The sticky substance can actually be lecanium scale excrement or "honeydew". Scale insects suck the juice out of newer plant growth on deciduous trees, particularly oak trees, and drop their waste. This waste will also float through the air. It starts out as a clear sap and then turns black from a sooty mold fungus on your furniture, vehicles, deck, and walkways.
Besides the unsightly fungus in your yard, if the lecanium scale presence is left uncontrolled, one or two major infestations can kill your trees. A properly timed horticultural oil spray will smother the scale which will help to alleviate this problem.