  This project aims to study the sedimentation of a surfactant-laden drop in a
liquid with particles. A 2D simulation is performed with MATLAB. The interface
is captured by the front-tracking method. The local viscosity depends on the
local particle concentration, which follows a power law. The surfactants not
only decrease the surface tension but also induce a surface tension gradient.
If the surface tension decreases, the settling velocity will decrease due to a
larger deformation of the drop (i.e., the drop becomes more flat).
Additionally, increasing the Peclet number (ratio of convection to diffusion on
the interface) for surfactants will reduce the settling velocity due to a
larger surface tension gradient.
