Combined effects of Marangoni, sedimentation and coffee-ring flows on evaporative deposits of superparamagnetic colloids

Evaporation of sessile colloidal droplets is a way to organize suspended particles. It is already known that the composition of the surrounding fluid modifies the dried deposit. For superparamagnetic particles, recent studies showed that external magnetic fields can act as remote controls for those deposits. In this paper, we study the configuration space given by the interplay of such fields and a modification of the fluid composition by considering various concentrations of phosphate buffered saline (PBS). We show that the magnetic field modifies the morphological properties of the deposit, while the composition (i.e. PBS concentration) modifies the density profile of the deposit. We then present an explanation of these influences considering the competition between (i) sedimentation, (ii) coffee-ring and (iii) Marangoni flows. From these considerations, we propose a master curve which should be able to model the deposit densities of any system where the above mechanisms compete with each other.