One of them wrote:
"I'm guessing you did this:True, I first dripped a couple of drops from a 10 mL medical syringe directly onto the surface. This is the most efficient method, but only allows the cooling of very small volumes.
You dripped some water from your fingers directly onto the surface of the peltier device, and it froze.
You then put some water in that small beaker, and place the beaker on the surface of the device. Well, unless your device surface and beaker bottom are absolutely flat, you're going to get somewhat poor thermal conduction. You could coat the interface with some of that thermally conductive paste if you are looking for a better result. It's the same white stuff you may see between the heatsink and the junction on the other side."
When I tried to freeze the water in the glass beaker, the surface of the beaker is not flat and the thermal conductivity between the beaker and the cooler was poor. Thermal paste would improve the results a bit, but as the glass itself has a poor thermal conductivity, It would still be inefficient. I think a better solution would be that instead of the beaker I use a smaller container with a thin bottom and made of some material with better thermal conductivity, for example aluminium or copper. Thermal paste could be applied as well if needed.
Another point was the inefficiency of the heatsink.
"Is there a fan on the heatsink on the hot side? Are you using that fan? Is it free to move air?"
"Well if I understand your setup photo, your fan is sitting directly on the table surface. That limits its ability to move air."There is a fan on the heatsink (that I got from an old computer lying in my attic), but a rather small one. I think it has 6 cm in diameter at most. During the experiment the heatsink was put on top of a large tube that allowed plenty of cool air to reach the heatsink. I am looking for a better heatsink but I don't want to spend money for it, because my resources are limited.