Woehl, Miao Jiang, Ning Wu, Clemens Bechinger Recent work shows that colloidal dimers with asymmetric geometric or interfacial properties can propel due to the unbalanced electrohydrodynamic (EHD) flow around the particles, offering a new avenue to engineer active particles. In this work, we present experimental studies on the active motion behaviors of boomerang-shaped colloidal particles driven by the EHD flow. We show that by breaking the symmetry of particle shapes, the boomerang particles exhibit several different modes of active motions which can be either translation and/or rotation, and these modes of motions can be fine-tuned or switched by the frequency and initial phase of the electrical fields. |
Thursday, March 7, 2019 9:12AM - 9:24AM |
R61.00005: Dynamics of colloids in external potentials created by light fields Stefan Egelhaaf We experimentally create different potential energy landscapes using extended laser light fields, ranging from periodic patterns (laser fringes) to random patterns (speckle patterns). The dynamics of colloidal particles in these potential energy landscapes are followed by video microscopy. Diffusive as well as sub- and super-diffusive behavior can be observed with the extent of the different regimes depending on the specific situation, such as the shape and modulation amplitude of the potential but also the composition of particle mixtures and the particle concentrations. We will review our findings from different experimental conditions and compare the experimentally observed behavior to simulation and theoretical results. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R61.00006: Novel dynamics of optical manipulation in nano-architected objects Ognjen Ilic, Harry Atwater Light can be a powerful tool to mechanically manipulate matter, as evidenced by concepts such as optical tweezers and optical traps utilized across biology, colloidal science, microfuidics, and elsewhere. Advances in nanofabrication have enabled the ability to engineer the phase front of light as well as structure the shape of the target object for more advanced mechanical manipulation. We show how the combination of phase-space topology and particle asymmetry can provide a powerful degree of freedom in designing nanoparticles for optimal external manipulation. In particular, we find counterintuitive dynamics where optically asymmetric particles become stable nanoscale motors even in a light field with zero angular momentum. Here, the wavelength of the incident light can be used to externally control the number, orientation, and the stability of the equilibrium states. Finally, we discuss how structural complexity on the nano-scale can enable rich dynamics of optical manipulation of objects much larger than a wavelength in size. |
Thursday, March 7, 2019 9:36AM - 9:48AM |
R61.00007: Aggregation and phase transition of colloids induced by optical tweezers and thermophoresis Nicolas Bruot, Hajime Tanaka Optical tweezers are an appropriate tool to manipulate colloidal systems. We present here a technique based on optical trapping and thermophoresis to create and manipulate large aggregates of colloidal particles. The system is a mixture in organic solvents of refractive index- and density-matched PMMA colloids that do not interact directly with the laser of the optical tweezers, and titania particles that can be trapped. Trapped titania colloids are heated by the laser beam of the optical tweezers and create a very localized temperature gradient in the fluid. Surrounding PMMA colloids experience this gradient and are subject to thermophoresis that causes their aggregation around the titania particle. With confocal imaging, we characterize the growth dynamics of such aggregates and show that they can induce the crystallization of colloids. The advantage of this method compared to direct optical trapping and direct heating of the fluid is that spherically symmetric aggregates can be created in the bulk of a sample. We suggest that this system could be used to study the evaporation of a nuclei of a dense phase in a colloidal gas by turning off the laser. |
Thursday, March 7, 2019 9:48AM - 10:00AM |
R61.00008: Shear Induced Particle Migration in Locally Driven Brownian Suspensions Ilya Svetlizky, Yael Roichman Complex fluids include elements of intermediate size; elements which are larger compared to the molecules forming the embedding liquid. Among many, colloidal dispersions are probably the simplest example. When these fluids are driven out of their thermal equilibrium, hydrodynamic interactions and the deformed microstructure of the particle phase result in a wide range of phenomenology. |
Thursday, March 7, 2019 10:00AM - 10:12AM |
