My question probably doesn’t even make sense so here is an example. If you pour water over a massive boulder and throw sand at it, the sand will stick to the rock better than it would without the water. For larger objects like your hand, it would make it much harder to climb that rock than without the water poured on it. Why is it the opposite?
In: Physics
Three terms – cohesion, adhesion, and friction.
Cohesion is a material’s tendency to stick to itself, for example, water likes to stick to itself in drops.
Adhesion is a material’s tendency to stick to other things. Water has decent adhesion, this is why water will stick to the sides of cup in tiny droplets (the droplets held together in cohesion).
Friction is the force responsible for the “stickiness” between objects rubbing against each other.
The issue here is water has pretty high strength in co- and adhesion. So water poured on a rock will stick a little bit and keep the rock wet, as opposed to all running off and leaving the rock dry. Adhesion can “hold” small things onto that water, like the sand in your example. Think of it as a rock/water/sand sandwich. The water sticks to the rock and the sand sticks to the water.
These “hesion” forces are really weak though. A large force, like your hands grabbing the rock, overwhelm the “hesion” forces and now Friction comes into play. Water acts as a weak lubricant, meaning it slightly reduces friction. If you imagine a rough material, like sandpaper, water’s ad/cohesive properties effectively allow the water fill in the gaps and *smooth* the surface on a microscopic level. This smoothness makes it harder to grab and makes you slip. A more highly co/adhesive material, like oil, does this even more noticeably.
If a surface is wet, the water reduces friction. Just like grease, water is not solid and basically allows you to slip off a surface, just like a boat can go over water, but not land.
Small objects stick to water, because of the surface tension. The object itself is not heavy enough to break the friction between water molecules, so the water and sand stay in place.
You could picture it as a steep incline with a very thin rope. If you are light enough, the rope will help you climb, but if you are too heavy the rope will break and you fall down.
Water really likes to bond to itself. It forms lots of intermolecular connections between various water molecules. This makes it hard to separate water.
Think of sticking two objects together with taffy. It adheres to both itself and the two objects and holds them in place. Water does the same thing, but to a lessor extent.
Water can manage to “hold on” to a very small object like a grain of sand. However, something larger will greatly overpower these bonds and then it just acts as a lubricant, with water molecules sliding over each other as your hand moves across the rock.
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