In a method to optimize transfer of a developer fluid on a printing substrate in an electrophoretic printing apparatus, a developer fluid containing at least carrier fluid and toner particles is used, the toner particles having sizes that are greater than diameters of capillaries in a surface of the printing substrate. The additive particles are added to the developer fluid before the transfer, sizes of the additive particles being selected such that the sizes are smaller at least in part in comparison to the diameters of the capillaries in the surface of the printing substrate.