Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R17: Colloids I: Scattering, Microscopy and Optical Traps |
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Sponsoring Units: GSOFT Chair: Vinothan Manoharan, Harvard University Room: 276 |
Thursday, March 16, 2017 8:00AM - 8:12AM |
R17.00001: Holographic Characterization of Colloidal Fractal Aggregates Chen Wang, Fook Chiong Cheong, David B. Ruffner, Xiao Zhong, Michael D. Ward, David G. Grier In-line holographic microscopy images of micrometer-scale fractal aggregates can be interpreted with the Lorenz-Mie theory of light scattering and an effective-sphere model to obtain each aggregate’s size and the population-averaged fractal dimension. We demonstrate this technique experimentally using model fractal clusters of polystyrene nanoparticles and fractal protein aggregates composed of bovine serum albumin and bovine pancreas insulin. This technique can characterize several thousand aggregates in ten minutes and naturally distinguishes aggregates from contaminants such as silicone oil droplets. [Preview Abstract] |
Thursday, March 16, 2017 8:12AM - 8:24AM |
R17.00002: Characterizing Scattering Properties of Fractal Systems via Their Light Localization Properties Ethan Avery, Peeyush Sahay, Matthew Robinson, Prabhakar Pradhan Self-similar characteristics of fractal systems mimic many random continuous optical media such as porous systems, glassy materials, biological tissues and cells, etc. We performed a rigorous statistical analysis of scattering properties of fractal systems via examining their `degree of structural disorder.' Our study focused on quantifying the degree of structural disorder of numerical model generated fractal matric lattices with systematic changes of the fractal dimensionality. For this, we evaluated the light localization properties of the media to determine the optimal scattering of the lattice media. The degree of structural disorder was calculated using the Inverse Participation Ratio (IPR) of the eigenfunctions of the light waves obtained from Anderson disorder tight binding Hamiltonian of the media with closed boundary conditions. The Fractal Dimension (FD) was calculated using the box counting method. The IPR was calculated for each system and plotted as a function of fractal dimension for two and three dimensional systems. The results show a non-monotonous behavior of the degree of structural disorder with the fractal dimension due to the competition between optical mass density points and the scattering capability of the system. [Preview Abstract] |
Thursday, March 16, 2017 8:24AM - 8:36AM |
R17.00003: Dynamics of self-assembly of colloidal clusters Solomon Barkley, Ellen Klein, Vinothan Manoharan We study how small numbers of attractive colloidal particles evolve into compact clusters. In our experiments, we arrange particles into a loosely connected structure, from which they can form additional contacts and fold into a cluster. We observe the folding process using digital holographic microscopy, which allows us to track the precise 3D position of each particle within the cluster during folding. Because the rearrangement process is stochastic, clusters with identical initial arrangements often end up in different final states. However, some final states are preferred over others, based on the number of folding pathways that lead to them. We show how the pathways change with the number of particles and which specific folding events determine the pathways that a cluster can follow toward its final state. [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 8:48AM |
R17.00004: Nanoparticle diffusion in complex and confined media Jacinta Conrad, Firoozeh Babayekhorasani, Dave Dunstan, Ramanan Krishnamoorti We identify distinct mechanisms controlling slowing of nanoparticle diffusion through complex media featuring both rigid geometrical confinement and soft mobile crowders. We use confocal microscopy and single particle tracking to probe diffusion of 400 nm nanoparticles suspended in water, in glycerol/water, or in a polymer solution through a packed bed of microscale glass beads. Long-time diffusive mobility of nanoparticles slows as the average pore size of the packed bed decreases for all solutions. The distribution of particle displacements is non-Gaussian, consistent with spatially heterogeneous confinement imposed by the bed. The slowing of nanoparticle mobility in all solutions follows predictions from hydrodynamic models. In polymer solutions, depletion interactions mediated by polymers result in temporary adsorption of particles onto the bead surface. Our results suggest that confined diffusive dynamics of nanoparticles in polymer solutions is controlled by two competing mechanisms: hydrodynamic interactions between particles and spatial obstacles, which dictate long-time slowing of diffusion, and depletion interactions between particles and confining walls due to macromolecules, which control transient adsorption and hence alter the statistics of the short-time motion. [Preview Abstract] |
Thursday, March 16, 2017 8:48AM - 9:00AM |
R17.00005: Role of Entropy in Self Purification of Colloidal Clusters Under Optical Trap Hreedish Kakoty, Rajarshi Banerjee, Chandan Dasgupta, Ambarish Ghosh Controlling the structure of a collection of colloidal particles under external forces can be helpful in developing soft nanomaterials with novel functionalities. How external impurities organize within such confined systems is of fundamental and technological interest, especially when the system sizes are so small that even a single dopant can interact with an appreciable fraction of the system. Experiments to specifically probe the behaviour of dopants have been relatively few. Here, we have used a defocused laser beam to form colloidal clusters in 2D with precise control over the size and phase of the assembly. Crucially, we could inject and subsequently study the behaviour of foreign dopants within these crystallites, revealing surprising position dependence in the fate of dopants getting either spontaneously expelled or permanently internalized. We have modelled this system numerically and found that this phenomenon arises due to the subtle interplay between the effects of external confinement and role of entropy in the thermodynamics of small assemblies of interacting particles. The studies presented here could be extended to host colloids of higher degree of complexity and the insight gained could be useful in designing and assembling new type of soft nanomaterials. [Preview Abstract] |
