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The transmission of viruses/ bacteria cause infection predominantly via aerosols. The transmission mechanism of respiratory diseases is complex, including direct or indirect contact, large droplet, and airborne routes apart from close contact transmission. With this pretext, we have investigated two modes of droplet evaporation to understand its significance in airborne disease transmission; a droplet in a contact-free environment, which evaporates and forms droplet nuclei, and a droplet on a hydrophilic substrate (fomite). The study examines mass transport, the deposition pattern of bacteria in the precipitates, and their survival and virulence. The osmotic pressure increases with the salt concentration, inactivating the bacteria embedded in the precipitates with accelerated evaporation. Further, the bacteria's degree of survival and enhanced pathogenicity are compared for both evaporation modes. The striking differences in pathogenicity are attributed to the evaporation rate, oxygen availability, and reactive oxygen species (ROS) generation.

Modification and control over the vaporization kinetics of microfluidic droplets may have strong utilitarian implications in several scientific and technological applications. The article reports the control over the vaporization kinetics of pendent droplets under the influence of competing internal electrohydrodynamic and ferrohydrodynamic advection. Experimental and theoretical studies are performed and the morphing of vaporization kinetics of electrically conducting and paramagnetic fluid droplets using orthogonal electric and magnetic stimuli is established. Analysis of the observations reveals that the electric field has a domineering influence compared to the magnetic field. While the magnetic field is noted to aid the vaporization rates, the electric field is observed to decelerate the same. Neither the vapour diffusion dominated kinetics nor the field induced modified surface tension can explain the observed vaporization behaviours. Velocimetry within the droplet shows largely modified internal ferro and electrohydrodynamic advection, which is noted to be the crux of the mechanism towards modified vaporization rates. A mathematical treatment is proposed and takes into account the roles played by the governing Hartmann, electrohydrodynamic, interaction, the thermal and solutal Marangoni, and the electro and magneto Prandtl and Schmidt numbers. It is observed that the morphing of the thermal and solutal Marangoni numbers by the electromagnetic interaction number plays the dominant role towards morphing the advection dynamics. The model is able to predict the internal advection velocities accurately. The findings may hold significant promise towards smart control and tuning of vaporization kinetics in microhydrodynamics transport paradigms.

In this article we report the atypical and anomalous evaporation kinetics of saline sessile droplets on surfaces with elevated temperatures. In a previous we showed that saline sessile droplets evaporate faster compared to water droplets when the substrates are not heated. In the present study we discover that in the case of 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 visualisations using particle image velocimetry PIV reveals 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 thermosolutal Marangoni based scaling analysis is put forward. It is observed that the Marangoni and internal advection borne of thermal and solutal gradients are competitive, thereby leading to the overall decay of internal circulation velocity, which reduces the evaporation rates. The theoretically obtained advection velocities conform to the experimental results. This study sheds rich insight on a novel yet anomalous species transport behaviour in saline droplets.

The present article experimentally and theoretically probes the evaporation kinetics of sessile saline droplets. Observations reveal that 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 behaviour 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 behaviour. Internal advection is observed using PIV to be generated within the droplet and is dependent on the salt concentration. The advection dynamics has been used to explain and quantify the improved evaporation behaviour 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 the advection behaviour. 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.

The article experimentally reveals and theoretically establishes the influence of electric fields on the evaporation kinetics of pendant droplets. It is shown that the evaporation kinetics of saline pendant droplets can be augmented by the application of an external alternating electric field. The evaporation behaviour is modulated by an increase in the field strength and frequency. The classical diffusion driven evaporation model is found insufficient in predicting the improved evaporation rates. The change in surface tension due to field constraint is insufficient for explaining the observed physics. Consequently, the internal hydrodynamics of the droplet is probed employing particle image velocimetry. It is revealed that the electric field induces enhanced internal advection, which improves the evaporation rates. A scaled analytical model is proposed to understand the role of internal electrohydrodynamics, electrothermal and the electrosolutal effects. Stability maps reveal that the advection is caused nearly equally by the electrosolutal and electrothermal effects within the droplet. The model is able to illustrate the influence played by the governing thermal and solutal Marangoni number, the electro Prandtl and electro Schmidt number, and the associated Electrohydrodynamic number. The magnitude of the internal circulation can be well predicted by the proposed model, which validates the proposed mechanism.

