Drying and deposition of poly(ethylene oxide) droplets determined by Peclet number

We report results of a detailed experimental investigation into the drying of sessile droplets of aqueous poly(ethylene oxide) (PEO) polymer solutions under various experimental conditions. Samples are prepared with a range of initial concentrations c(0) and are filtered to remove traces of undissolved PEO clusters. In typical experiments, droplets with initial volumes between 5 mu L and 50 mu L are left to evaporate while temperature and relative humidity are monitored. Droplets either form a disk-like solid "puddle" or a tall conical "pillar". The droplet mass is monitored using a microbalance and the droplet profile is recorded regularly using a digital camera. Subsequent processing of the data allows values of droplet volume V, surface area A, base radius R, contact angle theta and height h to be determined throughout drying. From these data we identify four stages during pillar formation: pinned drying; pseudo-dewetting; bootstrap building; solid contraction and propose physical models to explain key aspects of each stage and to predict the transition from each stage to the next. The experimental parameters of relative humidity, temperature, pressure, droplet volume and initial contact angle are all systematically varied and observed to influence the drying process and consequently whether the droplet forms a pillar or a puddle. We combine these parameters into a dimensionless Peclet number Pe, which compares the relative effects of evaporation and diffusion, and show that the drying behaviour is only dependent on c(0) and Pe.