# Bubble formation in long plastic tube with glass windows sealing the ends

Suppose you have a plastic tube that's length is much longer than its inner diameter sealed with glass on both ends. Inside the tube is high purity water with no bubbles i.e. all gases that remain in the water are dissolved and all the volume inside of the tube is filled with water.

When the temperature is increased sufficiently such that the water becomes saturated with the dissolved gases, what determines where bubbles will form first? Will bubbles form near the glass, along the plastic, or randomly along the length? What determines the bubble size?

Saturation of the water is determined by the partial pressure of the gas. In a sealed tube, heated or not, if the hydraulic pressure is high enough, bubbles won't form (won't displace liquid water) against that backpressure.

We have no way of knowing if the temperature rise expands the water more than the container (so gage pressure rises with temperature) or if the temperature rise expands the container more than the water (pressure drops with temperature).

Thus, as in boiling, there may be a superheated situation, where the water and its dissolved gasses are ready to erupt, but the container (like a pressure cooker) holds it all in.

When bubble-formation IS favored, there can be a variety of reasons that bubbles would form at a site. A surface that is 'wettable' by water, would inhibit bubble formation, while one that is hydrophobic would promote bubble formation (because there is less force holding the water to that surface). Any microbubble, perhaps protected by a crevice, can nucleate a stream of gas bubbles, as one sees in a pan of boiling water on a stove.

Or, the superheated liquid can form bubbles around the trail of ionizing particles, as in a bubble chamber invention of Donald Glaser.

Once a bubble forms and grows, the immediate vicinity of that bubble is no longer gas-saturated, so the liquid and bubble are not homogeneous with the rest of the container. The next bubble may form in a richer dissolved-gas portion of the liquid, elsewhere.

the bubbles will first form anywhere where pre-existing bubble nuclei are located on the surface of the tube or the glass. the perfect nucleus is a crevice or tiny pore in the plastic or glass surface in which a very small amount of air is present.

Bubbles that form in plastic tubes will grow to a size inconsistent with the amount of dissolved gas initially present in the water in the tube. this is due to diffusion of air into the tube which adds to the volume of air that already in the bubbles.

• Assuming a hermetically sealed system, would the size of the bubbles be largely determined by surface geometry? Assuming a horizontal orientation relative to gravity, would the bubbles move? – ZackWoodRD Feb 15 '18 at 21:48
• final bubble size would be determined by partial pressures and diffusion rates, about which I do not know enough to offer sane guesses... regarding bubble movement, it depends on the diameter of the tube. with small tubes (< 1/4" roughly) the bubbles tend to remain stationary; larger than that and the tube supports two-phase flow so the air and water can simultaneously move by each other in the tube. – niels nielsen Feb 15 '18 at 22:05