Quick question about "fingering"-- is there evidence that this effect occurs within a coffee puck? Know that it usually applies when two fluids are present, usually with differing viscosities. Not exactly what is happen in a coffee puck during extraction...but injected water coming in contact with the developing syrupy (viscous) aqueous coffee might qualify....??? So fingering, if it does occur, might be temporally driven-- it can come and go and vary in position with the puck over time?
Wow-- this is a very dynamic system! Very complicated.....neat.

Folks here probably have written about this--but pulsing the flow can play wonderful games with compressibility modulated flow....particle relaxation times vs. hydraulic impedance....

Side question:
??Can the Decent espresso be programmed to pulse the water flow??

doublehelix wrote:Quick question about "fingering"-- is there evidence that this effect occurs within a coffee puck? Know that it usually applies when two fluids are present, usually with differing viscosities.

Dammit. It was such a nice hypothesis - wasn't it?

https://ethz.ch/content/dam/ethz/specia ... entals.pdf

The fingering flows do appear in the text about washing the filtrate medium from the "dirty" higher viscosity liquor.

So which begs the question - after the puck is wet - are we washing it? Do fingering flows happen after PI?

Looks like I have a homework assignment. The article is is very comprehensive.
A quick look at the pictures indicates that the dark liquor was already present before the white liquid was pushed through the cake, to create fingered flow--I think? Coffee extraction is different since the liquor is dynamically forming-- different boundary conditions?--please correct me if I have this all wrong.
In any case- given that nothing is perfect, it's likely that there is some fingering during extraction-- the question might be how much and when--as you've just said.

doublehelix wrote: In any case- given that nothing is perfect, it's likely that there is some fingering during extraction-- the question might be how much and when--as you've just said.

All I can say is that the definitive fingering coffee seems to give me is "the finger" - and it seems to do so every time I get some semblance of confidence that I understand something. And come to think of it, Dave, you seem to have a lot to do with this.

But it wasn't all for naught - now I know how a thinner "finger flows" into a thick paint. That must come in handy sometime. Maybe for a post at www.home-painter.com.

BTW (and more seriously) - the reason I thought "fingering flows" or some other sort of "selective flows" would be convenient is that they can explain different EY and why PI and grind density homogeneity increases EY.

The same paper discusses the slowdown of flow through higher density filtrate. Thus came the concept of air being pushed out sealing the puck (if one forgets to PI). Since it is air - it should easily recover after being sealed by pulling back the pressure and continuing PI. I have done that experiment easily many times and am quite confident in it (hopefully you'll agree).

So if fingers are where density settles down (since the "inter" grind space is taken up by incompressible water) extracting at this point will press the air out of the spaces between the fingers, and they either won't be extracted - or be extracted later.

This is similar to the concept of channeling - except that it isn't one or two channels - but more like a probabilistic distribution of channels with air reducing the extraction between these channels.

Oh boy, Assaf, hate to be the person giving the "finger"--never my intention here . My humblest apologies .
I guess what you're speaking of is a three component system within the cake matrix? : hot water/extracted coffee/air; and the air disappears when the puck is totally wetted?
Guess channeling and fingering are related but different. Channeling speaks to puck defects; fingering is an intrinsic feature of a perfect coffee puck. And to climb up on a tall house of cards, one can have both channeling and fingering! FWIW-- the Illy book says that espresso has a viscosity @45deg C = 1.7 mPa(s); water is around 0.6 mPa(s). ...
Need to think about this.........


I'm a little confused about what you mean what you mean by "sealing the puck." Could you please clarify? What I get is that water goes in, air gets pushed out and maybe also mixed with the aqueous coffee (air/coffee emulsion). Maybe another good experiment is to measure the puck impedance to just air--no water--might have a dynamic response? Not sure how to do this????

doublehelix wrote:I'm a little confused about what you mean what you mean by "sealing the puck." Could you please clarify? What I get is that water goes in, air gets pushed out and maybe also mixed with the aqueous coffee (air/coffee emulsion). Maybe another good experiment is to measure the puck impedance to just air--no water--might have a dynamic response? Not sure how to do this????

So assume we grind fine and end PI too quickly. A few mm of puck got wet but the bottom is dry.

Now we up the pressure to 8-9 bars. The top is incompressible. Since it soaked up water (at least MOST of it is incompressible). But the bottom is dry and has air between the particles.

Now air is compressible. And 9 bars is A lot in pneumatics - so the air:
1. Air locked in voids Compresses down in volume (ideal has or close to it)
2. Some of it is squeezed out through the bottom of the filte basketr (the basket has holes)

So the puck - below the water line, compresses to a higher density. If we ground very fine, it was already pretty dense to begin with and now it won't allow any water through.

I think this is rather proven by the fact that if we make this mistake - we can back out easily. Drop the pressure, the air will be drawn back in and density more or less restored, finish PI (low flow) - and raise the pressure again.

That is why normalization of the grinds (the term used by the doomed book) is so important. Any density differential is amplified by the risk of trapped air being compressed an EY dropping like a brick.

I also think that this is why the crema devices work. They slow down flow at the bottom of the basket and thus ensure that all of the puck is wetted. That is enough to at least create a thin layer of whitish crema...

Got it-- makes perfect sense now-- many thanks! Not to be difficult here.. .but can't the air above the puck, under pressure, permeate (to some extent) through the wetted puck layer? Not sure the wetted puck layer is a perfect barrier? Consider the pressure differential operating across that boundary---kind of like carbonation through a fritted filter?
Heck, maybe we're seeing the trapped air doing the fingering?

Some air may be dissolved in the water above the puck - but it wont affect the barrier by much. (Perhaps when the pressure is released some air will fizz out of the water.)

Even more so, the compressed dry puck will remain mostly dry (I guess some seepage may continue - depending on the roast and grind levels). The wet layer pressing down on it will keep it so.

Neat! Good to know....heck, air does float on water--thanks for the clarifications! These coffee variables are fascinating to think about and maybe play with.
Still have more questions and thoughts--but without data, they're mere musings.