  In the presence of a chemically active particle, a nearby chemically inert
particle can respond to a concentration gradient and move by diffusiophoresis.
The nature of the motion is studied for two cases: first, a fixed reactive
sphere and a moving inert sphere, and second, freely moving reactive and inert
spheres. The continuum reaction-diffusion and Stokes equations are solved
analytically for these systems and microscopic simulations of the dynamics are
carried out. Although the relative velocities of the spheres are very similar
in the two systems, the local and global structures of streamlines and the flow
velocity fields are found to be quite different. For freely moving spheres,
when the two spheres approach each other the flow generated by the inert sphere
through diffu- siophoresis drags the reactive sphere towards it. This leads to
a self-assembled dimer motor that is able to propel itself in solution. The
fluid flow field at the moment of dimer formation changes direction. The ratio
of sphere sizes in the dimer influences the characteristics of the flow fields,
and this feature suggests that active self-assembly of spherical colloidal
particles may be manipulated by sphere-size changes in such reactive systems.

Soft Condensed Matter