The present article reports the hitherto unreported phenomenon of arrested evaporation dynamics in pendent droplets in an electric field ambience. The evaporation kinetics of pendant droplets of electrically conducting saline solutions in the presence of a transverse, alternating electric field is investigated experimentally. It has been observed that while increase of field strength arrests the evaporation, increment in field frequency has the opposite effect. The same has been explained on the solvation kinetics of the ions in the polar water. Theoretical analysis reveals that change in surface tension and diffusion driven evaporation model cannot predict the arrested or decelerated evaporation. With the aid of Particle Image Velocimetry, suppression of internal circulation velocity within the droplet is observed under electric field stimulus, and this affects the evaporation rate directly. A mathematical scaling model is proposed to quantify the effects of electrohydrodynamic circulation, electrothermal and electro-solutal advection on the evaporation kinetics of the droplet. The analysis encompasses major governing parameters, viz. the thermal and solutal Marangoni numbers, the Electrohydrodynamic number, the electro Prandtl and electro Schmidt numbers and their respective contributions. It has been shown that the electrothermal Marangoni effect is supressed by the electric field, leading to deteriorated evaporation rates. Additionally, the electrosolutal Marangoni effect further supresses the internal advection, which again arrests the evaporation rate by a larger proportion. Stability analysis reveals that the electric body force retards the stable internal circulation within such droplets and arrests advection.

The article reports the domineering governing role played by the direction of electric and magnetic fields on the internal advection pattern and strength within salt solution pendant droplets. Literature shows that solutal advection drives circulation cells within salt based droplets. Flow visualization and velocimetry 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, the electric field leads to deterioration of the same. The concepts of electro andmagnetohydrodynamics are appealed to and a Stokesian stream function based mathematical model to deduce the field mediated velocities has been proposed. The model is found to reveal the roles of and degree of dependence on the governing Hartmann, Stuart, Reynolds and Masuda numbers. The theoretical predictions are observed to be in good agreement with experimental average spatio-temporal velocities. The present findings may have strong implications in microscale electro and/or magnetohydrodynamics.

The article demonstrates that the internal circulation velocity and patterns in sessile droplets on superhydrophobic surfaces is governed by the surface curvature. Particle Image Velocimetry reveals that increasing convexity deteriorates the advection velocity whereas concavity augments it. A scaling model based on the effective curvature modulated change in wettability can predict the phenomenon, but weakly. Potential flow theory is appealed to and the curvatures are approximated as wedges with the rested droplet engulfing them partly. The spatially averaged experimental velocities are found to conform to predictions. The study may have strong implications in thermofluidics transport phenomena at the microscale.

Evaporation kinetics of pendant droplets is an area of immense importance in several applications in addition to possessing rich fluid and thermal transport physics. The present article experimentally and analytically sheds insight into the augmented evaporation dynamics of paramagnetic pendent droplets in the presence of a magnetic field stimulus. Literature provides information that solutal advection and solutal Marangoni effect lead to enhancement of evaporation in droplets with ionic inclusions. The major crux of the present article remains to modulate the thermosolutal advection with the aid of magnetic field and comprehend the dynamics of the evaporation process under such complex multiphysics 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 magnetophoretic 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 magnetothermal and magnetosolutal 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 magnetothermal and magnetosolutal Marangoni numbers, magneto Prandtl and magneto Schmidt numbers and the Hartmann number. The modeling establishes the magnetosolutal 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. The present study reveals rich insight on the magneto thermosolutal hydrodynamics aspects in paramagnetic droplets.