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Hi all, been learning Houdini since mid Feb and thought it time I better share something back as I've found the forum immensely useful ;-) RnD of Houdini particles flowing over a hard surface model to simulate a condensation/rain/rivulet/droplet effect. This is part of a larger RnD project I'll render & upload shortly (this is one section of a larger flying object - a cloud seeding drone... just an excuse to learn a load of Houdini). Partly inspired by Mark Theriault's Sucker Punch Intro Breakdown using Pflow (vimeo.com/39161208) and having just starting making the switch for fx from Maya to Houdini, this was one of those "I wonder if I can do that now in Houdini" things that I just had to try. I remember years ago doing something similar in Maya and it taking a heck of a lot longer! This effect took about a week of tweaking. The sim is basically just a popnet which relies on an SDF volume (isoOffset) for surface interaction and uses a voppop (VEX) to control the "drip off threshold". Once I had a working method, I then used 3 duplicate variations for the different rivulets (trail SOP) and finally added a random scatter SOP for some extra motionless droplets. As the final shot won't be so closeup, I have to option of increasing/decreasing the uniform sampling of the SDF to fit. When all this is done, I'll hopefully be taking this RnD further with a rain-on-a-window-pane high-detail/realism sim. Thanks to SpencerL for advice on the initial VEX methodology.