You might think that making ice is a simple business: just throw water into the freezer and it turns to ice. Simple, right? That's true if you just want to make a single tray of ice, but most of us prefer to have ice available on demand. That's why we have ice makers, devices that can make ice consistently for the many years that you will own your fridge. That takes a bit more engineering than a simple ice tray. Let's take a closer look at how the humble ice maker creates the ice to keep your summer drinks cool.
The basic ingredient of ice is, as you might expect, water. But you can't just throw any old water into an ice maker: good ice needs clean, fresh water. So, the first part of the ice maker is a filter that removes all of the solids (dust and dirt particles) and most of the dissolved chemicals as well. What comes out of this filter is crisp, clean, clear water that will make crisp, clean and clear ice. Assuming that you remember to change the filter, that is...
Now that we have clean water, we can start freezing it. Most ice makers are built around a tray that has semicircular or rectangular depressions in it. A valve above this controls the flow of water, filling the tray with enough water to fill the depressions, forming the shape of the ice cube. Because this tray is inside the freezer, it gets cold, and the water begins to freeze. Nearby is a sensor which is measuring the temperature of the water, waiting for it to reach a certain temperature (usually about -12°C/10°F).
Once it reaches this temperature, the water is solidly frozen and ready to remove. Ice is tricky, though: it sticks tightly to most surfaces, so you can't just tip it out. Instead, ice makers use heat to loosen the ice. Under the ice tray is a small electric heating element, which is triggered to heat the the tray slightly, creating a very thin layer of water between the ice and the tray. This is not enough to melt most of the ice, but the thin layer of water provides enough lubrication to allow a motorized arm to push the ice out, into the ice holding bucket.
The tray is then filled again, and the cycle continues until the ice bucket is full. On most cheap refrigerators, a wire pokes down into the bucket, and the ice piling up pushes this triggering a switch that stops the ice maker cycle. When you remove some ice, the wire falls down again, disengaging the switch and restarting the ice maker cycle. More expensive refrigerators will use infrared or other sensors that don't need a wire, but the principle is the same: they detect the level of the ice and disable the ice maker when it reaches a certain level.
Most of you will also be familiar with the curse of the cheap ice maker: you get a big block of ice because the ice cubes have stuck together. On the lower-end models that just dump the ice into a bucket, the ice cubes will stick together over time, eventually forming a solid lump of ice. This is a phenomenon called accretion, where the moisture in the air passing over the ice cubes bumps into the ice, and joins it. It's the same phenomenon that makes icicles on tree branches on a cold night: moisture in the air freezes and sticks together, forming ice crystals that then pick up more moisture, and so on.
Inside your ice maker, the ice cubes that are in contact will join together as they accrete new ice, forming a solid block. If you have a particularly bad refrigerator (or an old one), the ice cubes may also be melting slightly as the temperature of the freezer compartment rises. This creates a thin layer of liquid water on the surface of the ice cubes, which flows together, only to be refrozen when the freezer cools down again.
More expensive fridges deal with this problem by including a stirrer, a metal rod that is turned occasionally inside the ice bucket, separating the ice cubes. This also explains the mysterious noises that you may hear from your expensive fridge in the middle of the night: it isn't haunted, it is just stirring the ice. These more expensive fridges also separate the freezer and ice maker compartments, limiting the flow of cold air into the ice bucket so there is less moisture in the air to be captured.
So consider this the next time you grab a chunk of ice to chill your soda: the ice that you take for granted is the result of some sophisticated engineering and some basic science.