Internal thermo-solutal, electro and magnetohydrodynamic advection influenced evaporation kinetics of pendant droplets

Abstract

The present thesis reports the influencing role of dissolved ions in a polar fluid on its evaporation dynamics, and the nature in which the same may be modulated by induced electro and magneto-advection. The evaporation dynamics of pendant droplets of aqueous solutions of variant simple salts, concentrations and different physical stimuli have been experimentally studied. Initially, experiments were done without any physical stimuli, and the presence of salts is observed to enhance the evaporation rate (obeying the classical D2 law) and the enhancement has been found to hold a direct proportionality to the concentration of the dissolved salt. Furthermore, it is observed that the degree of enhancement in evaporation rate is also directly proportional to the solubility of the salt in question. The phenomenon is explained based on the chemical kinetics and thermodynamics of hydration of the ionic species in the polar fluid. The classical evaporation rate constant formulation is found to be inadequate in modeling the enhanced species transport. Additional probing via particle image velocimetry (PIV) reveals augmented internal circulation within the evaporating salt based drops compared to pure water. Mapping the dynamic surface tension reveals that a salt concentration gradient is generated between the bulk and periphery of the droplet and it could be responsible for the internal advection cells visualized. A thermo-solutal Marangoni and Rayleigh convection based mathematical formulation has been put forward and it is shown that the enhanced solute-thermal convection could play a major role in enhanced evaporation. The internal circulation mapped from experiments is found to be in good quantitative agreement with the model predictions. Scaling analysis further reveals that the stability of the solutal Marangoni convection surpasses the thermal counterpart with higher salt concentration and solubility. The present work sheds insight onto the possible domineering role of conjugate thermos-hydraulic and mass transport phenomena on the evaporation kinetics aqueous droplets with ionic inclusions. Next, the evaporation kinetics of pendant droplets is experimentally and analytically sheds insight into the augmented evaporation dynamics of paramagnetic pendent droplets in the presence of a magnetic field stimulus is provided. Previous study in this thesis provides information that solutal advection and solutal Marangoni effects lead to enhancement of evaporation in droplets with ionic inclusions. The major crux of this part remains to modulate the thermo-solutal advection with the aid of magnetic field and comprehend the dynamics of the evaporation process under such complex multi-physics interactions. Experimental observations reveal that the evaporation rate enhances as a direct function of the magnetic moment of the solvated magnetic element ions, thereby pinpointing at the magneto-phoretic and magneto-solutal advection. Additionally, flow visualization by PIV illustrates that the internal advection currents within the droplet are strengthened in magnitude as well as distorted in orientation by the magnetic field. A mathematical formalism based on magneto-thermal and magneto-solutal advection effects has been proposed via scaling analysis of the species and energy conservation equations. The formalism takes into account all major governing factors such as the magneto-thermal and magneto-solutal Marangoni numbers, magneto-Prandtl and magneto-Schmidt numbers and the Hartmann number. The modeling establishes the magneto-solutal advection component to be the domineering factor in augmented evaporation dynamics. Accurate validation of the experimental internal circulation velocity is obtained from the proposed model. This study reveals rich insight on the magneto-thermo-solutal hydrodynamics aspects in paramagnetic droplets. Later on, the behavior of pendant droplet evaporation of saline solution in the presence of electric field is investigated experimentally as well as analytically. The contribution of the electric field is observed to be negative, and it attempts to slow down the evaporation rate and it is experimentally observed. With flow visualization study, suppression of internal circulation velocity is reported under electric field which affects evaporation rate directly. Furthermore, a scaling model similar to previous cases is proposed to quantify the effect of electro- thermal as well as electro-solutal advection on evaporation of pendant droplet mathematically. The scaling takes account of involved all major parameters like thermal and solutal Marangoni, electro-hydrodynamic number, electro-diffusive Prandtl and electro-solutal term and their respective contribution. With the inclusion of electro-hydrodynamic term, Marangoni number scales down to poses unstable circulation within droplet concerning Rayleigh number. The effective solutal Marangoni effect dominates effective thermal Marangoni and thus, generates stable circulation, but its effect reduces as compared to solutal Marangoni effect without an electric field. This understanding of the electro-hydrodynamic behavior of pendant droplet leads to better implication while designing devices for either non- uniform electric field (dielectrophoretic force (DEP)) or non- uniform permittivity (electrostriction force). The last of the thesis reports the domineering role played by the direction of electric and magnetic fields on the internal advection pattern and strength within salt solution pendant droplets. The thesis work shows that solutal advection drives circulation cells within salt based droplets, even in the absence of any external field. An experimental study is performed, where electric and magnetic fields are applied across pendent droplets of salt solutions and their internal flow dynamics is observed. Flow visualization and velocimetry (two-dimensional) reveals that the direction of the applied field governs the enhancement/reduction in circulation velocity and the directionality of circulation inside the droplet. Further, it is noted that while magnetic fields augment the circulation velocity (with respect to the solutal advection already present in salt solution droplets at zero field); the electric field leads to deterioration of the same. The concepts of electro and magnetohydrodynamics of droplets are appealed to and a Stokesian stream function based mathematical model to deduce the field mediated velocities has been proposed. The roles of the governing Hartmann, Stuart, Reynolds and Masuda numbers is revealed by the proposed model. The theoretical predictions are observed to be in agreement with the experimentally determined averaged spatio-temporal circulation velocities. The present findings discussed in this thesis may have strong implications in design and development of systems employing microscale and interfacial electro and/or magnetohydrodynamics.