What determines water flow speed in the Clever Dripper coffee maker? Recently James Hoffmann (a YouTuber that has a coffee channel) posted a video on 'the ultimate technique of using the Clever Dripper'. Many users of this brewer have experienced 'choking' of the brew, where the coffee grounds basically prevent the water from passing through the filter. The change he recommends to solve this issue is putting the water in the brewer, before putting in the coffee. Indeed, this makes a massive difference. However, the filter after brewing does not look different between either of the two techniques. Also, the moment when the water is released from the brewer, the coffee grounds are in both cases settled to the bottom of the filter.
My question is: Why does the water flow speed increases so drastically by adding the water first? Since the location of the grounds are the same between the techniques, I think the answer is related to soft-matter physics of the ultra-fines in the grounds. Any thoughts?
This is the link to the video: https://www.youtube.com/watch?v=RpOdennxP24&ab_channel=JamesHoffmann
 A: What we drink as "coffee" is that which dissolves in hot water when we brew it. Coffee beans also have oils and plant matter that do not dissolve.  Adding water first allows the user to help protect the filter from clogging.  When the coffee grind is added, the "non soluble" items will tend to float and/or have less contact with the filter paper.  Adding coffee first will foul the filter from bottom to top as water is added.  Even if one stirs the brew at the end, adding water first will result in a faster "draw".  Great video!
A: Capillary pressure saturation (Pc–Sw) relationship plays a central role in the description of fluid flow in porous media. See https://doi.org/10.1007/s11242-019-01322-x
If air is present in the porous medium (the coffee ground mass before water is added), capillary surface tension effects provide an extra pressure resistance for water to enter the porous medium (particularly the smaller pores).  If the water is added first, the coffee grounds can settle through the water, and no air will be present within the pores when it settles.  So the porous coffee grounds medium will be fully saturated with water, and no capillary pressure saturation resistance will exist.
