What Causes Irregularities in Droplet Formation?

[Danger]

No, this is not about whether or not Farrah has an STD.
I don't know whether this belongs in Physics or Engineering, so feel free to move it.
In a normal day, I have a need to apply one drop of water at a time to something. A drip rate of anywhere from 1/2 second to about 3 seconds is acceptable. Faster results in too much saturation, and slower becomes tedious.
I have a basic understanding of droplet formation regarding surface tension and gravity, but I've noticed a couple of discrepancies.
The first is that the drip rate decreases over time. Even though it's the hot tap that I use, I assumed that the pressure from the heater would remain constant. If so, why would it slow down?
Secondly, there's an regular irregularity to the pattern (with a couple of taps, not all). There'll be 3 or 4 normal single droplets, then 2 or 3 come out right on top of each other, then the pattern repeats. What would cause that?

 

[Redbelly98]

The first is that the drip rate decreases over time. Even though it's the hot tap that I use, I assumed that the pressure from the heater would remain constant. If so, why would it slow down?

I've noticed this too. I figured the tap mechanism moves a little and takes some time to settle into place, after having been repositioned. I think the movement is so small, it's not noticeable unless the tap is nearly shut.

Just my guess.

Secondly, there's an regular irregularity to the pattern (with a couple of taps, not all). There'll be 3 or 4 normal single droplets, then 2 or 3 come out right on top of each other, then the pattern repeats. What would cause that?

I don't know, other than to say it's probably a chaotic system so that you wouldn't get simple, regular patterns.

... Farrah ...

LOL

 

[Danger]

Hmmm... I hadn't considered the tap movement possibility. It brings to mind something else as well. Perhaps the washer takes a while to expand after being decompressed from the seat.

 

[Cantab Morgan]

Hmmm... I hadn't considered the tap movement possibility. It brings to mind something else as well. Perhaps the washer takes a while to expand after being decompressed from the seat.

Very compelling! Before I read that I was going to suggest that it might have something to do with the faucet metal heating up from ambient temperature to the hot water's temperature. But washer "memory" taking a while to overcome seems more reasonable.

I now can't help thinking of Feynman's famous O-ring demonstration after Challenger.

 

[Danger]

it might have something to do with the faucet metal heating up from ambient temperature to the hot water's temperature

That makes a lot of sense, except that I never run enough water for it to get warm, let alone hot. It's strictly stuff that's been in the top of the pipe for hours.

 

[LURCH]

I believe this last statement is the key. Water has been in the pipe for hours, filling the pipe from bottom to top. Once the valve is opened a crack, the weight of the water inside the pipe is what is pushing the droplets out. This forc is being counteracted by atmospheric pressure, holding the water back so that you don't get a continuous flow. With every drop that is released, the amount of water inside the pipe decreases slightly, and therefore the pressure behind the valve diminishes slightly. As pressure decreases, drip rate slows.

For some of the faucets, releasing a critical number of droplets may allow a small pocket of air to enter the pipe. This would cause several droplets to release all at once which, in turn, would equalize the pressures once again, causing the drip rate to return to a steady pace.

All speculation, of course; this is just my guess.

 

[Danger]

Thanks, Lurch. That's another well-reasoned response.