Entropy is more than an abstract quantity but can result in measurable forces which can lead to surprising effects, e.g. that the amount of order can be increased when the entropy of the system rises. The principal phenomenon of such entropic forces which is important for phase separation, crystallization but also macromolecular crowding in biological systems is easily understood by considering two large spheres suspended in a fluid containing smaller spheres. If the distance between the larger ones is smaller than the diameter of the smaller particles, the latter are excluded in the green region and an effective osmotic imbalance causing a net attraction between the large spheres. In case of hard spheres this force is of entirely entropic origin. If other interactions are present the forces are no longer purely entropic but qualitatively very similar. The term “Depletion forces” covers this broader range of many body interactions. (more on depletion forces). Although the forces are typically on the order of pN we can measure this effect in colloidal systems by employing total internal reflection microscopy . With this method we are currently investigating depletion forces of hard and charges rod-like particles (more) as well as hard and charged spherical depletion agents.
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