Viscosity and Glass Innovation | Materials Science | Corning

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Viscosity

Go with the Flow

Go with the Flow

In creating pure and tough specialty glass, Corning innovators are masters of viscosity.

You can visualize one of the most important scientific concepts in glassmaking by considering something that’s probably in your kitchen cupboard right now.

Open a jar of honey or molasses and observe its thick, semifluid state. Tip it, and – at least at room temperature -- it doesn’t pour quickly. In scientific terms, it’s resistant to flow.

And as it makes its way somewhat reluctantly over the edge of the jar, it’s easy to imagine the internal friction building up at the molecular level within the thick, sticky mass.

Scientists measure that friction in terms of viscosity. The unit of measure is known as a poise – named for 19th-century French physicist Jean Leonard Poiseuille, who developed breakthrough studies of how human blood flows through veins.

When a fluid exhibits a lot of resistance to flow, the viscosity is high. Honey, for example, may have viscosity of about 10,000 centipoise (100 poise) at room temperature.

When a material flows freely, though, there’s very little internal friction and therefore a much lower viscosity. Water has a viscosity of about 1 centipoise at 70 degrees.

Generally, as a mixture heats up, its viscosity drops. Simmer some honey on the stove, and it’ll thin down – exhibiting  a much lower viscosity than it did in its room-temperature state.

What does this have to do with the making of specialty glass?

Just about everything, actually.

Take the natural tendency of glass to form microscopic crystals as it cools.

Each molten mixture has a specific point – known as the liquidus temperature – at which it is highly vulnerable to crystal formation through the bonding of silicon and oxygen atoms. If that happens in the making of ultra-pure optical glass, for example, an entire batch is ruined.

Corning scientists measure the viscosity of the mixture when it reaches liquidus temperature, and have found that crystals are less likely to form when viscosity is higher, rather than lower. The higher viscosity acts as a kinetic barrier, causing the silicon and oxygen atoms to move too slowly to find each other and crystallize.

So, to the extent our scientists carefully calibrate these variables, they can more quickly produce stunningly pure glasses.

A deep understanding of viscosity is also key to the success of the glass ion-exchange process, which results in the tough, damage-resistant properties of Corning® Gorilla® Glass.

In ion exchange, glass is immersed in a molten salt solution. Potassium ions (electrically charged particles) in the solution migrate into the glass surface, replacing the smaller sodium ions originally in the glass.

As the glass cools, the larger potassium ions compress the glass together, and this layer of compressive stress at the surface of the glass is what protects it from damage.

This is also the point at which viscosity plays a big role in the toughness of the glass.

A lower-viscosity glass can accommodate the larger potassium ions without the stress layer as part of a phenomenon scientists call “stress relaxation.” This loss of compression results in a glass surface more vulnerable to nicks and scratches.

So designing  ion-exchangeable glasses with higher viscosity maximizes the desirable compression, allowing Corning to produce the tough, pristine glasses our customers need.

Computer modeling of glass formulas helps our scientists more accurately predict the viscosity of different glass compositions at all stages of the manufacturing process. Corning has developed the world’s most advanced viscosity model, which – unlike other commonly used models – offers an accurate prediction of the behavior of viscous materials even at low temperatures.

The model – developed by Dr. John Mauro, Dr. Doug Allan, and Dr. Marcel Potuzak – has helped significantly increase the rate of new-glass innovation at Corning. In considering new compositions to meet demanding requirements, our scientists are able to get very close to predicting glass viscosity before reaching the experimental stage in the lab.

That, in turn, is another reason Corning is able to keep producing specialty glasses that are changing the way we live today.