From cement cohesion to DNA condensation, a proper statistical physics treatment of systems with long range forces is important for a number of applications in physics, chemistry, and biology. We compute here the effective force between fixed charged macromolecules, screened by oppositely charged mobile ions (counterions). We treat the problem in a one dimensional configuration, that allows for interesting discussion and derivation of exact results, remaining at a level of mathematical difficulty compatible with an undergraduate course. Emphasis is put on the counter-intuitive but fundamental phenomenon of like-charge attraction, that our treatment brings for the first time to the level of undergraduate teaching. The parity of the number of counterions is shown to play a prominent role, which sheds light on the binding mechanism at work when like-charge macromolecules do attract.
I. INTRODUCTIONSoft matter refers to a wealth of systems that may seem disparate at first sight: foams, glues, paints, liquid crystals, colloids, polymers etc. [1]. What they have in common is threefold: a) they are sensitive to 'gentle' mechanical of thermal fluctuations; b) they feature at least two distinct lengthscales (one being microscopic, the other being intermediate between microscopic and macroscopic; c) they often exhibit strong collective effects, such as the property to self assemble into complex entities. As a consequence, their properties are often difficult to rationalize and the sole knowledge of the basic interaction forces between the individual components is not enough to understand the resulting properties. Here, our interest goes to one such collective effect, that challenges intuition and takes place when charges are considered. It is at the root of key phenomena, outlined below. Soft Matter is in itself a relatively new field, but it is starting to enter the classroom [2][3][4][5][6][7][8].In vacuum, two like-charges interact via Coulomb law, which yields a repulsive force. In a solvent like water, containing mobile cations and anions, the situation becomes more complex. It is highly relevant in soft matter physics [1], and more precisely for colloidal suspensions, where large charged macromolecules, typically of micronsize, are surrounded by much smaller ions, called microions. As microions move on a much shorter time scale than the macromolecules, the relevant concept to discuss interactions is that of effective potential: the direct interactions between the charged macromolecules are modified as a result of the fluctuations of the medium. This yields an effective interaction which is mediated by the microions, and it is obtained by performing a thermal equilibrium average over the fluctuations of the positions of the microions [9]. A first effect of the microions is to screen the bare Coulomb repulsion, resulting in a short range effective interaction between the macromolecules [10]. In a small coupling regime (when the typical electrostatic energy is smaller than the thermal one), the effective interaction rem...