We capture digital holograms of micrometer-scale silica rods and sub-micrometer-scale Janus particles freely diffusing in water, and then fit numerical scattering models based on the discrete dipole approximation to the measured holograms.
This inverse-scattering approach allows us to extract the position and orientation of the particles as a function of time, along with static parameters including the size, shape, and refractive index. The best-fit sizes and refractive indices of both particles agree well with expected values.
Furthermore, the measured translational and rotational diffusion coefficients for the silica rods agree with hydrodynamic predictions for a spherocylinder to within 0. We also show that although the Janus particles have only weak optical asymmetry, the technique can track their 2D translation and azimuthal rotation over a depth of field of several micrometers, yielding independent measurements of the effective hydrodynamic radius that agree to within 0.
The internal and external consistency of these measurements validate the technique. Because the discrete dipole approximation can model scattering from arbitrarily shaped particles, our technique could be used in a range of applications, including particle tracking, microrheology, and fundamental studies of colloidal self-assembly or microbial motion. We discuss digital holographic microscopy DHM , a 3D imaging technique capable of measuring the positions of micron-sized colloidal particles with nanometer precision and sub-millisecond temporal resolution.
We use exact electromagnetic scattering solutions to model holograms of multiple colloidal spheres. While the Lorenz-Mie solution for scattering by isolated spheres has previously been used to model digital holograms, we apply for the first time an exact multisphere superposition scattering model that is capable of modeling holograms from spheres that are sufficiently close together to exhibit optical coupling.
Tutorial on off-axis electron holography.
We study both dimers and triangular trimers of spheres, for which no analytical calculations of the diffusion tensor exist. We observe anisotropic rotational and translational diffusion arising from the asymmetries of the clusters. In the case of the three-particle triangular cluster, we also detect a small but statistically significant difference in the rotational diffusion about the two in-plane axes. We attribute this difference to weak breaking of threefold rotational symmetry due to a small amount of particle polydispersity.
Our experimental measurements agree well with numerical calculations and show how diffusion constants can be measured under conditions relevant to colloidal self-assembly, where theoretical and even numerical prediction is difficult. The technique is able to track the center of mass of the rod to a precision of 35 nm and its orientation to a precision of 1. Digital holographic microscopy is a fast three-dimensional 3D imaging tool with many applications in soft matter physics.
Recent studies have shown that electromagnetic scattering solutions can be fit to digital holograms to obtain the 3D positions of isolated colloidal spheres with nanometer precision and millisecond temporal resolution. Here we describe the results of new techniques that extend the range of systems that can be studied with fitting.
We show that an exact multisphere superposition scattering solution can fit holograms of colloidal clusters containing up to six spheres. We also introduce an approximate and computationally simpler solution, Mie superposition, that is valid for multiple spheres spaced several wavelengths or more from one another.
We show that this method can be used to analyze holograms of several spheres on an emulsion droplet, and we give a quantitative criterion for assessing its validity. This has been used to explain why colloids often bind to liquid interfaces, and has been exploited in emulsification, water purification, mineral recovery, encapsulation and the making of nanostructured materials. However, little is known about the dynamics of binding. The relaxation appears logarithmic in time, indicating that complete equilibration may take months. Surprisingly, viscous dissipation appears to play little role.
Instead, the observed dynamics, which bear strong resemblance to ageing in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. These results may provide clues to longstanding questions on colloidal interactions at an interface. Holographic microscopy is a unifying theme in the different projects discussed in this thesis.
The technique allows one to observe microscopic objects, like colloids and droplets, in a three-dimensional 3D volume. Therefore, one can capture 3D information at video frame rates. The price for such speed is paid in computation time. The 3D information must be extracted from the image by methods such as reconstruction or fitting the hologram to scattering calculations. Using holography, we observe a single colloidal particle approach, penetrate and then slowly equilibrate at an oil—water interface.
Because the particle moves along the optical axis z-axis and perpendicular to the interface holography is used to determine its position. We find that the capillary force pulling the particle into the interface is not balanced by a hydrodynamic force.
An introduction to holography
A separate project discussed here also examines colloidal particles and fluid-fluid interfaces. But the fluids involved are composed of colloids. With a colloid and polymer water-based mixture we study the phase separation of the colloid-rich or liquid and colloid-poor or gas region. In comparison to the oil—water interface in the previously mentioned project, the interface between the colloidal liquid and gas phases has a surface tension nearly six orders of magnitude smaller.
