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For scientific applications I mean mostly things I've just played with (e.g. I once had an app for interacting with 1D and 2D FFTs, using numpy), or more recently I've been trying to get Vispy working so that I can have apps demonstrating real time visualisations. I don't have much on google play, but PlaneWaves is an old example where I was playing with opengl.
I don't really have any guidelines for bigger projects, it's not something I've done much of, but you'd probably get a better answer on the kivy mailing list or irc (the latter in particular is very active), as other devs will be able to answer there.