The Physics of Kettle Streams

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For a while now, I have wanted to dig deeper into the physics of kettle streams and how they affect agitation when we brew coffee. It is currently hard to find any information about the effects of pour height and pour rate on coffee brewing, and yet in my experience it can definitely have an impact on the resulting cup of coffee. It probably comes at no surprise if I tell you that pouring way too fast can dig a hollow in the coffee bed and lead to clogging a filter, or to an unevenly extracted and astringent brew. What may surprise some readers a bit more is that measuring my pour rate and pour height to keep them stable is one of the things that really helped me achieve more repeatable brews. I discussed this a bit more in previous posts about what affects brew time and the repeatability of manual pour overs.

Apart from affecting the height of water at any given moment and therefore indirectly affecting brew time, the rate at which you pour water onto your coffee slurry (i.e., the mix of water and coffee grounds) also has an important effect on how much you agitate it, and whether you end up with a hollow in the coffee bed. But this is also linked to how high you pour from, and what kettle you are using ! More specifically, the width of the gooseneck, how smooth it is at its tip, how vertical the stream is, and how fast you move the kettle around will all change the properties of the water stream and how it interacts with the slurry.

When a stream of water leaves the gooseneck, it will usually have a relatively smooth and cylindrical shape. Any tiny imperfections at the tip of the gooseneck will still be imprinted on the shape of the stream, which initially do not show much, but as the stream falls down further, its imperfections will gradually grow in size. This is due to a property of fluids called the surface tension. Molecules of water are slightly attracted to each other because they are polar, which means they have a slightly negative electric charge near the oxygen atom, and slightly positive charges near the hydrogen atom.

This means that water is slightly attracted to itself, a property called cohesion. It can also be similarly attracted to other materials: we call this adhesion. Water experiences very little adhesion with ambient air, and as a consequence the surface of water can seem to behave like a stretched elastic membrane, for example by forcing the water to shape itself into droplets. This effect is what we call surface tension, and it is thus simply a consequence of water having more cohesion than it has adhesion with air.

Now back to our slightly imperfect stream of water; because of water’s surface tension, the regions where the stream has a bit more curvature will experience a slightly decreased inward force, and vice versa. This is what will slowly drive the imperfections to grow until they are noticeable by eye, and further cause the stream to break into droplets. This phenomenon is called a Plateau-Rayleigh instability, and can be observed easily by pouring water with a gooseneck kettle from high up above a sink.

The length at which the perturbations become large enough to break the stream into droplets is typically called the breakup length of a stream of water. This length is affected both by the smoothness of the gooseneck and the pour rate. As you can imagine, a rougher gooseneck will have larger initial perturbations in the stream shape, and therefore a shorter breakup length. The effect of pour rate on breakup length is however much less straightforward; in fact, for each gooseneck type there is an “optimum” pour rate where the breakup length is very long, and pouring either slower or faster will reduce it.

I decided to check this relation with the Brewista Artisan 0.6L kettle I used for the past year or so, with and without the flow restrictor I described in this earlier blog post. For a while now I have suspected that the Fellow kettles actually do better than Brewista in terms of their gooseneck smoothness, and Fellow generously offered to send me their Stagg EKG 0.6L model for me to do a direct comparison. In each case, I poured water from above a tall mason jar placed on an Acaia Pearl S in flow rate mode, starting low and then raising up the kettle while keeping the same flow rate. When I started hearing a splattering sound, this indicated that the breakup length coincided with the surface of the water in the mason jar, so I stopped and measured both the absolute height of the tip of the gooseneck, as well as the height of the water. Subtracting these two values allowed me to estimate the breakup length associated with a given pour rate. By repeating this with several different pour rates, I was able to verify the predictions based on the physics papers above, as well as measure a net improvement in breakup length for the Stagg EKG over the Brewista, as I suspected !

Understanding breakup length is important because the smoothness of the water stream when it lands on the slurry will completely change how much, and how deeply the slurry gets agitated ! Engineers have already figured out these details for the construction of plunging jet reactors, which aim to maximize the rate of chemical reactions between a gas and fluid with a stream of fluid falling in a still pool and entraining some gas bubbles with it (for example, see Yin et al. 2018).

