Q: The pattern of bubbles in ice cubes has always fascinated me. Why is the highest concentration in the centre, with spines of progressively smaller bubbles radiating outwards?
A: Your correspondent has observed two effects at work in the solidification of ice 鈥� the segregation of a solution into two separate physical forms or phases on solidification, and the way in which crystals tend to grow.
Tap water contains, on average, 0.003 per cent dissolved air by weight but the solubility of air in ice is very small. When you put your ice tray in the freezer, heat flows away from the water through each of the outer faces of the cube and ice crystals containing almost no air will start to form from each of these outer surfaces. These will tend to grow inwards in the form of columnar crystals.
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The unfrozen water, lying between these crystals and in the centre of the cube, gradually becomes completely saturated with the air that is driven from the earlier forming ice.
When the concentration of air in the remaining water reaches 0.0038 per cent it forms what is known as a eutectic composition 鈥� a mixture that has the lowest freezing point of all possible mixtures of that particular substance. When this occurs it all freezes at once, separating into a mixture of approximately 2.92 per cent air by volume, and ice. The smaller bubbles radiating outwards are from the eutectic which was trapped between the columnar crystals and the high concentration in the middle is where the remaining bulk of the eutectic had been trapped.
An extreme example of the formation of columnar crystals can be seen if you bite into an iced lollipop. The segregation of dissolved elements and the formation of columnar crystals also occurs in the casting of metals.