Thomas A Caswell

The physics of "human" scale systems far from equilibrium is vital to our every day lives, but is not thoroughly understood. Examples include fluid singularities and disordered solids like granular packing and glassy systems. Recent advances in instrumentation allow access to systems at smaller spatial and temporal scales, as will as greatly increasing the volume of raw information gathered.

I am interested in studying human scale physics, pushing the limits of instrumentation, and developing better methods of handling and analyzing large data sets.

I study jamming in dense soft colloidal systems in three dimensions. Using N-isopropylacrylamide (NIPA), a temperature responsive polymer, colloids the size of the colloids, and hence the volume fraction can be continuously changed while the system is being observed. As the volume fraction is change the colloids go from being a gas, simple Brownian motion, to a jammed state where the colloids movement has become highly restrained. The idea of jamming and the jamming phase diagram, introduced by Nagel and Liu, provides a frame work to understand this change. Using the language of the jamming phase diagram, the system is at finite temperature, zero sheer stress and has a tunable volume fraction. Using high speed confocal microscopy I can image deep with in the sample. From the images individual particles can be identified and tracked in time. This allow both dynamic and static signatures of the jamming transition to be studied.

The idea of jamming has been proposed as a way of understanding the glass transition. By better understanding amorphous colloidal systems at the jamming transition we may be able to better understand the glass transition, which is one of the last unsolved problems in condensed matter physics.

Drops placed on a sufficiently hot surface will levitate above the surface on a vapor layer. This is known as the Leidenfrost effect and can easily be observed with water in a hot frying pan or liquid nitrogen on the floor. The vapor layer is generated by the drop as it evaporates and serves to both insulate the drop, prolonging the life time, and lubricates it motion, making the drop effectively friction-less. Recent work done in the Nagel lab has demonstrated that high-speed interference imaging can be used to measure the shape of the bottom of a Leidenfrost drop. I have worked to automate the identification of the interference fringes to allow us to track the shape over time.

The algorithms for finding and tracking particles in microscopy were developed by Crocker and Greir and have been widely used by the community. There are a number of implementations of the algorithm in IDL and MATLAB, however the execution time of these implementations becomes unreasonably long for extremely large data sets.

Building on the work of Peter Lu, who reimplemented the particle location algorithm in C++, I have reimplemented the Crocker Grier algorithm in C++ along with a framework for efficiently computing spatial and temporal correlations. Simple testing suggests the c++ code is ~20x faster than equivalent MATLAB or Python code.

Documentation and source code are available. If you do use this code, please contact me. I have also written a Python implementation of the tracking algorithm. While slower than the C++, it is significantly easier to use.

At Cornell I worked for Professor Sol Gruner with the detector development group on Pixel Array Detectors (PADs), a class of high-speed direct detection x-ray detectors. I primarily worked on the control systems and experimental applications of PADs, participating in several experiments conducted at the Cornell High Energy Synchrotron Source (CHESS) and adapting a PAD for use in an electron microscope.

In the summer of 2005, I helped prepare for and execute a two week experimental run at CHESS. In collaboration with Jin Wang's group from Argonne National Lab and Visteon Corporation we conducted two radiography experiments. The first experiment was time resolved tomography of the spray cone of a low pressure injection nozzle for gasoline engines. In the second experiment, we studied shock waves from fuel injection in a simulated diesel engine.

For my undergraduate senior thesis I finished adapting a 16x16 PAD for use in scanning transmission electron microscopy (STEM) in collaboration with Peter Ercius from Professor David Muller's group. In STEM, a small electron probe is rastered across the surface of a sample and a full diffraction pattern is formed at each raster position. Commercial STEM systems use point detectors to image the transmitted electrons reducing the information to one or two channels. Point detectors sum intensity across their active area, losing almost all of the spatial information. However, commercially available available area detectors are not practical for use in STEM due to low frame rates. The PAD used for this work is capable of framing at 1.1kHz, making it's use in STEM practical. A practical area detector for STEM could revolutionize the field of electron microscopy by allowing access to orders of magnitude more information from each sample with comparable collection times to current point detectors.