  Impact of droplets of varying surface tension and subsequent spreading over a
solid surface are inherent features in printing applications. In this regard,
an experimental study of impact of two drops of varied surface tension is
carried out where the sessile water droplet on a hydrophilic substrate is
impacted upon by another droplet of sequentially lowered surface tension. The
impacts are studied for different impact velocities and offsets with respect to
the mid-plane of the two colliding droplets. Sodium Dodecyl Sulfate (SDS) is
used to alter the surface tension without altering the viscosity, to study the
various parameters affecting the spreading length viz. the surface tension,
offset between the drops, and impact velocity. The spreading lengths are
obtained through image processing of the captured footage of the impact
dynamics by a high-speed camera. It is found out that upon lowering the surface
tension, the maximum and equilibrium spreading length varies to a significant
extent also the nature of the spreading dynamics changes. Both side and
top-view imaging are performed to understand the overall hydrodynamics. There
is also a substantial change in drawback when dissimilarity is surface tension
between the impacting droplets exist. Finally, a fit model is obtained to
predict the maximum spread length of the various cases.
