Ice Melting

by Oz

Here is our presentation slides, which include the images:

Heat actuator Water Crystallization SMALL

Water droplets were frozen at -78C for 10-30 minutes, before being imaged under 625x microscope.  Some water droplets were subjected to negative pressure for the freezing process (.5 and .33 atm respectively), and their crystal size compared with 1atm frozen water.  Glycerin and PEG (Poly ethylene glycol) were also frozen.

The point to take away from these images is that the ice crystals (the small, granular, sharp angled shapes) melt, turning into water droplets, followed by water droplets conglomerating into a larger droplet.  The first part of the crystal to melt are the fractal “arms” which extend past the center of the crystal; these are pointed out in a couple images.  The effect of the greater “stickiness” (surface attraction) from the wetted crystals, as well as a bulk amount of liquid outside the focus of the microscope, causes more crystals to move into the focus of the microscope as the melt continues.  The end of the melt process results in many individual domains of mini-water droplets; these then combine to form macroscopic droplets.  The initial size of these droplets is indicative of general crystal size; lower pressure results in smaller crystals.  These smaller crystals are somewhat difficult to image as droplets, as there is greater surface tension and thus reason to form larger droplets quickly.  In general, there were issues with imaging since the ice melted so quickly; using a controlled, sub-zero C environment for the entire microscope set-up and environment would help with this issue, and allow for greater time to find a perfect focus and thus greater possible resolution.  In such a scenario, direct measurements and analysis of the ice crystals could be readily performed, as opposed to the analysis of their post-melt state.

Neither PEG nor Glycerol formed large crystals similar to those of the water.  The glycerol is either very poorly crystalline or amorphous; it seems as though the PEG has an even weaker trend towards crystallinity.

Ice is a readily available material that illustrates many of the principles behind designing solid-liquid heat controlled actuators.