Why does the water diffuse at right angles in my coffee? [closed]

I purchase an espresso shot each morning at my local coffee shop, and I usually water it down slightly to take the edge off as well as to cool it. Recently, I have noticed that when I pour the cold water in, it seems that the surface of my espresso is suddenly disturbed in a lattice-like configuration \begin{align*} \nearrow \searrow \nearrow \searrow \\ \nwarrow\swarrow\nwarrow\swarrow \end{align*} i.e., water moving up at a $45^\circ$ angle and down at a $45^\circ$ angle along the surface. Googling water diffusion in coffee has been fruitless unfortunately. Any thoughts? Regards, Zvpunry.

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Can you post a picture of the coffee? It's not clear from your illustration what you're observing. –  John Rennie Jun 5 at 16:12
My first suspicion is that you have neat little convection cells... –  dmckee Jun 5 at 17:00
It's not quite clear what you're referring to, but I'll go make myself an espresso and watch. This might be fun to animate with 3D graphics, to show what's really going on. –  DarenW Jun 5 at 17:51
Hmm, I didn't see anything like that. I'll go make yet another cup of espresso.... Ok, some questions. Do you see this only with the espresso, or with other fluids (milk, lemonade, tea, beer, soup ...?) How much fluid in what size and shape of cup? How are you pouring it in - a nice steady stream (diameter?) or splashy drops? Is this straight espresso with no additives such as cream, flavor syrups? A video would be helpful, even a crappy cell phone one. –  DarenW Jun 5 at 18:10
Thanks all for your input. I'm going to post a video of this soon, at least by tomorrow (if I have another espresso shot today that might be the end of me). –  Zvpunry Jun 5 at 18:52