Thursday, March 16, 2017 9:00AM - 9:12AM |
R17.00006: Traction force rheology: a new technique to understand the mechanics of colloidal solids. Zsolt Terdik We present a new technique, traction force rheology, to directly measure the mechanical response of colloidal solids (crystals and glasses) in response to imposed shear strain, while observing the dynamics of individual colloidal particles. The technique consists of forming a composite bilayer consisting of a colloidal solid on top of a soft, polymer gel with embedded tracer particles. A precisely controlled shear strain is applied to the bilayer leading to controlled deformation of the colloidal crystal/glass. In addition to directly observing rearrangements and defects that occur within the colloidal crystal/glass during plastic deformation, we also measure the deformation of the tracer particles embedded in the polymer gel. Given the observed deformation of the polymer gel and the measured mechanical modulus, the traction forces exerted on the polymer gel by the colloidal solid can be inferred using traction force microscopy. Experimental details, challenges, and current results will be discussed. [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:24AM |
R17.00007: Effects of multiple scattering on structural color in disordered colloids Victoria Hwang, Anna Stephenson, Vinothan N. Manoharan Disordered packings of colloidal spheres can show structural colors that are independent of the angle between light source and observer (E.R. Dufresne et al, Adv. Mater. 2010, XX, 1–6). This phenomenon arises from constructive interference of scattered light, but the disordered structure produces homogeneous colors, in contrast to the angle-dependent, or iridescent, colors of colloidal crystals. Although the color can be understood qualitatively through single-scattering models, these systems also show weak multiple scattering where neither single scattering nor diffusive transport assumptions are valid. To understand the effect of multiple scattering on the color, we perform polarization experiments to characterize multiple scattering in structurally-colored samples. We find that multiple scattering dominates at short wavelengths. In the observed reflection spectrum, this contribution adds to the single scattering from individual particles and from interference between scattered waves. Because multiple scattering reduces the saturation of color, we seek to minimize its effects for applications. To do this, we calculate the transport length of disordered colloids using Mie theory and use microfluidics to find the regimes of sample thickness that lead to optimal color saturation. [Preview Abstract] |
Thursday, March 16, 2017 9:24AM - 9:36AM |
R17.00008: Light scattered by `hedgehog' particles Joong Hwan Bahng, Douglas Montjoy, Wei-Shun Chang, Stephan Link, Nicholas Kotov Sensitive to even a small perturbation in its construct, particles provide versatile and compact platforms with which to design electromagnetic responses. With great advances in the nanofabrication, diverse particle types exhibiting unique and useful scattered radiation patterns have been realized or theoretically predicted. In particular, particles exhibiting broadband scattering with flexibility to suppress backscattering and enhance forward scattering hold promises in a diverse array of photonics devices. Recently, we have reported the `hedgehog' particles whose high aspect-ratio surface roughness elicits anomalous dispersion behavior that breaks the well-known ``similarity rule''. The `hedgehog' particles represent a novel class of ``rough'' particles comprised of all dielectric components that lies within the Mie scattering regime due to wavelength comparable dimensions. In this research, in addition to deviation in the interaction potential as reported previously, we will show that high aspect ratio nano-topography also modifies electromagnetic responses from what is predicted by Mie theory for smooth dielectric particles. In detail, the high aspect-ratio interfacial nano-corrugation 1) educes broadband scattering at the visible spectrum, 2) suppresses resonant modes within the `hedgehog' particles and 3) creates near-field profiles that elicits broadband suppression of backscattering and enhancement of forward scattering at visible spectral range and above. [Preview Abstract] |
Thursday, March 16, 2017 9:36AM - 9:48AM |
R17.00009: Coupling between absorption and scattering in disordered colloids Anna Stephenson, Victoria Hwang, Jin-Gyu Park, Vinothan N. Manoharan We aim to understand how scattering and absorption are coupled in disordered colloidal suspensions containing absorbing molecules (dyes). When the absorption length is shorter than the transport length, absorption dominates, and absorption and scattering can be seen as two additive effects. However, when the transport length is shorter than the absorption length, the scattering and absorption become coupled, as multiple scattering increases the path length of the light in the sample, leading to a higher probability of absorption. To quantify this synergistic effect, we measure the diffuse reflectance spectra of colloidal samples of varying dye concentrations, thicknesses, and particle concentrations, and we calculate the transport length and absorption length from our measurements, using a radiative transfer model. At particle concentrations so high that the particles form disordered packings, we find a minimum in the transport length. We show that selecting a dye where the absorption peak matches the location of the minimum in the transport length allows for enhanced absorption. [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:00AM |
