Pair Interaction of Catalytically Active Colloidal Particles

An increasing number of experiments on catalytically-driven (active) colloidal particles have shown that the interaction of chemically active particles is more complicated than usual interaction of two nonreactive (passive) particles. Indeed, each chemically active particle changes the distribution of reactants which, in turn, generates an overall force on other particles. First, we consider a pair of spherically symmetric catalytic particles, which are far from each other, in a colloidal dispersion of reactants and products. In this case there appears a force which can be either attractive or repulsive depending on the stoichiometry factor of the reaction. In fact, the interaction force can be thought of as a force between two charged particles which can bear charges of either the same or opposite signs depending on the stoichiometry factor. Next, we deal with interaction between catalytic and passive (cargo) particles. It is demonstrated that the force on a cargo is exactly the same as the force imposed by a catalytic particle on another one. On the other hand, the force on a catalytic particle imposed by the cargo is much smaller. Within the above-mentioned electrostatic analogy, the cargo particle is equivalent to a particle of vanishing permittivity.