Dynamic behaviors of impinging viscoelastic droplets on superhydrophobic surfaces heated above the boiling temperature

Abstract The impact dynamics of viscoelastic droplets is investigated on a superhydrophobic surface heated beyond the boiling temperature and up to the Leidenfrost temperature. Traditional impact outcomes, including rebound and deposition, are identified and analyzed. While the lower threshold Weber number for droplet rebound decreases with the surface temperature up to the Leidenfrost temperature (but not appreciably at higher temperature) but increases with the aqueous polymer concentration, the upper threshold stays nearly constant. Further analyses on the impact processes reveal that both the surface temperature and the polymer concentration have negligible effects on the droplet spreading, and the maximum spreading factor can be well described by a semi-empirical equation obtained from a simple energy-conservation modeling. By contrast, an increase in the surface temperature or a decrease in the polymer concentration would lead to a speed up of the droplet retraction, resulting in an increase in the restitution coefficient for bouncing droplets due to the suppression of the viscoelastic ligament formation. Moreover, the post-impact oscillation of depositing droplets was analyzed using a damped-harmonic-oscillator model, which shows that both the spring constant and the damping coefficient decrease with the surface temperature, suggesting a decrease in the viscous dissipation and the contact-line friction.