The phenomenon you're observing with inverted icicles in your ice cubes is known as "negative growth" or "regelation." It occurs due to the unique properties of water and the way it behaves when it freezes and melts under pressure.
When water freezes, it expands and takes up more space. This expansion can create pressure within confined spaces, such as ice cube trays or small openings within the ice. As the pressure increases, it can cause the ice to melt locally, creating small channels or fissures within the ice cubes.
In your freezer, when the temperature drops below the freezing point, the water in the ice cube trays starts to freeze from the outside. As the ice grows inward, the water inside the ice cubes is still in a liquid state due to the insulating effect of the ice surrounding it. This liquid water can be forced through the channels or fissures within the ice cube, where it encounters extremely cold temperatures. This rapid freezing of the liquid water forms small spikes or inverted icicles, which grow inward rather than outward.
Negative growth or regelation can also occur when two ice cubes come into contact with each other. The pressure at the point of contact can cause melting, and when the pressure is released, the water refreezes, creating a bond between the two ice cubes.
It's important to note that negative growth or inverted icicles are not commonly observed in everyday situations and may depend on specific conditions, such as the temperature, pressure, and the configuration of the ice cubes.