Anyone up for a bit of arithmetic? - Page 1
Forum section: electronics / general

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 03, 2021, 08:30:55 am
content: 30 years ago I bought a bottle of silver paint from RS. I have none left so needed some more.Bit of a shock looking at the price of the stuff so settled on buying a tub of MG chemicals silver plated copper particle water based stuff. The silver stuff has acetone so it has never been a problem drying out, one just adds acetone and it all dissolves again.Still way too expensive in small bottles I decided to buy a can, 850 ml for several hundreds pounds, decant it into 30 ml bottles and get some of my money back via ebay. That aint going to happen now as ebay broke itself on Friday and though I can sell stuff I cannot get the money out because of rule changes, it is no wonder they make so much money when they steal 100% of ones hard earned cash.Anyhow, that's nowt to do with this really.So, I have these 30 ml bottles full of predominantly water with metal particles. I know from manufacturers data that 100 grams of the liquid, when dried should weigh 42 grams. I know that the density of a 50% copper silver mix is ten ish (I have no way to get that guess any better than that, obviously water is density one. 30 ml in these bottles is 50 mm tall.As this stuff settles out of solution really quickly I wanted to make sure that I had a fair amount of mix, it takes ages to actually mix the paint and it only stays mixed for 5 minutes.With the above data I calculate that I should have about 7 mm of sediment in each bottle if the sediment were solid. Clearly it isn't solid so let's say double it for a reasonable approximation.This is not what I am seeing. This picture is after 24 hours.Other info: On a balance beam with the water 68 mm from pivot the paint is 53 mm from pivot in order to balance.All of this data should corroborate no matter how one does it but it just not adding up. So. moment, density, volume and practise, the picture being the actuality of the situation.Not important, just irritating and I just know that someone on here will take on the challenge..

author: ajb
author_rank: Super Contributor
timestamp: May 03, 2021, 06:06:16 pm
content: Short of mechanical compression or centrifuging, I don't think even a 2x solid density approximation is likely to apply here. Presumably the particles are quite small, and fluid mechanics at that scale are not the same as they are at larger scales more familiar to humans, so it would probably take much much longer than 24hrs to get it to settle to the point you're expecting, if it ever will. Plus, is the paint really just water and metal? I would expect there to be some sort of binder in there as well, otherwise the dried paint would just brush off the surface. That would confound your density estimates as well as possibly affect the settling of the metal. (Even water-based paints will sometimes include additional solvents that evaporate as part of the drying process, hence why you can't just wash the paint off the walls of your house with water.)

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 03, 2021, 06:12:03 pm
content: There is a binder of some sort but I assume it is similar in density to water and way different from the solids.I don't think the binder will affect much anyway, It is probably the "settling" that is making rubbish of the calculations but it seems excessive to me even after week it is not much lower than it is after a day and it is 5 times as much as my calculations expect it to be.Just a conundrum. Good job really, would life be worth living if we understood everything, I think not.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: May 03, 2021, 06:31:44 pm
content: This is basic colloid science. The particles are inherently attracted to each other and, without some mechanism to prevent it, they will quickly aggregate leading to a loose structure of particles that appears to occupy a much larger volume than they would if perfectly separated. I doubt the paint is formulated to be particularly stable. A downside of preventing the aggregation is that the particles sediment as individual particles and have time to very closely pack themselves together. This leads to "caking". If you have ever had to shake the crap out of bottle of calamine lotion, you'll know what I mean.If you know the true density of the particles, you can determine the solids content readily using a glass pycometer and a scale with 0.01g readability.Check out my free, no-sign-up-required courseIn particular, this lecture:https://enlightenscientific.teachable.com/courses/improving-the-quality-of-colloid-based-products/lectures/6738810