By varying many experimental parameters (temperature, pressure, humidity, contact angle, concentration and volume) we discover that whether evaporating droplets of PEO polymer solution deposit tall solid pillars or flat puddles is controlled by the dimensionless Péclet number, relating flux to diffusion.This article will appear in issue 17 of Soft Matter, a themed issue on Dynamics and Rheology of Fluid Interfaces. AbstractWe 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 L and 50 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 θ and height h to be determined throughout drying. From this 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 Péclet 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.

Microlitre polymer droplets deposit solid conical structures, via a novel bootstrap drying mechanism, over a range of initial conditions. Accepted for publication in Phys. Chem.Chem.Phys, 2010, DOI: 10.1039 Abstract Sessile droplets of aqueous poly(ethylene oxide) solution, with average molecular weight of 100kDa, are monitored during evporative drying at ambient conditions over a range of initial concentrations c 0 . For all droplets with c 0 ≥ 3%, central conical structures, which can be hollow and nearly 50% taller than the initial droplet, are formed during a growth stage. Although the formation of superficially similar structures has been explained for glass-forming polymers using a skin-buckling model which predicts the droplet to have constant surface area during the growth stage (L.Pauchard and C. Allain, Europhys. Lett., 2003, 62, 897-903), we demonstrate that this model is not applicable here as the surface area is shown to increase during growth for all c 0 . We interpret our experimental data using a proposed drying and deposition process comprising the four stages: pinned drying; receding contact line; "bootstrap" growth, during which the liquid droplet is lifted upon freshly-precipitated solid; and late drying. Additional predictions of our model, including a criterion for predicting whether a conical structure will form, compare favourably with observations. We discuss how the specific chemical and physical properties of PEO, in particular its amphiphilic nature, its tendency to form crystalline spherulites rather than an amorphous glass at high concentrations and its anomalous surface tension values for M W = 100 kDa may be critical to the observed drying process.