I’m very interested by the concept of an “entropic force” – that is, when the tendency for a system to move towards a state of larger disorder (entropy) manifests itself in a form that seems, for all practical purposes, to be a new force of nature, even if there is no “real” force acting. An important example (maybe) in all living organisms is osmosis, which, according to an new article by Eric Kramer, is probably misunderstood by almost everyone:(1) “The first misconception is that osmosis is limited to liquids,” he says. “But it works just fine for gases, too.”
(2) “Another misconception that osmosis requires an attractive force,” he says. “It doesn’t. When water fills the bag of sugar, it’s not because the sugar is pulling the water in. That’s not part of the explanation.”
(3) “A misconception is that osmosis always happens down a concentration gradient,” he says. “When you dissolve something in water, the water doesn’t necessarily get more diluted. Depending on the substance, it can get more concentrated.”
(4) “Anther misconception is that you don’t need to invoke a force to explain why the water flows into the bag. It’s thought that, like diffusion, it’s a spontaneous process,” he says. “But, in fact, there is a force. It’s complicated how it happens, but it turns out that the membrane – or the bag, in the familiar lab demonstration – exerts a force that pushes the water in.”
A better way to understand osmosis is to think about how the system can minimize the free energy. Although it appears that the system is an a high energy state while it supports an unbalanced column of water of height h on the left side (see figure), by allowing water to flow from the hypotonic to hypertonic side of the membrane, the free energy is reduced because the entropy of mixing is increased between the water and the solute. This explains why there does not need to be any “attractive force” between the water and the solute, but it can be reversed, depending of the details of the thermodynamic situation. The process continues until it is balanced by the regular gravitational potential energy of the excess water column. The author claims that there IS in fact a force acting… its the force of the membrane keeping the solute on the left side. This force then gets transferred from the solute molecules to the water. This is in contrast to diffusion, in which no (directed) force acts at all… just the random motion of the molecules creates the appearance of a force that disperses the molecules.