How Tree Free Adhesion Works
If you have ever held a tree frog or seen one up close you will realize they are amazing creatures, because they can climb right up your hand or any other smooth surface. You may have even seen one that has attached itself to your glass sliding door. In this blog post I will explain how a tree frog uses wet adhesion to stick to virtually any smooth surface. The Green Tree frog pictured left and below is able to stick to any smooth surface through wet adhesion. You can see from these photos that tree frogs have long limbs they can use for jumping and climbing , which have large pads on their hands and their feet. Scientist at the University of Glasgow in Scotland have found using a scanning electron microscope that frogs have very tiny microscopic Nanopillars on the bottoms of their hand and foot pads. These frogs have hexagonal pads that appear to be smooth, but under view from a scanning electron microscope they actually have groups of Nanopillars which have small dimples in the end of them that create friction against the surface that the tree frog is on. The space in between these hexagonal pads are channels that become filled with a mucus like substance that is 1.5 the viscosity of water. This liquid creates a surface tension that helps the Macropillaars on the tree frogs pads grip by creating friction between the nano pillar and surface the frog is sticking too. 
Sources
Marcus Adam, Sticky Buisness. Scientific American. http://www.scientificamerican.com/article.cfm?id=tree-frogs-hang-tight-but-how
lastmilephotography.wordpress.com 