Tuning evaporation driven deposition in sessile drops via electrostatic hetero-aggregation†

The desiccation of microliter drops containing colloidal particles often results in the coffee-ring effect, where non-volatile particles deposit at the drop periphery. Such deposits form primarily due to a radially outward flow generated within the drop during the drying process. In this work, we consider drying drops containing mixtures of oppositely charged species as a universal method to tune the morphology of dried deposits, including a complete suppression of the coffee-ring effect. We show how a variety of dried deposit patterns can be engineered by manipulating the mixing fraction (ω_f) – the ratio of the mass of positively charged species to the total particle mass of the dispersed phase, and the total concentration of particles in the drop (C_T). Using video microscopy, electrophoretic measurements, particle tracking microrheology, and bulk rheology, we elucidate how the interplay between sedimentation, capillary flow, and gelation in the evaporating colloidal drop governs the drying patterns formed in the systems containing oppositely charged species. The fundamental insights gained through this study were further applied to achieve uniform deposition of micronutrients on the surface of leaves for efficient surface deposition.