How to freeze bottles without making a mess

Have you ever left a bottle of liquid in the freezer, only to find it cracked or shattered? To save you from tedious freezer cleanups, researchers at the University of Amsterdam have investigated why this happens, and how to prevent it. They discovered that while the liquid is freezing, pockets of liquid can get trapped inside the ice. When these pockets eventually freeze, the sudden expansion creates extreme pressure – enough to break glass.

“Newton had an apple fall on his head. I found my freezer full of broken glass,” jokes Menno Demmenie, first author of the new study that was recently published in Scientific Reports.

He continues, more seriously: “The usual explanation for frost damage is that water expands when it freezes, but this does not explain why half-filled bottles also burst in our freezers. Our work addresses how ice can break a bottle even when it has plenty of space to expand into.”

To understand this process, the researchers used a special dye, methylene blue, to track freezing in open cylindrical glass containers. The dye easily dissolves in water and turns it blue. The dye becomes transparent when the water freezes, as it gets pushed out of the ice crystals. This allows the researchers to see exactly when and where ice forms.

Having filmed tens of samples of blue-dyed water freezing in a –30 °C environment, the researchers cracked the case. Ice breaks glass when the top surface of the water – the one open to air – freezes first. The rest of the water naturally freezes from the outside in, creating a pocket of liquid water surrounded by ice on all sides. When this pocket freezes too, it exerts an extreme amount of pressure on its surroundings, in many cases enough to break glass.

The researchers estimate the pressure exerted by the ice in their experiments to be around 260 megapascals, enough to dent high-strength steel and four times as much as their glass vials can withstand.

Smaller, water-repellent bottles can save the day

By testing glass containers of different sizes and with different surface coatings, the research team discovered that there are two ways to reduce the risk of trapped pockets of water forming.

The first way is to ensure the water gets colder before it begins to freeze. While water can start freezing at 0 °C, it is possible for liquid water to get ‘supercooled’ to subzero temperatures. Freezing needs to start somewhere, and the start of the phase transition can be delayed.

Supercooled water freezes differently than water that freezes closer to the freezing point. Rather than growing as a crystalline block, it freezes along fingerlike branches (‘dendrites’). In the experiments, this type of freezing turns the dyed water a darker shade of blue before it freezes completely.

The researchers discovered that this unusual ice growth results in a large amount of small air bubbles getting trapped within the ice, something which appears to relieve enough pressure to prevent fracturing. “The discovery of the link between these air bubbles and the freezing of supercooled water came as a surprise, and we hope to investigate this in more detail in future work,” comments Demmenie.

Supercooling and the subsequent bubble formation was seen more often in narrower bottles. Comparing two bottles of different sizes filled with the same amount of water, the water in the smaller bottle cools down faster thanks to the larger surface it has per unit volume. This increases the chance of water cooling further below 0 °C before it starts to freeze.

The second way to prevent trapped liquid pockets is to make sure that not the top surface, but the bottom of the container freezes first. So long as the top surface doesn’t freeze over before the rest of the water does, the ice will simply expand into the open space above.

The team found that the shape of the water’s surface plays a key role. In untreated glass containers, and in those coated with a layer that attracts water (being hydrophyilic), the water surface curves up against the glass. Thanks to the water molecules at the edge having less freedom to move, this region tends to be the first to freeze.

Water in containers with water-repelling (hydrophobic) coatings instead has a flat surface. Thanks to this, it is much more likely to freeze from the bottom up, preventing trapped liquid pockets from forming and reducing the risk of breakage.

So, what do we learn from this? If you don’t want shattered bottles in your freezer, choose smaller bottles and ones with more water-repelling surfaces. (Many plastics are more water-repelling than glass - think of the PET bottles that many soft drinks come in, or of hard plastic like the PP used for Dopper bottles.) Beyond avoiding messy kitchen disasters, these new findings will also help with understanding and preventing frost damage in other places, including buildings, roads and historical artifacts.

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