Elasto-hydrodynamic and thermofluidic instabilities in complex fluid droplets

Abstract

This dissertation presents research on the impact dynamics of non-Newtonian fluids on superhydrophobic surfaces, and the use of external magnetic and electric fields to control droplet rebound and reduce ink wastage in alternative inkjet printing technologies, including MHD and EHD inkjet printing. The findings of the study are presented in detail, and a new magnetic Bond number(Bom), magnetic Weissenberg number(Wim) and electric Eotvos number(Eoe) and electrical Weissenberg number (Wie) are introduced to understand the effect of external fields on non-Newtonian droplets due to magnetoelastic and electro elastic instabilities respectively. The study also delves into the realm of thermofluidic instabilities arising from Leidenfrost dynamics by employing complex f luid droplets such as surfactants, nanocolloids, and nanobubbles dispersed fluids. The research aims to suppress or delay the Leidenfrost effect during spray cooling processes and introduces novel coolants or alters the substrate’s microstructure to enhance heat transfer. The first two objectives aim to address elastohydrodynamic instabilities and their role in determining the magneto-elastic and electro-elastic effects that contribute to suppressing droplet rebound. The first objective of the dissertation presents experimental and scaling analysis to propose the existence of magneto-elastic effects in the impact hydrodynamics of non-Newtonian ferrofluid droplets on superhydrophobic surfaces under a magnetic field. The study investigates the role of We, Bom, polymer concentration, and Fe3O4 concentration in ferrofluids. Adding polymers causes rebound suppression of droplets at lower Bom, compared to Newtonian ferrofluid droplets. Moreover, increasing magnetic nanoparticle concentration triggers earlier rebound suppression with increasing Bom for a fixed polymer concentration and We. The interplay between the elastic effects of polymer chains and magnetic nanoparticles is named the magneto-elastic effect, leading to rebound suppression. A scaling relationship shows that the rebound suppression is observed as the onset of magneto-elastic instability when the magnetic Weissenberg number (Wim) exceeds unity. The study also proposes a phase map to identify the various regimes of impact ferrohydrodynamics of droplets. The second objective of this dissertation involves the investigation of the electrohydrodynamics of non-Newtonian dielectric fluid droplets impacting superhydrophobic surfaces. The results of the experiments reveal the occurrence of an electro-elastic effect that can lead to anti-superhydrophobicity. The study examines the electrohydrodynamics of non-Newtonian dielectric fluid droplets on superhydrophobic surfaces and provide a detailed analysis of the role played by various parameters such as electric Eotvos number Eoe, Weber number We, dielectric particle concentration TiO2, and polymer concentration (PEG-400) on droplet dynamics. The study proposes that the interplay of elastic relaxation dynamics of polymer chains and dielectric particles in the external electric field ambiance leads to droplet rebound suppression on superhydrophobic surfaces. The final three objectives of the dissertation present the potential role of thermofluidic instabilities of complex fluid droplets in suppressing the Leidenfrost effect by delaying the onset of dynamic Leidenfrost temperature during the impact of droplets on a hot surface. The third objective of the dissertation proposes the addition of anionic and cationic surfactants to water droplets as a novel method to increase the dynamic Leidenfrost temperature TDL, and the effect of surfactant concentration, Weber number(We), and Ohnesorge number(Oh) on the dynamic Leidenfrost temperature is experimentally investigated. The study also proposes a scaling relationship for TDL in terms of We and Oh, and presents a regime map of different boiling regimes as a function of impact We and substrate temperature.The fourth objective of the dissertation reports that the addition of Al2O3 nanoparticles to water alters the behavior of droplets impacting on a superheated substrate, delaying the onset of dynamic Leidenfrost temperature and suppressing the overall Leidenfrost regime. The study find that increasing nanoparticle concentration delays the onset of TDL at a specific Weber number (We), but for a constant concentration, the onset of TDL decreases with increasing impact We. The authors observe that the colloid droplets exhibit vigorous spraying behavior due to the nanoparticulate residue deposition during the spreading and retraction stages. The residue on the heated substrate changes the departure diameter of the vapor bubbles during boiling, prevents bubble coalescence and vapor layer formation, and reduces the likelihood of attaining the dynamic Leidenfrost regime. The authors use scaling analysis of TDL with impact We to explore the thermo-hydrodynamic behavior of impacting colloid droplets on a superheated substrate. Finally, the study segregate the different boiling regimes of colloid droplets over various impact We.The fifth objective of the dissertation proposes a new method for increasing the dynamic Leidenfrost temperature TDL by using nanobubble-dispersed droplets and investigates the influence of impact Weber number (We), Ohnesorge number (Oh), and nanobubble concentration on TDL. The findings suggest that using nanobubble-dispersed droplets can significantly delay TDL and that the TDL increases with increasing nanobubble concentration but decreases with increasing impact velocity for a particular concentration. The phase map presented in the study provides insights into the overall boiling behaviors of nanobubble-dispersed fluid droplets with substrate temperature against varied impact We. The overall investigation of the final three objectives of the dissertation has potential implications for thermal management systems operating at high temperatures.