Why does flow from my kitchen sink's tap have to be turbulent to create dishsoap bubbles?

October 24, 2005 4:35 AM

Why does the formation of soap bubbles in my dishwater depend on turbulent flow in the water coming out of the tap?

The hot water tap on my kitchen sink has pretty low water pressure and a reasonably laminar flow. I notice that when I put some dishsoap into the bottom of the sink and then only run hot water into the sink, then barely any soapbubbles are produced. But when I turn the cold-water tap on somewhat, thereby turning the flow more turbulent (think this is the appropriately opposite word, I'm relying here on the Wikipedia entry on fluid dynamics), then the flow starts to create lots of bubbles. This is annoying, because adding enough cold water to the flow makes the water too cold for my dish-washing taste. I've also tried just putting my hand into the flow to make it more turbulent; this works a bit, but not nearly as well.

Anybody know why it works like this?

posted by paul! to science & nature (15 comments total)

I expect that increased turbulence in the water increases the amount of air already in the water, and causes the soap to get moved around in a way more likely to form bubbles. The laminar flow from your cold tap is probably just moving the soap around more than mixing it up in a bubble forming kinda way.

I have no clue about fluid dynamics.
posted by alexst at 4:49 AM on October 24, 2005

The way I describe the problem, another obvious possible explanation would be a consequence of the change in water pressure and not on flow laminarity. This might be a key factor, I don't know, but there's something really non-linear about the way bubble formation depends on changes in the flow. When I add the cold water I might be increasing the flow by 30% or so, but bubble formation is clearly increased by orders of magnitude across this transition, whereas the amount of cold water I add beyond that doesn't seem to make such a big difference to the amount of bubble formation. My vague idea is that non-linear changes in the flow itself, like the transition from laminar to non-laminar flow, are a better candidate to explain this kind of relationship. But hopefully somebody who actually knows what they're talking about is around to make things clearer.
posted by paul! at 4:50 AM on October 24, 2005

The way I describe the problem, another obvious possible explanation would be a consequence of the change in water pressure and not on flow laminarity.

(Yikes. Note to self: reread, then post.) I meant to say here that the change in bubble formation would be a consequence of the change in water pressure and not of the change in laminarity.
posted by paul! at 4:52 AM on October 24, 2005

You realise bubbles aren't required for washing up anyway right? The soap will still do it's job.
posted by alexst at 4:54 AM on October 24, 2005

Higher water pressure will result in increased turbulence in the bowl though won't it?
posted by alexst at 4:55 AM on October 24, 2005

The turbulent jet of water entraps much more air along its convoluted surface than the smooth, laminar jet. The air is dragged below the water surface, and when it resurfaces it gets 'trapped' in the bubbly soap structure.

Furthermore, the soap get thoroughly mixed in the turbulent case, but I guess that's only of minor importance.
posted by swordfishtrombones at 5:08 AM on October 24, 2005

You realise bubbles aren't required for washing up anyway right? The soap will still do it's job.

I've even heard it suggested that soap will work better if there are no bubbles. My problem with this isn't so much that I doubt it's true - although I do, maybe just by sheer force of habit - as that I just find washing dishes with bubbles more enjoyable. I guess it's an aesthetic thing.

That issue about the need for bubbles actually warrants a thread of its own. But if anybody reading this thread knows about that too, bonus.
posted by paul! at 5:10 AM on October 24, 2005

Get a little strainer, pour your dish soap into that and hold it under the running water. I bet that gets you bubbles.
posted by duck at 5:53 AM on October 24, 2005

You could also get a strainer that screws onto the end of your faucet that will aerate the water. Or just agitate the water in the sink with your hand.

(But the answer is: more air and more agitation=more bubbles. That's the way it is.)
posted by OmieWise at 6:46 AM on October 24, 2005

You could also get a strainer that screws onto the end of your faucet that will aerate the water. Or just agitate the water in the sink with your hand

Good point...those little tap aerators also reduce your water consumption. In Toronto, the hydro company goes door to door installing these for anyone who wants them for free (along with new shower heads, pipe insulation, toilet dams, compact flourescent light bulbs, furnace filter alarms, and all sorts of goodies).
posted by duck at 6:54 AM on October 24, 2005

Yeah, the bubbles are your source of turbulence. You're using the terms more or less correctly: "laminar", which just means a smooth flow all going in one linear direction, and "turbulent" are approximate opposites.

The aerator is a great idea. They're very cheap, easy to install and will give you what you want: more bubbles. Get a gooseneck one so you can clean your sink with it.

It's true that soap doesn't need bubbles to work, but they do serve a purpose. When your dishwater stops foaming, no bubbles, that means that the soap has been exhausted and you need another squirt of liquid to keep cleaning your dishes. If you're washing pots and pans, it's easy to have too much cooking oil for a single squirt of soap to deal with.
posted by bonehead at 7:46 AM on October 24, 2005

If your hot water tap really does have a smooth stream (like this) then that implies that you are missing an aerator. Without one, you get a solid stream; their purpose is to chop up the stream and give it lots of air bubbles. (This also saves some water, since the stream "feels" like it has more water than it does.) I always find that using sinks / faucets that are missing the aerator to be unplesant - the water stream is too powerful and much of the flow is wasted. With an aerator it just feels like it's easier to get a good lather and get things wet quickly.
posted by Rhomboid at 7:51 AM on October 24, 2005

Even better than an aerator, because you can use it for other stuff, is one of these, with or without the little extension hose.
posted by mendel at 8:57 AM on October 24, 2005

posted by weapons-grade pandemonium at 9:54 AM on October 24, 2005

I like the suds myself. I've been known to whip them up with a whisk when I need a lot of lather for cleaning rugs and upholstery, but an aerator is great for lots of reasons.

The one I use, because it's compact yet still swivels in all directions, is made by Moen. A lot of add-on aerators hand way down and get in your way when you have to wash big pans.

Not recommending the retailer, just the plumbing attachment here.
posted by wryly at 1:37 PM on October 24, 2005