Modulation of evaporation-affected crystal motion in a drying droplet by saline and surfactant concentrations

Crystallization of evaporating droplets has attracted intense interests thanks to its broad applications in bioengineering, coating technologies and pharmaceutical industries. We investigate the effects of saline and surfactant concentrations on the evaporative dynamics and crystallization patterns of sessile droplets. The droplet exhibits three regimes during evaporation. In regime I, the droplet transits from pinning to depinning, and then the commence of crystallization near the contact line launches regime II, during which the crystal exhibits more or less inward motion. In regime III, the crystal becomes immobilized until the evaporation completes. In the surfactant-free saline droplet, the crystal exhibits a weak gathering motion towards the center at a low saline concentration, resulting in the mild discrete crystalline deposition. Whereas the crystal movability is highly enhanced that they gather as a chunk at a high saline concentration, resulting from the larger droplet contact angle and higher supersaturation at the beginning of crystallization, and the higher liquid-vapor surface tension. These factors increase the horizontal component of the capillary force and solutal Marangoni force which drive crystals inwards. In the presence of surfactant, the crystals stay almost immobilized in regime II compared to the ones of the surfactant-free droplet, leading to discrete crystal morphologies. Surfactant reduces the liquid-vapor surface tension and the contact angle at the beginning of crystallization which are responsible for the smaller inward driving force.