Question: When you say that forces build up after the frogs secrete a fluid, what forces are these and how do these forces stick to surfaces?

Diana Samuel answered on 22 Jun 2011:

Hmm, its a little tricky to explain, but I’ll give it a shot 🙂

Put a drop of water on the tip of your finger, then gently press the bottom of a glass (or other flat-bottomed object) against the drop. You should notice that the droplet deforms; it will originally have had an outward facing curve (i.e. its a convex shape), but upon deforming it it now curves inwards around its edge (making it concave – you’d might need a microscope to see this!).

Starting off with the first force – capillarity – lets say your finger is the toe pad and the flat-bottomed object is a surface the toe pad wants to stick to; the water droplet is some mucus thats been secreted. Outside the droplet, there’s air. Surface tension is present at the air-mucus interface (i.e. around the edge of your water droplet), and this force resists external pressure. The pressure inside the droplet, however, is lower than this external pressure, and its this difference in pressure (called the Laplace pressure) that helps the toe pad and the surface to stay together.

The other force, Stefan adhesion, is a time-dependent force. Take 2 flat sheets of material (e.g. 2 CD’s or plates of metal) and dunk them in a container filled with water, making sure you’re holding the sheets one above the other with a small gap between them (the bottom sheet represents a surface, the top sheet represents a toe pad). Now try to pull the sheets apart. Repeat the experiment a few times, varying the distance you’re holding the sheets apart. You should notice that its easier/quicker to separate them if you’ve held them quite far apart to begin with. The physics behind is that when you’re separating them, the fluid surrounding them needs time to flow into the widening gap you’re creating. Thus, the force is partly dependent on how far apart they were to begin with (so the 2 surfaces will adhere together better if the distance between them is small). The other factor it depends on is the viscosity of the surrounding medium (i.e. how resistant it is to flow). As you can imagine, the time to separate the 2 sheets would be much longer if you were using e.g. honey instead of water. the fluid secreted by the toe pads (mucus) is more viscous than water.

Putting it all together, the toe pads are quite soft, so they can make pretty good contact with surfaces; this means the distance between the toe pads and the surface is quite small. The mucus they secrete is quite viscous, and a pressure difference is generated when the ‘blobs’ get deformed (i.e. when the wet toe pad presses against the surface). Thus, the forces of capillarity and Stefan adhesion help the frogs to stick to surfaces.

I hope that was clear, let me know if I need to explain anything.