At approximately this time of year, it is possible to start foraging for damsons in the UK countryside. These small plums make lovely cakes and muffins and, very importantly, great damson gin. A bit like sloe gin but, in my opinion, better. All this is a digression. When I found out about a cafe called Damson & Co I had to try it, purely for the name which brings back fond memories of country walks and gin shared with friends. However, even armed with its address and location on a map I missed it! Damson & Co is very inconspicuous in the way that it is situated on the street. Just as with its wild counterpart, it is easy to walk past without noticing that it’s there but once you’ve seen it, it is obvious, a location that you mark down in order to return to it again and again.
Inside, lavender decorated the table tops in the small but extremely friendly cafe. We enjoyed an Americano, an iced latte and a lovely chocolate brownie that had been warmed almost to the point of melting. They were also extremely helpful when I asked the dreaded “does it contain nuts?” question, checking the ingredients, informing me of the (obligatory) “it may have had contact with a nut at some point in its manufacture” line, but ultimately helping me to choose what was a great nut-free cake. Complementary water was automatically put onto the table and so we had, for the brief moment before I ate the cake, a range of ‘phases of matter’ on the table. Water in the forms of liquid in the bottle, solid ice and steam rising up from the coffee and a brilliantly gooey, viscous chocolate cake somewhere between liquid and solid. At that point it was quite clear what the physics bit of this cafe-physics review would have to be: phases of matter and phase changes.
As ice melts into water, or evaporates to form steam, it undergoes many changes in its properties: Ice is of course solid; liquid water conducts heat much readily more than steam (more on this another day). Another property that changes is the heat capacity of the ice/water/steam. The heat capacity is the amount of energy that it takes to heat a substance by one degree in temperature. At the temperature that the substance changes, say between a liquid and a solid, there will frequently be a spike in a plot of “heat capacity” vs. temperature. This tells us that, as the solid changes to a liquid (or vice versa) the response of the material to being heated changes. Physicists often measure the heat capacity of substances to see if any phase changes occur. A phase change does not necessarily mean that the substance goes from liquid to solid or to gas. A substance will be said to undergo a phase change if it becomes ferromagnetic (like iron at room temperature) or if it becomes superconducting (like aluminium at approximately -272C). Back in the 1920s it was the investigation of the heat capacity of liquid helium that helped to suggest that there was a new form of matter lurking at extremely low temperatures.
Heike Kamerlingh Onnes (a great physicist and apparently a very nice man) had managed to liquify helium gas in 1908. Helium gas becomes a liquid below -269C. By the 1920s it was clear that something very strange happened to liquid helium if you cooled it even more, to temperatures below -271C. The behaviour of the heat capacity spiked indicating that the helium was undergoing another phase change, but to all appearances it was still a liquid. There was no indication that the helium was solidifying, what could it be? More experiments revealed that below -271C the helium liquid started to behave very strangely indeed. It climbed up the walls of its container ‘by itself’ and it managed to leak through minuscule cracks in the glass containers that it was kept in (for a video click here). Cracks that could not be detected before the ultra-cold helium started to leak through them. It took until 1937/38 before this new state of matter was named and it is still not clear that we understand it.
There is so much more to the phases of matter than meets the eye while watching ice melt in a glass of water on a hot summer’s day.
Damson & Co can be found at 21 Brewer St. London