Delayed Leidenfrost phenomenon during impact of elastic fluid droplets

Abstract

This article highlights the role of non-Newtonian (elastic) effects on the droplet impact phenomenon at temperatures considerably higher than the boiling point, especially at or above the Leidenfrost regime. The Leidenfrost point (LFP) was found to decrease with an increase in the impact Weber number (based on the velocity just before the impact) for fixed polymer (polyacrylamide) concentrations. Water droplets fragmented at very low Weber numbers (approx. 22), whereas the polymer droplets resisted fragmentation at much higher Weber numbers (approx. 155). We also varied the polymer concentration and observed that, up to 1000 ppm, the LFP was higher than that for water. This signifies that the effect can be delayed by the use of elastic fluids. We have shown the possible role of elastic effects (manifested by the formation of long lasting filaments) during retraction in the increase of the LFP. However, for 1500 ppm, the LFP was lower than that for water, but had a similar residence time during the initial impact. In addition, we studied the role of the Weber number and viscoelastic effects on the rebound behaviour at 405°C. We observed that the critical Weber number up to the point at which the droplet resisted fragmentation at 405°C increased with the polymer concentration. In addition, for a fixed Weber number, the droplet rebound height and the hovering time period increased up to 500 ppm, and then decreased. Similarly, for fixed polymer concentrations like 1000 and 1500 ppm, the rebound height showed an increasing trend up to certain a certain Weber number and then decreased. This non-monotonic behaviour of rebound heights was attributed to the observed diversion of the rebound kinetic energy to rotational energy during the hovering phase. Finally, a relationship between the non-dimensional Leidenfrost temperature and the associated Weber and Weissenberg numbers is developed, and a scaling relation is proposed.

Magneto-Elastic Effect in Non-Newtonian Ferrofluid Droplets Impacting Superhydrophobic Surfaces

“…In this context, it is worthwhile to mention that the relaxation time of the polymeric fluids increase with the polymer concentration. The role of We in setting up the critical shear rate required for non-Newtonian dynamics to be strong enough to slow down the retraction dynamics of the droplets during retraction is also well documented in earlier studies. ,− Now the product of the relaxation time and the shear rate (inverse of the flow time scale) is the classical Wi . Clearly, any phenomenon governed by Wi can be referred as elastic effects. , The present study is also focusing the modulation of drop impact dynamics due to varying magnetic field strengths (varying Bo m ).…”

“…It is at this instance that the plausible role of viscosity of the fluids on the rebound suppression dynamics needs to be discussed. In our previous reports , on elastic fluids, we have shown conclusively that the increase in viscosity of the fluids due to the presence of polymer may alter the impact hydrodynamics but does not trigger rebound suppression under any circumstances. In the present study, we have performed detailed rheology and magneto-rheology to understand the role played by viscosity (Figure S3, Supporting Information).…”

“…From Figure , it was observed that for a fixed magnetic particle concentration, with increasing Weber number, the critical Bo m for the rebound suppression is reduced. Therefore, it is evident that the non-Newtonian effects responsible for droplet rebound suppression ,− are becoming more potent with the increase of polymer concentrations as well as We . In this context, it is worthwhile to mention that the relaxation time of the polymeric fluids increase with the polymer concentration.…”

“…They further showed that the change in retraction dynamics of the non-Newtonian droplets can also be used to morph the thermo-species transport behavior, such as delaying the droplet Leidenfrost effect. 14 Zang et al 15 observed a transition from droplet rebound to sticking by the addition of PEO to Newtonian fluids and reported the importance of sliding angle of the fluid droplet, in addition to the contact angle. Yun et al 16 reported the effect of the electric field on nonaxisymmetric droplets toward inhibiting droplet rebound and also investigated the impact dynamics of ellipsoidal drops as a function of the geometric aspect ratio and impact Weber number.…”

“…Based on the shear rate during onset of retraction and the relaxation time scale of the elastic fluid, they showed that the onset of rebound suppression occurs only under circumstances where the governing Weissenberg number ( Wi ) exceeds one. They further showed that the change in retraction dynamics of the non-Newtonian droplets can also be used to morph the thermo-species transport behavior, such as delaying the droplet Leidenfrost effect . Zang et al observed a transition from droplet rebound to sticking by the addition of PEO to Newtonian fluids and reported the importance of sliding angle of the fluid droplet, in addition to the contact angle.…”