Marangoni flows and deposits in colloidal binary droplets*

Droplet evaporation is a versatile process to deposit solute materials on a substrate unless it induces solute accumulation at the contact line because of outward capillary flows. Generating inward Marangoni flows by surface tension gradients is a good candidate to suppress outward capillary flows. Controlling Marangoni flows would be available by adopting binary solvents but actually difficult in colloidal binary droplets, which have been recently studied to uniformly deposit colloidal particles. To understand and manipulate Marangoni flows in colloidal binary droplets, we investigate droplet evaporation and deposition dynamics by changing relative mass ratio of binary solvents. Particularly, we demonstrate that island deposits are created when Marangoni flows overwhelm capillary flows during most of evaporation times. Formation of island deposits in colloidal binary droplets would be a helpful route to achieve uniform deposits of colloidal particles or nanoparticles.