Jerome Fung

Assistant Professor, Physics and Astronomy

Title

Research in my group focuses on colloidal suspensions and light. Colloidal suspensions, such as paint, consist of small particles (typically, at least 10 times smaller than the diameter of a human hair!) suspended in a fluid. The particles are large enough, however, to interact strongly with light and can often be seen under a microscope.

Optical Trapping

Optical tweezers use a focused laser beam to exert and measure forces on colloidal particles -- like tractor beams in science fiction movies, albeit on a microscopic scale. Optical tweezers can be used to measure the forces exerted by biological molecules, assemble objects at the nanoscale, and study how colloidal particles interact with each other. While the behavior of spherical particles in optical tweezers is well-understood, the behavior of non-spherical particles in optical tweezers (or in even more complicated optical beams) is not.

We have a custom, home-built optical tweezers microscope in the lab that can create multiple optical traps. We're using this setup to explore how non-spherical particles such as clusters of spheres can be manipulated using light.

Video of a 1-micrometer-diameter silica sphere in our optical tweezers microscope while the trapping beam is blinked on and off.

Interactions and Rheology of Soft Colloidal Spheres

Why does glass behave like a solid even though it has the disordered structure of a liquid? This is one of the biggest open questions in condensed matter physics. We are starting a new project to see whether a system of soft colloidal spheres could be useful as a model system for studying the behavior of glasses.

Specifically, we're investigating the properties of a recently-developed particle that consists of an elastic, nearly-transparent polymer shell surrounding a small hard core. These particles change their size with temperature (allowing us to control the phase behavior of suspensions) and can be imaged with microscopy and manipulated with optical tweezers. This could eventually allow us to see exactly what individual particles do when you mechanically deform a glassy suspension of these particles -- something that is impossible to do in a molecular glass.

Currently, we're working on preparing the particles, measuring their pair interactions using optical tweezers, and measuring the mechanical behavior of suspensions of the particles.

Computational Modeling of Optical Trapping & Manipulation

In parallel to our experimental efforts, we're also using computers to model how particles might behave in our optical tweezers setup. Specifically, while we can't perform pencil-and-paper calculations, we can numerically predict the forces and torques that a given particle in a given beam will experience. We combine the predictions of the optical forces with calculations of hydrodynamic friction and with thermal fluctuations to predict how particles such as clusters of spheres behave in optical tweezers and optical vortex beams.

These modeling efforts rely on the ability to predict how oddly-shaped particles interact with (or scatter) light. This turns out to be harder than it might sound, particularly for colloidal particles that are comparable in size to the wavelength of the light. So we also work on extending the capabilities of software tools that can perform such calculations using algorithms such as the extended boundary condition method and the discrete dipole approximation.

Simulated trajectory of a cluster of two rigidly-bound, 1.6-micron-diameter silica spheres held in optical tweezers. This cluster is stably trapped, but turns out to both wobble and spin in the beam.