(624e) Interaction of Two Particles in a Chemically Reactive Medium

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. In this case there appears an attractive force decaying inversely proportional to the squared distance between the particles. This effect was first demonstrated by Wei & Jan [1]. We generalize their calculations to a more realistic case, when both reactant and product species are present in the dispersion; here the force can be either attractive or repulsive depending on the stoichiometry factor introduced in [2]. 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. Another analogy is so-called secondary Bjerkness force between two bubbles oscillating in fluid. The leading term in interaction of two osmotic motors (Janus particles) is the same as for two particles uniformly covered with catalyst. The force does not depend on the mutual orientation of self-propulsion. 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; it is inversely proportional to quintic degree of the interparticle distance. Note that if the cargo and catalytic particles are connected by a rigid thin rod, the pair starts moving; thus it operates as a catalytic motor [3]. Within the above-mentioned electrostatic analogy, the cargo particle is equivalent to a particle of vanishing permittivity. We also generalize the model to finite ratio of particle radii and the interparticle distance and to include hydrodynamic interactions.

1. H.-H. Wei & J.-S. Jan, J. Fluid Mech. 657, 64--88 (2010).

2. U. M. Cordova-Figueroa, & J. F. Brady, Phys. Rev. Lett. 100, 158303 (2008).

3. S. Thakura & R. Kapral, J. Chem. Phys. 133, 204505 (2010).