R61.00009: Repulsive Interactions between non-Brownian particles with non-uniform density during sedimentation Xiaolei Ma, Justin Burton We report experimental results on the sedimentation of non-Brownian particles with variable density distributions at low Reynolds number (10-3). Particle doublets with uniform density were composed of equal-sized aluminum-aluminum spheres or steel-steel spheres, whereas doublets with non-uniform density were composed of equal-sized aluminum-steel spheres. We observed an effective in-plane repulsive force during the settling of two aluminum-steel doublets due to their tendency to align with gravity, in agreement with a recent prediction [1], in contrast to settling dynamics of two doublets with uniform density that align with the fluid flow. For 10 or more interacting doublets in 2D channel flow, non-uniform particles tended to organize into equally-spaced paths determined by the particle size, whereas particles with uniform density had much stronger local interactions and often crossed paths and flipped orientation. For 100 or more particles settling in a cylindrical tube, the resulting distribution of particle positions at the bottom after sedimentation was more uniform for aluminum-steel doublets, suggesting that the 2-body repulsive force plays a role in the many-body interactions during sedimentation. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R61.00010: Collective motions of externally driven colloids with different sizes Taichi Matsubayashi, Norihiro Oyama, Takashi Taniguchi The collective motions of externally driven Np-colloidal systems (2 ≤ Np ≤ 3) in a confined viscous fluid between two parallel plates with a separation distance Lz have been investigated using three-dimensional direct numerical simulations with fully resolved hydrodynamics. We consider a system which consists of colloid particles with two different sizes (L: Large, S: Small, the size ratio is 1.25). In the two-particle system consisting of L- and S- particle, a stable doublet cluster whose leading particle is the small particle is always formed. In a three-particle system consisting of the two large particles and one small particle, although only the LSL-cluster have been observed in an experiment, we found that two types of stable clusters (SLL, LSL) appears and LLS-cluster does not appear, where (XYZ) means that X is the leading particle, Y the middle and Z the rear particle. We also found that SLL more probably appears in a higher external driven force and a larger Lz. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R61.00011: Light-induced Convective Segregation of Different Sized Particles Raj Kumar Manna, Oleg Shklyaev, Joshua Kauffman, Benjamin Tansi, Ayusman Sen, Anna Christina Balazs By heating an aqueous solution of gold nanoparticles by light, this solution and the suspended microparticles can be driven to flow within the confines of a microchamber. The convective flows are generated by thermal buoyancy effects and can be controlled by varying the intensity of imposed light and the concentration of the nanoparticles in the solution. Using computer simulations, we focus on a microchamber that is inclined at an angle with respect to the horizontal direction and demonstrate that the thermally-driven convective flows in this chamber can be harnessed to separate micrometer-sized particles along the bottom, inclined wall. A competition between the drag force imposed by the fluid flows and forces from gravity acting on the different sized particles gives rise to this particle separation. Furthermore, we show that the magnitude of the separation between the different particles increases with the increase of the inclined angle and the relative difference in the particle sizes. The separation of particles can also be tuned by changing the intensity of light. Our simulation results are supported by experimental observations. The behavior of the systems is highly robust and controllable and thus allows it to be used to a wide range of particle sorting applications. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R61.00012: Spontaneous pulse generation in a steady channel flow of a colloidal suspension Suin Shim, Howard A Stone We present experiments demonstrating the spontaneous generation and traveling of a colloidal pulse in a steady channel flow. When deionized (DI) water with suspended positively-charged particles flows steadily through a single channel, a pulse of particles is generated, which then flows through the channel at a slower speed than the mean flow velocity. With detailed experimental investigations and quantified results, we rationalize our observations by considering CO2 driven diffusiophoresis. The concentration gradient of ions in the liquid phase is created by the leakage of CO2 through the permeable PDMS walls. Mathematical models for early stage particle focusing and the traveling pulse will be compared with the experimental observations. |
Thursday, March 7, 2019 10:48AM - 11:00AM |
R61.00013: Programmable nano-hydrogels for flow control in porous media Liyuan Zhang, Shima Parsa Moghaddam, David A Weitz We study the effect of injection of programmable nano-hydrogels on the dynamic of the flow through the 3D micromodel porous medium. We develop a core-shell nano-hydrogel with programmable swelling behavior. Using confocal microscopy, we visualize flow in a 3Dmicromodel before and after injection of the nano-hydrogels. After injection of the suspension of nano-hydrogels, the shells degrade and nano-hydrogels swell up to 6 times. Hence, by injection of a small amount of the suspension, the resistance of the medium increases up to 80% resulting in significant changes in the flow paths and diversion of the flow towards unexplored regions. These nano-hydrogels show great promise for application in enhanced oil recovery. |
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