The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.
This award will support a twenty-four-month research fellowship by Dr. Anthony Anderson to work with Dr. Grae Worster at the University of Cambridge (DAMTP) in the United Kingdom.
Colloidal suspensions do not freeze uniformly. Instead, the frozen phase (e.g. ice) becomes segregated, trapping bulk regions of the colloid within, and a fascinating variety of patterns in the structure of the segregated ice emerge. These patterns depend on the freezing condition, particle concentration, and other properties of the colloidal suspension. Recent efforts by Prof. Worster and his collaborators to model the freezing of hard-sphere colloidal suspensions demonstrate that a planar ice interface can become thermodynamically unstable and break down spatially during solidification. The central aim of the current investigation is to identify the extent to which this thermodynamic mechanism for morphological instability underlies pattern formation in colloidal systems in general.
The investigation relies on a combination of theory and experiments. The Directional Solidification facility housed in DAMTP at the University of Cambridge is being used to perform the necessary experimental tests of the theoretical predictions of morphological transitions. This combination of mathematical analysis and experimentation has proven to be a very powerful approach in the development of the theory of alloy solidification, which shares several analogous features with the solidification of colloidal suspensions.
Many natural and technological processes involve the solidification of particle suspensions. In particular, ?freeze-casting? technology relies on the patterns of segregated ice as templates for engineering advanced composite materials. The phenomenon of ice segregation also underlies frost heave, whereby saturated soils expand as they freeze, which can lead to beautifully patterned ground, but cause damage to engineering structures. Other examples of interest include the preservation of cells, tissues, and perishable foods. The development of a theory to predict the conditions under which various patterns occur will consequently impact several applications.
This research project focused on the freezing behavior of fine-grained particle (colloidal) suspensions. When such suspensions are frozen the process typically occurs nonuniformly, whereby the frozen phase (e.g. ice) segregates from the suspension, trapping bulk regions of colloid within. Ice segregation produces a fascinating variety of patterns in frozen suspensions and has important implications for a number of applications, both natural and technological. For example, ice segregation underlies frost heave and expedites erosion in the cold regions of the earth, it is a critical factor in the efficacy of cryopreservation, and it can be utilized in a process known as "freeze-casting," which can be used to template bio-inspired composite materials. This project combined theory and experiment in order to elucidate the dynamical processes that lead to ice segregation. We developed a directional solidification protocol to freeze carefully prepared colloidal suspensions with precise control over the freezing conditions. This protocol enabled us to build a phenomenological map of the ice segregation patterns as a function of important control parameters: freezing rate, temperature gradient, and properties of the suspension, like concentration and particle size. Though the freezing behavior is sensitive to many parameters, the freezing rate is particularly important. In a series of experiments varying only the freezing rate, we identified two important regimes of ice segregation. At slow freezing rates, a mode of ice segregation known as ice lensing was observed, which is analogous to what is found in freezing soils -- the most forceful frost heave is manifested by ice lenses. Above a critical freezing rate, a separate mode ice segregation occurs, that features periodic alternating layers of ice and particles oriented perpendicular to the temperature gradient. The particle-rich layers in this regime appeared to have further ice segregation at a finer scale in the form of vertical dendritic ice crystals. This latter mode is relevant to "freeze-casting," where dendritic ice segregation is wanted, however the horizontal bands must be avoided. A principal theoretical challenge in freezing colloids involves encoding the physics at the scale of individual particles into model that can describe ice segregation at the macroscopic scale. The development of such homogenized continuum models was a major objective of this project. We have extended continuum models of frost heave to make predictions of ice lens formation that could be directly compared with experiments. To our knowledge, this is the first attempt to quantitatively test such models. Since most of the ice segregation behavior is nonlinear and spatially complex, I have also begun developing more general models for numerical simulations in collaboration with the University of Oxford. Such efforts will be valuable in developing the theory of freezing colloids in tandem with experiments and also in improving technologies like "freeze-casting," which require detailed control of the ice segregation process over a range of processing conditions.
