Interfacial- and electro-ferro-hydrodynamics driven vaporization of sessile droplets

Abstract

The present thesis reports the influencing role of solvated ions in DI water on its evaporation kinetics and internal hydrodynamics with/without the presence of external physical stimuli on different wettability substrates. The evaporation dynamics of sessile droplets of aqueous solutions of various salts with different concentrations under the varying strength of applied external stimuli is studied experimentally on the hydrophilic and superhydrophobic substrate (SHS). Initially, experiments were conducted with salt solution droplets in the absence of external physical stimuli and the results were compared with the DI water droplet. Observations reveal that the presence of solvated ions leads to modulated evaporation kinetics, which is further a function of surface wettability. On hydrophilic surfaces, increasing salt concentration leads to enhanced evaporation rates, whereas on superhydrophobic surfaces, it first enhances and reduces with concentration. Also, the nature and extents of the evaporation regimes (constant contact angle or constant contact radius) are dependent on the salt concentration. The reduced evaporation on superhydrophobic surfaces has been explained based on observed (via microscopy) crystal nucleation behavior within the droplet. Purely diffusion-driven evaporation models are noted to be unable to predict the modulated evaporation rates. Further, the changes in the surface tension and static contact angles due to solvated salts also cannot explain the improved evaporation behavior. Internal advection is observed (using PIV) to be generated within the droplet and is dependent on the salt concentration. The advection dynamics have been used to explain and quantify the improved evaporation behavior by appealing to the concept of interfacial shear modified Stefan flows around the evaporating droplet. The analysis leads to accurate predictions of the evaporation rates. Further, another scaling analysis has been proposed to show that the thermal and solutal Marangoni advection within the system leads to advection behavior. The analysis also shows that the dominant mode is the solutal advection and the theory predicts the internal circulation velocities with good accuracy. The findings may be of importance to microfluidic thermal and species transport systems. Next, we report the complex evaporation kinetics of saline sessile droplets on surfaces with elevated temperatures. Our previous studies show that on non-heated substrates, saline sessile droplets evaporate faster compared to their water counterparts. In the present study, we discover that on heated surfaces, the saline droplets evaporate slower than the water counterpart, thereby posing a counter-intuitive phenomenon. The reduction in the evaporation rates is directly dependent on the salt concentration and the surface wettability. Natural convection around the droplet and thermal modulation of surface tension is found to be inadequate to explain the mechanisms. Flow visualizations using particle image velocimetry (PIV) reveal that the morphed advection within the saline droplets is a probable reason behind the arrested evaporation. Infrared thermography is employed to map the thermal state of the droplets. A thermo-solutal Marangoni-based scaling analysis is put forward and the major governing non-dimensional numbers have been accounted for in the analysis. It is observed that the Marangoni flow and internal advection borne of thermal and solutal gradients are competitive, thereby leading to the overall decay of internal circulation velocity compared to the equivalent pure water case, which reduces the evaporation rates. The theoretically proposed advection velocities conform to the experimental results. This study sheds rich insight on a novel species transport behavior in saline droplets. Further, we report the morphing of the evaporation kinetics of paramagnetic saline sessile droplets in the presence of a magnetic field stimulus. We explore the evaporation kinetics both experimentally and theoretically and study the kinetics on hydrophilic and superhydrophobic substrates for various magnetic field strengths. We show that the evaporation rates of the paramagnetic droplets are augmented significantly, and are observed to be a direct function of the magnetic field strength. Additionally, we note the modulation of the contact line transients due to the presence of the field. The influential role of solvated ions in modulating the flow behavior, and subsequently the evaporation, of droplets, is present in literature. Taking a cue, we show using particle image velocimetry and infrared thermography that the magnetic field augments the thermo-solutal advection within the droplets. A mathematical analysis, based on the different internal advection mechanisms has been proposed. We reveal that the magneto-thermal and magneto-solutal modes of internal ferrohydrodynamics are the dominant mechanisms behind the augmented evaporation dynamics. The experimentally obtained internal velocities are in excellent compliance with the model predictions. Furthermore, the enhanced evaporation rates are predicted accurately using a proposed model to scale the interfacial shear modified Stefan flow. The inferences drawn from these findings may hold several important implications in magnetic field modulated microfluidic thermal and species transport systems. In the next study, we report the atypical phenomenology of suppression of the evaporation kinetics of electrically conducting saline sessile droplets in the presence of transverse electric field stimulus. Our experimental results show that the evaporation rates (on hydrophilic and superhydrophobic surfaces) of saline droplets are higher than water droplets, but the rates reduce when an alternating electric field is applied across the saline droplets. The reduction is noted to be a direct function of the electric field strength, and the contact line evolution dynamics during evaporation are grossly altered. The classical vapor diffusion-driven evaporation models fail to explain the morphed evaporation rates. The interfacial property variations with electric field are also unable to predict the physics. Subsequently, flow visualization and infrared thermography were performed to diagnose the internal thermo- and solutal-electrohydrodynamics. The electric field is observed to suppress the convection within the evaporating droplet. A scaling analysis model is proposed based on the internal advection mechanisms, to quantify the role of electrohydrodynamics, electro-thermal, electro-solutal convection, and the Electrohydrodynamic number on the internal circulation velocity and evaporation rates. The model incorporates the effects played by the governing Marangoni, Capillary, evaporative Jacob, electro-Prandtl, and electro-Schmidt numbers towards morphing the thermo-solutal advection. The predicted convection velocities are in good agreement with the experimental values. An interfacial shear modified Stefan flow analysis is put forward to determine the morphed evaporation rates, and a good match with the experimental observations is obtained.