Observation of bubble growth in evaporating polymeric droplets via skin formation

We report the observation of bubble formation, growth, and different modes of rupture/breakup in isolated evaporating polymeric droplets. The accumulation of polymer at the droplet surface leads to bubble formation, growth, and subsequent breakup of the evaporating droplet. At high heating rates and for low concentrations of polymer, uninterrupted evaporation of droplets occurs while bubble growth and breakup at droplet surface occur for high polymeric concentration droplets. In contrast, rapid bubble growth from the droplet center is initiated for solutions with high polymer concentration under low heating rates. The bubble growth in droplets occurs due to skin formation following the accumulation of polymer during the evaporation process. The polymer concentration and temperature profiles of droplets for different heating rates are obtained numerically. The numerically obtained timescales for skin layer formation agree well with the experimental bubble ejection timescales.