Experimental investigations of drying droplets containing microparticles and mixtures of Poly(ethylene oxide) microparticles

Ring-stains are seen when droplets of liquid containing particles are left to dry on a surface: a pinned contact line leads to outward radial flow, which is enhanced by the diverging evaporative flux at the contact line. As a result, suspended particles in the drops are transported to the edge of the droplet, and deposited in a circular stain. In the first section of this study, we investigated how the width and height of ring in water droplets containing suspensions of polystyrene microparticles with diameters ≤0.5μm vary with experimentally controlled parameters, including particle size, contact angle, concentration, evaporation rate and orientation of the droplets. Our studies found, for the first time, that the drying rate plays an important role in determining the shape of the final deposit which may contribute to a better understanding of a coffee ring effect. At low drying rates, nearly all the particles are deposited in the ring and the width and height of the ring follow a power law. However there is a significant deposition of particles in the center of the droplets at the fastest drying rates. In the second section, we investigated the drying dynamics of the droplets using optical microscopy and optical coherence tomography both at low and high drying rates. The results indicate that, as expected, the outward capillary flow to the edge dominates for 0.5m and 2m particles at low drying rates. However, at the highest drying rates, particle flow is reversed towards the center for 0.5m and 2m particles and is attributed to Marangoni flow driven by a high temperature gradient between the edge and apex of the drop. For 5m at low drying rates, sedimentation is found to be more significant than outward flow and at higher drying rates the outward flow dominates over sedimentation. The final section involved mixing polystyrene microparticles with Poly(ethylene oxide)(PEO) polymer. Drying droplets of aqueous solutions of PEO is known to form either the familiar "coffee-ring" or a tall "central pillar" depending on the concentration of the polymer, its molecular weight, humidity and drying rate. The objective of this study was to investigate the effect of particle sizes and concentration on drying of water droplets containing a water/PEO solution mixture. Results show that the four stages in the pillar forming of PEO drying process is disrupted, and even completely destroyed, with the additional of polystyrene microparticles. The extent at which the pillars are destroyed depends on both the concentration and sizes of the polystyrene microparticles as well as on PEO concentration in the mixture. A range of deposit was observed, as a result, including the standard particle ring stain, polymer pillars and puddles. We present a novel and versatile technique, using the skewness of the height profile, to distinguish quantitatively between the various deposits. We show that this simple parameter seems to illustrate the observed deposit types, with a positive skewness for pillars, close to zero for flat puddle deposits and negative skewness for ring stains. The skewness values have the potential to be useful for characterizing deposits in a wide variety of system