Insights into evaporation, atomization and precipitate formation of polymer droplets

Abstract

Evaporation of polymer droplets is an active area of interest due to its applications in systems such as ink jet printing, thin films, spray combustors to name a few. Thus, understanding complex dynamics of evaporating polymer droplets in different experimental configurations is crucial to cater to the wider industrial applications. This study investigates evaporation and subsequent dynamics of two classes of polymeric droplets-low viscoelastic (PAM) and high viscoelastic (PEO) in two different experimental configurations: Laser induced evaporation of droplet under acoustic levitation, natural evaporation of droplet on hydrophobic surfaces. In the first part, we investigate the interaction of an aqueous low viscoelastic polymer droplet (PAM) with a tunable continuous laser in an acoustically levitated environment. Depending on the laser irradiation intensity and polymer concentration, we observe four temporal phases: droplet evaporation, vapor bubble growth followed by membrane inflation, bubble/membrane rupture through hole nucleation, and droplet breakup. During the initial droplet evaporation phase, concentration build-up at the droplet surface beyond a critical limit lead to the formation of a skin layer. It is revealed that at a given location inside the droplet, hot spots occur, and the maximum temperature at the hot spots scales linearly with irradiation intensity until a bubble nucleates. The low-intensity laser interaction leads to symmetric membrane inflation that eventually forms holes at droplet poles and cracks on the shell surface. On the contrary, high intensity causes early bubble nucleation followed by asymmetric membrane inflation that eventually ruptures through multiple hole formation. Furthermore, the growth and rupture of the membrane is followed by a catastrophic breakup of the droplet. Two dominant atomization modes are reported at significantly high irradiation intensities: stable sheet collapse and unstable sheet breakup. The evolution of droplets into a stable/unstable sheet follows universally observed ligament and hole dynamics. A regime map is shown to describe the influence of polymer concentration and irradiation intensity on the strength and mode of droplet atomization. In the second part, we investigate the interaction of a aqueous high viscoelastic polymer droplet (PEO) with a tunable continuous laser in an acoustically levitated environment. Depending on the laser irradiation intensity, we observe nucleation of a bubble in the dilute regime of polymer concentration, contrary to the previously observed bubble nucleation in a semi-dilute entangled regime for low viscoelastic modulus polymer droplets. After the bubble nucleation, a quasi-steady bubble growth occurs depending on the laser irradiation intensity and concentrations. Our scaling analysis reveals that bubble growth follows Plesset-Zwick criteria independent of the viscoelastic properties of the polymer solution. Further, we establish that the onset of bubble growth has an inverse nonlinear dependence on the laser irradiation intensity. At high concentrations and laser irradiation intensities, we report the expansion and collapse of polymer membrane without rupture, indicating the formation of an interfacial skin with significant strength. The droplet oscillations are primarily driven by the presence of multiple bubbles and, to some extent, by the rotational motion of the droplet. Finally, depending on the nature of bubble growth, different types of precipitate form contrary to the different modes of atomization observed in low viscoelastic modulus polymer droplets. In the third part, we experimentally report the concentration and molecular weight dependence of the deposit patterns of low viscoelastic evaporating polyacrylamide (PAM) droplets on hydrophobic surfaces. We find that with an increase in non-dimensional concentrations c⁄c^* ranging from 0.16 (dilute) to 66.66 (semi-dilute entangled) there is a gradual transition from ring to uniform precipitates. However, with a decrease in the molecular weight of the polymer by one order, the coffee ring formation was not suppressed for the reported range of concentration. We attribute these results to the role played by the critical overlap concentration (c^*) and diffusion coefficient of polymer along with the evaporation modes. Lastly, the authors report the experiments on the precipitate formation of evaporating (PEO) droplets on hydrophobic surfaces. We observe the final precipitates to be deformed with the formation of central dip over the concentrations ranging from semi-dilute unentangled to semi-dilute entangled. Overall, this study provides valuable insights into the complex phenomenon of evaporation of polymer droplets in different configurations and its importance in various industrial and natural processes. The findings can help optimize these processes and improve our understanding of them.