I saw Manu’s recent video showing snowflakes (ice crystals) melting to form water. I thought I’d try something similar with salt crystals, which slowly dissolve and re-form when placed in water containing a high concentration of dissolved salt:
All videos shown at 120x actual speed.
I made a super-saturated salt solution by pouring two tablespoons of ordinary table salt into about 5 tablespoons of water, microwaving the mixture, and then (carefully) stirring the boiling solution rigorously until the salt stopped dissolving. I then poured a tiny amount of the solution onto the sticky side of a slide prepared using scotch tape. I did not place another piece of tape over the droplet to “seal” the slide because I wanted the solution to be in contact with air—the evaporation of the water (which leaves behind the dissolved salt) is what drives the formation of salt crystals.
The second video shows a big one of the resulting crystals slowly changing shape due to salt being deposited in some parts and re-dissolved from other parts. The different rates of mass deposition in different locations on the crystal actually result in a small torque being exerted on the crystal, and so you can even see it spin slightly over the course of the video. Near the end of the video the water around it mostly evaporates and it approaches its steady-state shape, which is noticeably more square-shaped.
In order to image the evaporation and crystal formation, I used my iPhone’s “timelapse” feature, which records videos at roughly 1 frame per second. One thing I thought was awesome is how the dissolution/reformation of the salt crystals looks very similar to the behavior of the ice crystals in Manu’s snowflake video—the main difference is that the process happens much, much slower for salt crystals, hence why I needed to make a timelapse video. Both phenomena are examples of phase transitions, and so their underlying physics is very similar.
Here are some other videos from my experiments. You can see directed motion of small salt crystals, which are due to small forces that act on them due to differences in salt concentration throughout the solution.