Evaporation-induced deposition morphology of suspension droplets on hydrophobic surfaces manipulated by controlling the relative humidity

•The relative humidity manipulates the deposition morphology of suspension drops.•For polystyrene, a uniform pattern is formed at a relative humidity of 15%.•The coffee ring effect is suppressed simply by manipulating the relative humidity.•A mass conservation model is applied to calculate the capillary flow velocity inside an evaporating drop.•The capillary flow velocity increases dramatically as the evaporation process near the drop self-pinning. The deposition morphology of drying suspension droplets has attracted significant attention due to its practical applications in inkjet printing, painting, and coating technologies. In this study, the deposition morphology of silica/polystyrene suspension droplets on hydrophobic surfaces is explored and manipulated by controlling the relative humidity. The deposits are ring-like for the silica suspension droplets and become thicker and smaller under higher relative humidity. On the other hand, a uniform deposition is observed for the polystyrene suspension droplet evaporated at a relative humidity of 15%, i.e., the coffee ring effect is suppressed simply by manipulating the relative humidity. We applied a mass conservation model incorporated with the image analysis of an evaporating droplet to calculate the capillary flow velocity distribution inside a three-phase-contact-line (TPCL) shrinking droplet. The capillary flow velocity is higher when the relative humidity is lower. While the capillary flow velocity is higher to bring more particles towards and accumulate along the TPCL of the droplet, and the particles would pin the droplet earlier. The capillary flow velocity diagram also shows that the velocity is higher at the beginning of the mixed mode in the evaporation process and when the droplet starts to self-pin (at the end of the mixed mode) along the TPCL.