Ferro and electrohydrodynamics of impacting droplets

Abstract

The aim of this thesis is to understand the external field induced (gravitational, electric and magnetic fields) spreading dynamics of drops on surfaces of varying degrees of wettability. In pursuit of this, the present study explores the roles of surface wettability, substrate inclination, electric and magnetic field strengths on the post-impact dynamics of droplets. In the subsequent paragraphs, the effects of external fields are investigated in details. In the first study, experimental investigations were carried out to elucidate the roles of surface wettability and inclination on the post-impact dynamics of droplets. The maximum spreading diameter and spreading time were found to decrease with increasing inclination angle and normal Weber number (Wen) for superhydrophobic (SH) surfaces. The experiments on SH surfaces were found to be in excellent agreement with an existing analytical model, albeit with the incorporation of modifications for the oblique impact conditions. The energy ratios and elongation factors were also determined for different inclination angles. On inclined SH surfaces, different features like arrest of secondary droplet formation, reduced pinch-off at the contact line and inclination dependent elongation behavior were observed. On the contrary, the hydrophilic surfaces show opposite trends of maximum spreading factor and spreading time with inclination angle and Wen, respectively. This is caused by the dominance of tangential kinetic energy over adhesion energy and gravitational potential at higher inclination angles. Further, the influence of the surface tension (using surfactant solutions, without significantly changing the viscosity) and viscosity (using colloids, without significantly changing the surface tension) for impact on SH and hydrophilic surfaces are probed. The exercise allows better insight on the exact hydrodynamic mechanisms at play during the impact events. This is followed by studying the aspects of post-impact hydrodynamics of ferrofluid droplets on SH surfaces in the presence of a horizontal magnetic field. A wide gamut of dynamics was observed by varying the impact Weber number (We), the magnetic field strength (manifested through the magnetic Bond number (Bom), which is defined as the ratio of magnetic force to surface tension force), and the Hartmann number (Ha), defined as the ratio of magnetic force to the viscous force). For a fixed We ~60, the current study observed that at moderately low Bom ~300, droplet rebound off the SH surface is suppressed. The noted We is chosen to observe various impact outcomes and to reveal the consequent ferrohydrodynamic mechanisms. It is found that ferrohydrodynamic interactions leads to asymmetric spreading due to variation in magnitude of the magnetic force; and the droplet spreads preferentially in a direction orthogonal to the magnetic field lines. The present study shows analytically that during the retraction regime, the kinetic energy of the droplet is distributed unequally in the transverse (orthogonal to the external horizontal magnetic field) and longitudinal (along the direction of the magnetic field) directions. This ultimately leads to suppression of droplet rebound. In addition, the study also investigates the role of Bom at fixed We ~60, and observed that the liquid lamella becomes unstable at the onset of retraction phase, through nucleation of holes, their proliferation and rupture after reaching a critical thickness only on SH surfaces, but is absent on hydrophilic surfaces. Based on the experimental observations, an analytical model was formulated to predict the onset of instability at a critical Bom. The model shows that the critical Bom is a function of the impact We, and the critical Bom decreases with increasing We. A phase map encompassing all the post-impact ferrohydrodynamic phenomena on SH surfaces was developed for a wide range of We and Bom. Finally, the present study reports experimental and semi-analytical findings to elucidate the electrohydrodynamic (EHD) of a dielectric liquid droplet impact on SH and hydrophilic surfaces. The study observed that for a fixed We~60, droplet rebound on SH surface is suppressed with increase of electric field intensity (increase of Cae). At high Cae, 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, inducing large electrical stresses at the liquid rim facing the electrodes. 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 Cae~4.5, 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 Cae. It is also observed that the energy conservation based analytical model holds reasonably good agreement with the experimental maximum spreading diameter. Finally, a phase map was developed to explain the post impact droplet dynamics on SH surfaces for a wide range of We and Cae.