coating

This implies that you are immersing your substrate in a liquid (which one?) and observing bubbles.

How small can the bubbles be and still be acceptable? Zero is not a believable answer. Why do the bubbles matter? What is the substrate?

These types of questions are impossible to answer upfront because 1) there is not enough background information, and 2) you are already proposing most of the answer

John Aspen Research, -

Opinions expressed herein are my own and may not represent those of my employer.

Water, everyone's favorite.

I was kind of hoping for zero. The problem is optical, the bubbles scatter light.

ABS. Polishing the surface sort of helps but is not reliable. I was thinking a hydrophilic coating might work but it would have to durable, like in years. So far, I have just found hydrophobic coatings and, as expected, those do not work.

Silane? Would it be durable? I'll check it out.

It will be in cold water all the time. We did try flame treatment, mainly because it was easy to have done.

Sorry that I haven't been able to reply all week. Rolf has done a great job as always.

A quick-and-dirty route to silane is to use "Rain-X" or any of the other consumer products that are applied to windshields. Rain-X may not be the most durable, but again, as a first shot, it would be fast.

Longer term, there are plenty of silanes available from GE, Dow Corning and Gelest and others. Gelest in particular has lots of great technical literature explaining in detail the technology, the chemical reactions,... as well as a great selection of products, both commerical and developmental.

the tank (or do whatever it is that you are doing) in order to prevent bubbles? Is degassing the water an option? That would not help with bubbles of air formed during the filling, but it would prevent additional bubble from forming at a later date.

Lastly, what I was getting at at the bottom of my earlier post is the thought expressed by Eisenhower: if you can't solve the problem, try solving a bigger problem. You defined the problem very narrowly for us. Is there something in the larger process that can be changed/ altered/added to give us more wiggle room?

John Aspen Research, -

Opinions expressed herein are my own and may not represent those of my employer.

I can't give a lot of details (deep, dark industrial secret). A better design would at least reduce the problem but not without multiplying the cost and size. Fortunately, the problem only happens in a few installations, where the process water is cold and our device is located far enough away for the water to warm up. Right now the only solution is to locate our equipment closer, but where it may be difficult to service. Degassing is a possibility but our customers are very sensitive to up-front cost. The good news is our competitors have the same problem and they have never bothered to solve it. I was hoping for a quick, painless fix.

This sounds like your issue has to do with surface tension of the water, since warm water has a lower surface tension than cold water. Therefor, in case you are allowed to and it is practical, can't you add "enough drops" of a tenside (e.g. Pril detergent) to the cold water to lower its surface tension sufficiently?

You could also add a deaerating agent to the cold water, which causes rapid accumulation (enlargement) of the gas bubbles dispersed in a liquid, which results in considerably improved escape of air from the liquid (see

Are these air bubbles only on the surface of the coated plastic or also on the inside walls of the water container and do the air bubbles prefer certain areas on the surface of the coated plastic? Is the coated plastic surface perfectly clean, i.e. stain- & grease-less?

I reckon the flame treatment you tried did not do the trick ...?

What about PTFE coating? Fairly Inert and durable, low surface tension....

By the decrease in solubility of air as the water warms up.