  Recent experimental results indicate that mixing is enhanced by a reciprocal
flow induced inside a levitated droplet with an oscillatory deformation [T.
Watanabe et al. Sci. Rep. 8, 10221 (2018)]. Generally, reciprocal flow cannot
convect the solutes in time average, and agitation cannot take place. In the
present paper, we focus on the diffusion process coupled with the reciprocal
flow. We theoretically derive that the diffusion process can be enhanced by the
reciprocal flow, and the results are confirmed via numerical calculation of the
over-damped Langevin equation with a reciprocal flow.
