Effect of Immiscible Secondary Fluid on Particle Dynamics and Coffee Ring Characteristics during Suspension Drying.

Particle motion and coffee ring patterns in water-borne suspensions of polystyrene (PS) particle added with small amounts of secondary hydrophobic decalin are investigated during the drying of the suspension droplets, mainly employing light scattering methods. Very tiny secondary fluid insertions via high-speed agitation effectively link the particles through hydrophobic dissolution leading to the formation of multimodal particulate clusters, with resistance to the outward capillary flow and suppression of coffee ring formation after drying. The impact of decalin on particles is corroborated by actual images acquired from an optical profiler and a scanning electron microscope (SEM). The average particle motion inside the suspension changed by decalin was expressed in terms of mean square displacement (MSD) based on diffusing wave spectroscopy (DWS). Employing multispeckle diffusing wave spectroscopy (MSDWS), the rapid motion or β-relaxation of particles in various suspensions with and without decalin is quantified in early lag time during the drying of droplets. The change in particle dynamics during suspension drop drying, when adding a small secondary fluid, plays a key role in tuning coffee ring patterns.