  In the present study, the role of surfactants on the cross-stream migration
of droplets is investigated both experimentally and theoretically. For
experimental analysis, sunflower oil is used as the carrier phase and DI water
as the dispersed phase which is intermixed with Triton X-100 that acts as the
surfactant. A T-junction is used in the microchannel for the purpose of droplet
generation. Presence of an imposed pressure driven flow induces droplet
deformation and disturbs the equilibrium that results in subsequent
surfactant-redistribution along the interface. This further induces a gradient
in the surface tension, thus generating a Marangoni stress that significantly
alters the droplet dynamics. On subsequent experimental investigation, it is
found that presence of surfactants reduces the cross-stream migration velocity
of the droplet. Further, it is shown that the effect of surfactants in reducing
the cross-stream migration is significantly enhanced for a larger droplet as
compared to a smaller one within the same time span, provided the channel
height is kept constant. In addition, a larger surfactant concentration is
found to induce a greater retardation in cross-stream migration of the droplet,
the effect of which is reduced when the initial transverse position of the
droplet is shifted closer to the channel centerline. The present analysis can
be applied to various droplet based microfluidic as well as medical diagnostic
devices where manipulation of droplet trajectory is a major issue. For the
theoretical counterpart, an asymptotic approach is adopted in the presence of
bulk-insoluble surfactants and under the assumption of negligible fluid
inertia. A good match between our theoretical prediction and the experimental
results is obtained.
