Horizontal Freezing Stage

Overview: M.S. Research

M.S. Advisor: Professor Ulrike G.K. Wegst | April 2015 - August 2016

For my M.S. research, I designed, built, and tested new, multipurpose freeze casting equipment to address the current limitations in freeze casting with two main aims:

Aim 1: Enable experiments that increase the fundamental understanding of the freeze casting process.

Aim 2: Improve the manufacturing process by creating equipment capable of fabricating larger samples in new shapes.

My overarching research approach was to iterate efficiently while keeping designs and experiments simple and cost effective. The goal was to identify and tackle as many problems as possible and prototype solutions that would guide future work.

Background: Freeze Casting Process

Freeze Casting is a material processing technique that utilizes directional solidification and phase separation in solutions or slurries to fabricate microporous structures with highly aligned porosity. The process consists of three main steps: solution/slurry preparation, unidirectional solidification with a thermal gradient (freezing), and lastly sublimation of solvent.

The image shows the typical microstructure of a sodium alginate freeze cast material. The polymer solution was unidirectionally solidified and then the ice was sublimated away, leaving the resulting structure.

Schematic of a typical single side freeze caster and freezing process. The slurry is poured into a PTFE mold with polished copper bottom. PID controlled resistive heaters control temperature. The copper rod is immersed in liquid nitrogen and acts as… — Schematic of a typical single side freeze caster and freezing process. The slurry is poured into a PTFE mold with polished copper bottom. PID controlled resistive heaters control temperature. The copper rod is immersed in liquid nitrogen and acts as a heat sink. (b) Controlled freezing results in the formation of lamellar ice crystals that expel the particles and/or solutes and are concentrated in the space between the crystals. (c) The crystals template the structure and after sublimation the resulting structure is highly-aligned and porous, as the SEM images show. Reproduced from *U. G. K. Wegst, H. Bai, E. Saiz, A. P. Tomsia, and R. O. Ritchie, "Bioinspired structural materials," Nat Mater, vol. 14, pp. 23-36, 01//print 2015.*

Horizontal Freezing Stage

The horizontal freezing stage (HFS) was designed to facilitate experiments to study the kinetics of ice-phase growth and mechanisms of ice templating in ice-polymer composites. In-situ light microscopy of controlled directional solidification would be the basis of such experiments. The primary goals were to:

Observe the formation and progression of Mullins-Sekerka instabilities that cause the primary dendrites to form as a function of polymer concentration and cooling rates over long relative distances (~50 mm).
Study conditions to enable single domain growth using microfluidic channels and Hele-Shaw cells.
Create planar, cellular, and dendritic solidification fronts to observe, under controlled conditions, the microstructure interactions of the growing ice crystals.

Theses primary goals required a new solidification stage to be designed. The need for a new stage created an opportunity to pursue new and improved capabilities for fabricating freeze cast materials. By designing a multipurpose device, secondary goals could be pursued such as:

Demonstrate a modular cold block design that could allow for multiple cold blocks to be configured in a flexible manner for emerging needs and experiments.
Fabricate larger samples in new shapes, specifically sheet materials.
Double sided freezing capabilities.