Electrohydrodynamics of dielectric droplet collision on different wettability surfaces

Abstract

In this article, we report experimental and semi-analytical findings to elucidate the electrohydrodynamics (EHD) of a dielectric liquid droplet impact on superhydrophobic (SH) and hydrophilic surfaces. A wide range of Weber numbers (We) and electro-capillary numbers (Ca e ) is covered to explore the various regimes of droplet impact EHD. We show that for a fixed We~60, droplet rebound on SH surface is suppressed with increase of electric field intensity (increase of Ca e ). At high Ca e , instead of the usual uniform radial contraction, the droplets retract faster in orthogonal direction to the electric field and spread along the direction of the electric field. This prevents the accumulation of sufficient kinetic energy to achieve the droplet rebound phenomena. For certain values of We and Ohnesorge number (Oh), droplets exhibit somersault-like motion during rebound. Subsequently we propose a semi-analytical model to explain the field induced rebound phenomenon on SH surfaces. Above a critical Ca e ~4.0, EHD instability causes fingering pattern via evolution of spire at the rim. Further, the spreading EHD on both hydrophilic and SH surfaces are discussed. On both wettability surfaces and for a fixed We,, the spreading factor shows an increasing trend with increase in Ca e . We have formulated an analytical model based on energy conservation to predict the maximum spreading diameter. The model predictions hold reasonably good agreement with the experimental observations. Finally, a phase map was developed to explain the post impact droplet dynamics on SH surfaces for a wide range of We and Ca e .

Triggering of electro-elastic anti-superhydrophobicity during non-Newtonian droplet collision

“…As a result, the retraction velocity reduces with increase in E 2 ε. This has been observed in our previous study [21], where rebound suppression on SH surface was observed with increase in electric field intensity for a Newtonian dielectric field. In the present scenario, we have noted that the onset of rebound suppression is largely reduced to lower values of Eo e when the non-Newtonian component is present in the fluids.…”

“…As discussed earlier [21], the dielectric force acting on the droplet is proportional to (E 2 ε). This dielectric force is responsible for elliptic spreading of the droplets.…”

“…Figure 2 a–d illustrates the influence of the Eo e on the spreading behaviour of non-Newtonian dielectric droplets by varying particle ( TiO 2 ) concentration at constant polymer ( 5%V/V ) concentration and We ∼ 65. In this work, we do not focus on the spreading electrohydrodynamics of Newtonian dielectric droplets, as has been discussed in detail in our previous work [21]. Figure 2 a shows the spreading behaviour of pure polymer (P5-T0) droplets.…”

“…The experimental set-up is similar to our earlier study on droplet impact [21]. Figure 1 illustrates the schematic of the experimental set-up, in which two aluminium strips (50 × 20 × 2 mm) with attached wires were used as electrodes to generate a direct electric field of up to a maximum of 8 kV.…”

“…Besides, many researchers have experimentally elucidated the impact or dynamic EHDs of dielectric droplets. Sahoo et al [21] experimentally investigated the impact EHDs of dielectric liquid droplets on both hydrophilic and superhydrophobic (SH) surfaces. They reported the influence of the electric Capillary number on the onset of droplet rebound suppression.…”