Basically, the rougher a stream is where it lands on the slurry, the more air bubbles will be transported in the slurry (see Baylar et al. 2016). The air bubbles will cause friction with the flow of water below the surface, as the bubbles themselves are slowed down by buoyancy forces pushing them upwards. At the depth where water has slowed down enough, only buoyancy remains as an acting force on the air bubbles and they will rise back up to the surface. This rapid breaking of the underwater current can cause the flow to become turbulent, which is a good thing for coffee extraction because turbulence is great at distributing fresh water between all particles of coffee, and make sure that no one particle remains under-extracted.

If the stream of water is extremely irregular where it lands however, a lot of air bubbles will be dragged into the slurry and the breaking will happen so fast that agitation will not reach very deep down, potentially leaving the coffee bed undisturbed. At its most extreme, pouring from higher than the breakup length will cause only tiny droplets of water to fall on the slurry, and no agitation at all will be able to reach the coffee bed and make extraction more even.

If we take the opposite approach and pour water at a fast rate by holding the kettle just above the surface of the slurry, the stream will be very smooth while it enters the slurry and almost no air will be dragged into it. This means that the underwater current will not slow down as much before it reaches the coffee bed, and you will instead slowly dig a hollow in the coffee bed using a laminar flow (i.e., a “not turbulent” flow). This is not very efficient at separating all coffee particles to extract them evenly, and the hollow will potentially later cause a channel and local over-extraction unless you swirl the dripper to make the coffee bed flat again.

Now that we saw how breakup length affects agitation, it should be clearer that in order to obtain a turbulent agitation of the coffee bed, we need the stream of water to be a little rough as it enters the slurry, but not too much. To little will result in non-turbulent flows, and too much will only agitate the surface of the slurry. In practice, I found best results by pouring from the height that will place your breakup length just below the surface of the slurry. You can achieve this by pouring just a bit lower than the point where you can hear a splattering sound as the stream enters the slurry. In other words, you do not want to hear that splattering sound, but you want to be pouring just below the point where you would hear it.

Depending on how fast you are pouring and what kettle you are using, this pour height that almost causes splattering sounds will be different, because the breaking length of your stream will be different. These effects were explored by Qu et al. (2011) and Baylar et al. (2016) in the context of plunging jet reactors, and are illustrated below:

The longer your breakup length is, the more velocity your stream of water will have as it enters the slurry, because it had more time to be accelerated by gravity. This means that selecting a longer breakup length will cause stronger and deeper agitation, and will agitate your coffee bed more. Unfortunately, I do not think there is a “best answer” when it comes to what should be your breakup length. If you have a breakup length that is very long, and end up pouring from very high up (without splatter !) as a result, you might impart so much agitation to the coffee bed that you could end up clogging your filter because very fine coffee particles will more easily fall at the bottom.

In my experience, I like using a breakup length of about 18-19 cm, which I achieve by pouring at 5 g/s with my Stagg EKG 0.6L kettle, and pour from a height just below that which causes splattering sounds. This works very well with my current set-up (detailed here on my blog), but you may want to select shorter break-up lengths if your grinder or coffee generates a lot of fines. I sometimes reduce my pour rate to 3-4 g/s with some Ethiopian coffees that clog easily, and crank it up to 6-7 g/s with some naturals or South Americans that have insanely fast drawdown times. I recommend experimenting with your own kettle and determine the pour rate that suits you best.

There is another detail we did not discuss that will affect the breakup length of your kettle. How vertical your falling stream of water is important – generally, I found that the most vertical streams are easier to control, preserve a good breakup length, and cause less hollows in the coffee bed. To make your stream as vertical as possible, I recommend moving your kettle relatively slowly to leave the stream undisturbed, and keep it at a relatively stable height during your brew. One nice advantage of having a high quality kettle is that they are designed to minimize the teapot effect. This is another phenomenon caused by the surface tension of water, causing it to want to stick to the outer wall of the kettle, and make the stream of water less vertical especially at slow pour rates.

Now I know we discussed a lot of different things in this blog post, and it can be a little hard to keep all these variables in mind when you try to apply these concepts to your brewing method. To help you a little, I created some 2-dimensional maps of how much agitation you will cause in the slurry, and whether that agitation is turbulent or laminar. Your exact map will depend on the kettle you are using, but this should give you a good idea of how to explore your options. As I mentioned, I personally found the best results by maximizing turbulence by pouring just below the splattering point, and finding the flow rate that causes the right amount of agitation to avoid clogging and maximize evenness of extraction.

I would like to thank Fellow who generously sent me a Stagg EKG 0.6L gooseneck kettle for free so that I could compare its performance with the Brewista artisan. This gift came without any obligation on my part regarding the content of this or further blog posts.