Suspensions of microscopic particles, such as reaction mixtures with catalyst particles and nutrient solutions with bacteria, often contain heterogeneous distributions of solute. The concentration gradients that result from these inhomogeneities can put the particles in motion through a process called phoresis, or chemotaxis in the case of living systems. Through this process of chemotaxis white blood cells manage to chase bacteria and bacteria know in which direction to escape. It was recently proposed that solute-mediated interactions could be used to drive similar predator-prey behavior in synthetic systems as well.
In this work, we have employed a model system of oil droplets dissolving in an aqueous surfactant solution to study the forces that particles exert on each other through the formation of local concentration gradients. We found that the solute-mediated interactions can be repulsive or attractive depending on the rate of dissolution. Indeed, when the type of oil droplets and surfactant are chosen appropriately, one type of droplet chases after the other type of droplet, similar to the way white blood cells chase after bacteria.