Jerome Fung

Colloidal self-assembly at an interface

Mix a drop of water into a vial of oil. With some surfactant and a vigorous shake, that one droplet has become thousands, and the total interfacial area has increased by an order of magnitude or more. Like the folded membranes in our mitochondria, the alveoli in our lungs, and the catalytic converters in our cars, oil-water emulsions contain a vast reservoir of interfacial area that can be used to control and transform the things that encounter it. The oil-water interface is especially well-suited to directing the assembly of colloidal particles, which bind to it rapidly and often irreversibly.

Measuring translational, rotational, and vibrational dynamics in colloids with digital holographic microscopy.

We discuss a new method for simultaneously probing translational, rotational, and vibrational dynamics in dilute colloidal suspensions using digital holographic microscopy (DHM). We record digital holograms of clusters of 1-μm-diameter colloidal spheres interacting through short-range attractions, and we fit the holograms to an exact model of the scattering from multiple spheres. The model, based on the T-matrix formulation, accounts for multiple scattering and near-field coupling. We also explicitly account for the non-asymptotic radial decay of the scattered fields, allowing us to accurately fit holograms recorded with the focal plane located as little as 15 μm from the particle. Applying the fitting technique to a time-series of holograms of Brownian dimers allows simultaneous measurement of six dynamical modes - three translational, two rotational, and one vibrational - on timescales ranging from 10(-3) to 1 s. We measure the translational and rotational diffusion constants to a precision of 0.6%, and we use the vibrational data to measure the interaction potential between the spheres to a precision of ∼50 nm in separation distance. Finally, we show that the fitting technique can be used to measure dynamics of clusters containing three or more spheres.

Imaging multiple colloidal particles by fitting electromagnetic scattering solutions to digital holograms

Abstract 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.

Design and Synthesis of Model Transparent Aqueous Colloids with Optimal Scattering Properties

We demonstrate the synthesis and self-assembly of colloidal particles with independently controlled diameter and scattering cross section. We show that it is possible to prepare bulk colloidal suspensions that are nearly transparent in water, while the particles themselves can be individually resolved using optical microscopy. These particles may be ideal model colloids for real-space studies of self-assembly in aqueous media. Moreover, they illustrate the degree to which the optical properties of colloids can be engineered through straightforward chemistry.

Holographic Measurements of Anisotropic Three-Dimensional Diffusion of Colloidal Clusters

We measure all nonzero elements of the three-dimensional diffusion tensor D for clusters of colloidal spheres to a precision of 1% or better using digital holographic microscopy. 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.

Generalization of the optical theorem for light scattering from a particle at a planar interface

The optical theorem provides a powerful tool for calculating the extinction cross section of a particle from a solution to Maxwells equations, relating the cross section to the scattering amplitude in the forward direction. The theorem has been generalized by a number of other workers to consider a particle near an interface between media with different refractive indices. Here we present a derivation of the generalized optical theorem that is valid for a particle embedded in the interface, as well as an incident beam undergoing total internal reflection. We also obtain an additional useful physical result: we show that the far-field scattered field must be zero in the direction parallel to the interface. Our results enable the verification of computations of scattering by particles embedded in interfaces and may be relevant to experiments on colloidal particles at fluid interfaces.

STUDYING THE DYNAMICS OF COLLOIDAL PARTICLES WITH DIGITAL HOLOGRAPHIC MICROSCOPY AND ELECTROMAGNETIC SCATTERING SOLUTIONS

Digital holographic microscopy (DHM) can measure the 3D positions as well as the scattering properties of colloidal particles in a single 2D image. We describe DHM and our analysis of recorded holograms with exact scattering solutions, which permit the measurement of 3D particle positions with ∼10 nm precision and millisecond time resolution, and discuss studies of the Brownian dynamics of clusters of spheres with DHM.