R17.00010: A New Technique for Measuring Concentration Dependence of Self and Collective Diffusivity by using a Single Sample Krittanon Sirorattanakul, Chong Shen, Daniel Ou-Yang Diffusivity governs the dynamics of interacting particles suspended in a solvent. At high particle concentration, the interactions between particles become non-negligible, making the values of self and collective diffusivity diverge and concentration-dependent. Conventional methods for measuring this dependency, such as forced Rayleigh scattering, fluorescence correlation spectroscopy (FCS), and dynamic light scattering (DLS) require preparation of multiple samples. We present a new technique to measure this dependency by using only a single sample. Dielectrophoresis (DEP) is used to create concentration gradient in the solution. Across this concentration distribution, we use FCS to measure the concentration-dependent self diffusivity. Then, we switch off DEP to allow the particles to diffuse back to equilibrium. We obtain the time series of concentration distribution from fluorescence microscopy and use them to determine the concentration-dependent collective diffusivity. We compare the experimental results with computer simulations to verify the validity of this technique. Time and spatial resolution limits of FCS and imaging are also analyzed to estimate the limitation of the proposed technique. [Preview Abstract] |
Thursday, March 16, 2017 10:00AM - 10:12AM |
R17.00011: Direct measurement of ballistic to diffusive crossover in colloidal particles in simple and complex fluids Andrew Hammond, Eric Corwin Brownian motion was famously used by Einstein to demonstrate the molecular nature of fluids in 1905. Only recently has it become possible to examine the short time behavior of a single particle, albeit using an optical trap. The use of an optical trap to confine the particles allows an extremely high precision measurement of the short-time ballistic motion but at the cost of a loss of information about the crossover to diffusive behavior. We present a new measurement method which allows us to resolve the full transition between both ballistic motion and long-time diffusive motion. This technique is broadly applicable to many different types of liquids, from simple Newtonian fluids such as water to complex Maxwell fluids and beyond. Measuring the full transition provides a direct measurement of the temperature and rheological response of the fluid which control the shape of the transition. We comment on the usefulness of this measurement technique to study the glass transition. [Preview Abstract] |
Thursday, March 16, 2017 10:12AM - 10:24AM |
R17.00012: Long-Range Tractor Beams for Colloidal Particles Argha Mondal, Aaron Yevick, David Grier Optical micromanipulation is an increasingly powerful platform for research in soft-matter science. We recently extended the technique of holographic optical trapping to create practical implementations of tractor beams, modes of light that transport illuminated objects upstream along their entire length. These proof-of-concept demonstrations were limited by poor diffraction efficiency. Here, we introduce the technique of intermediate-plane holographic trapping that offers a hundred-fold increase in diffraction efficiency, trapping strength and transport speed. While the present talk focuses on transport in tractor beams, the same technique can be used to improve the performance of other modes of optical micromanipulation. [Preview Abstract] |
Thursday, March 16, 2017 10:24AM - 10:36AM |
R17.00013: Precision Particle Characterization with Holographic Video Microscopy Mark Hannel, David B. Ruffner, John C. Crocker, David G. Grier We previously have introduced methods for tracking and characterizing individual colloidal spheres through quantitative analysis of holographic video microscopy images. Here, we demonstrate that the performance of holographic particle characterization can be improved substantially by accounting properly for wavefront propagation through the microscope's imaging system. This analysis provides new design criteria for the microscope that we have implemented using a Shack-Hartmann wavefront sensor. The improved measurement system achieves consistent precision and accuracy throughout a measurement volume extending to 100 micrometers on a side. Experiments on well-characterized model systems confirm this technique's ability to track a micrometer-scale sphere to within 3 nanometers, while simultaneously measuring its radius with nanometer precision and its refractive index to within a part per thousand. [Preview Abstract] |
Thursday, March 16, 2017 10:36AM - 10:48AM |
R17.00014: Measuring nm-scale Interaction Potentials with a Microscope: Light Microscopy at Maximal Precision Brian Leahy, Matthew Bierbaum, James Sethna, Itai Cohen Tremendous effort has been put into improving both microscope design and imaging techniques over the past few decades, resulting in an enormous increase in image quality and resolution. We show that a similarly large improvement can be achieved in the analysis of microscopy images. We demonstrate our approach on an image of colloidal particles, improving the measurement of object positions and radii in a microscope image by up to a factor of 100 over current methods. We measure object properties by fitting experimental images to a detailed model of the physics of image formation, a method we call Parameter Extraction from Reconstructing Images (PERI). This unprecedented resolution immediately opens a new window into colloidal science. We use this resolution to measure 10 nm screening lengths in colloidal pair potentials from direct imaging of a colloidal suspension. Importantly, the ideas behind our technique can be readily applied to other imaging modalities such as brightfield microscopy or even STEM and STM. [Preview Abstract] |
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