So interfacial fluctuations are observable under microscopy. We also use holographic microscopy to study this system but not to track particles with great time and spatial resolution. Rather, holography allows us to observe nucleation of the liquid phase occurring throughout our sample volume. Young's law predicts that a colloidal sphere in equilibrium with a liquid interface will straddle the two fluids, its height above the interface defined by an equilibrium contact angle.
But little is known about the dynamics of binding, or any aspect of the interaction between a particle and an interface outside of equilibrium. Chapter 2 reviews the importance of interfacial particles in materials science, and serves as a partial motivation for the work presented here. Chapter 3 describes the apparatus and experimental procedures employed in the acquisition of our data, with a short review of experiments that led to the current set.
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Special attention is paid to the optical apparatus and the custom sample cells we designed. Chapter 4 deals with non-adsorptive interactions between colloidal particles and liquid interfaces. A theoretical discussion founded on but not wedded to classical DLVO theory is presented before the results of our experiments are analyzed.
It is shown that particle interface interactions may be purely repulsive or contain an attractive component that results in binding to the interface that is not associated with breach. In chapter 5 the adsorption of polystyrene microspheres to a water-oil interface is shown to be characterized by a sudden breach and an unexpectedly slow relaxation. Particles do not reach equilibrium even after seconds, and the relaxation appears logarithmic in time, suggesting that complete equilibration may take months.
Instead, the observed dynamics, which bear strong resemblance to aging in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. We discuss a new method for simultaneously probing translational, rotational, and vibrational dynamics in dilute colloidal suspensions using digital holographic microscopy DHM.
The model, based on the T-matrix formulation, accounts for multiple scattering and near-field coupling. We measure the translational and rotational diffusion constants to a precision of 0. Finally, we show that the fitting technique can be used to measure dynamics of clusters containing three or more spheres. We have built a simple holographic microscope completely out of consumer components.
Tutorial on off-axis electron holography.
We obtain at least 2. Micrometer-sized colloidal particles are a model system for understanding self-assembly in condensed matter. Here I present the results of digital holographic microscopy experiments that probe the 3D structure and dynamics of these systems. Skip to main content. Main Menu Utility Menu Search. An introduction to holography Dennis Gabor developed holography in the s as a way to improve the resolving power of electron microscopes. If the two waves are coherent meaning the phase is well-defined at each point in space , they can interfere, or beat, with each other to produce interference fringes, like this: In the picture above, the small blue particle is illuminated by the red beam.
Electron holography microscope with spatial resolution down to one atom
In-line holographic microscopy The in-line holographic microscope operates as described above. See also: Current research areas , Holographic microscopy. Publications Wang, A. Contact-line pinning controls how quickly colloidal particles equilibrate with liquid interfaces. Share on Facebook.
Share on Twitter. Share on Linkedin. Dipolar Magnetism in Self-assembled nanoparticle Systems. Carbon nanotubes structure vs. Counting Statistics and Accuracy within Electron Microscopy. ESEM — Reaction cell for in situ studies of catalyst. Thin film functional oxides for catalytic water splitting-characterization. For these purposes, a robust building for housing the electron microscope was constructed. In particular, acoustic-absorption materials were placed on the interior walls of the building to reduce sound noises, and precise-room-temperature control units were installed; moreover, the electron microscope was covered with magnetic-shielding "permalloy" to reduce ambient magnetic field effects.
For performance evaluation of the developed electron microscope, its information transfer, indicating a specimen fine structures transfer capability to its camera, was measured by using a tungsten single crystal. The results indicated that the electron microscope can transfer crystal-structure information of a world-finest resolution of 43 pm under the corrected spherical aberration condition.
Furthermore, gallium-nitride GaN single crystal observation showed the atomic resolution of 44 pm, i. This result demonstrates that the developed electron microscope can visualize specimen structures and electromagnetic fields at the atomic level. In particular, at RIKEN, scientific achievements in the following three areas were realized: three-dimensional 3D observations of internal electric potentials in and around materials, fine magnetic behavior observations of microscopic region within magnets, and observations of magnetic vortices called "skyrmions", promising candidates for memory devices with ultra-low power consumption.
Ongoing forward, Hitachi will collaborate with world-first-class research institutes including RIKEN, and will use the developed holography electron microscope to study quantum phenomena of emergent materials by measuring their electric and magnetic fields at the atomic scale, for example, high-performance magnets, large-capacity rechargeable batteries, low-power-consumption memory devices, and high-temperature superconductors.
And in doing so, Hitachi will contribute to the advancement of quantum mechanics, condensed-matter physics, materials science and technology, and other fields while developing new materials that will support a sustainable society.
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