  We consider analytically and numerically head-on collision between two
self-propelled drops. Each drop is driven by chemical reactions that produce or
consume the concentration isotropically. The isotropic distribution of the
concentration field is destabilized by motion of the drop which is itself made
by Marangoni flow from concentration-dependent surface tension. This
symmetry-breaking self-propulsion is distinct from other self-propulsion
mechanisms due to the intrinsic polarity such as squirmers and self-phoretic
motion; there is a bifurcation point below which the drop is stationary and
above which it moves spontaneously. When two drops moving along the same axis
with opposite direction, the interactions arise both from hydrodynamics and
concentration overlap. We found that two drops exhibit either elastic collision
or fusion depending on the distance from the bifurcation point controlled, for
instance, by viscosity. The elastic collision results from the balance between
dissipation and energy injection by chemical reactions. We derive the reduced
equations for the collision between two drops and analyze the contributions
from the two interactions. The concentration-mediated interaction is found to
dominate the hydrodynamic interaction.
