Evaporation of a single droplet of a pure liquid in a confined chamber under atmospheric ambient condition is expected to be purely controlled by the rate of diffusion of the vapor into the surrounding. But, it is seen from the experimental results presented in this paper that for several liquids the process is faster than a theoretical estimate of the diffusion-driven process. It is seen from the visualization inside the droplet that these liquids exhibit intense internal circulation during evaporation. From a scaling analysis the temperature variations within the droplet due to surface traction and buoyancy-driven convection during evaporation is estimated. Marangoni and Rayleigh numbers are also obtained from these estimates. The values of these numbers indicate that Marangoni convection aided by buoyancy is probably the reason for the internal circulation induced within the droplet. The average velocity of the internal circulation is measured and is found to compare well with the velocity scale for Marangoni convection. © 2012 Elsevier Ltd.

Shamit Bakshi

Department of Mechanical Engineering

Ambient conditions

Average velocity

Buoyancy-driven convection

CLOSED CHAMBERS

DIFFUSION-DRIVEN

Droplet evaporation

INTERNAL CIRCULATIONS

MARANGONI

Marangoni convection

PURE LIQUIDS

Rayleigh

Rayleigh number

Scaling analysis

SINGLE DROPLET

SURFACE TRACTIONS

Temperature variation

Velocity scale

Drops

Evaporation

Liquids

Phase Transitions

Vapors

Visualization

Heat convection

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

International Journal of Multiphase Flow

Sessile droplets seated on superhydrophobic surfaces are known to exhibit internal circulation patterns. The present article experimentally demonstrates and theoretically confirms, for the very first time, that the nature and velocity of the internal circulation in sessile droplets seated on superhydrophobic surfaces are strongly governed by the curvature of the surface and its directionality. Sessile droplets were rested on concave and convex superhydrophobic surfaces, and both with one curvature (cylindrical) and two curvatures (spherical) and varying droplet diameter to curve diameters were studied. Particle image velocimetry (PIV) was employed for flow visualization and quantification. It was observed that increasing convexity of the surface leads to deterioration in the velocity of advection within the droplet, whereas increasing concavity of the surface augments the velocity of circulation. A scaling model based on the effective curvature-modulated change in wettability has been put forward to predict the phenomenon, but it was found to be weak in deducing the circulation velocities. Consequently, potential flow theory is employed and the curvatures are approximated as equivalent wedges, with the rested droplet engulfing the wedge partly. Based on the curvature of the surface, the equivalent included wedge angle is deduced. Flow theory over wedged structures is employed to deduce the changes in the internal velocity in the presence of curved surfaces. The spatiotemporally averaged experimental velocities are found to conform to predictions from the proposed model, and good agreement between the theoretical predictions and experimental observations is achieved. The present findings may have strong implications in thermofluidic transport phenomena or multiphase transport processes at the interfacial and/or microscale. © 2019 American Chemical Society.

Langmuir

Superhydrophobic Surface Curvature Dependence of Internal Advection Dynamics within Sessile Droplets

The influence of the suspender on the evaporation of droplets under quiescent atmospheric conditions was studied experimentally using pendant droplets of ethanol (volatile) and water (non-volatile). Conducting and non-conducting suspenders of different materials and configurations (a steel needle, glass needle, steel rod, and glass rod) were evaluated. For both liquids, it is observed that the rate of evaporation is higher with steel suspenders. More importantly, for ethanol droplets, there is a difference in the evaporation rates between the needle and rod configurations of the same material. Thus, it is shown for the ethanol droplet that the configuration (needle versus solid rod) also plays a role in the evaporation process, apart from the material. The water droplet does not show this dependence. Visualization of internal flow and heat transfer calculations explain this observation. The results will be useful in choosing an appropriate suspender while using the pendant droplet method for studying complex interactions in an evaporating droplet. © 2019 Elsevier Masson SAS

International Journal of Thermal Sciences

Influence of the suspender in evaporating pendant droplets

It is known from recent studies that evaporation induces flow around a droplet at atmospheric conditions. This flow is visible even for slowly evaporating liquids like water. In the present study, we investigate the influence of the ambient gas on the evaporating droplet. We observe from the experiments that the rate of evaporation at atmospheric temperature and pressure decreases in a heavier ambient gas. The evaporation-induced flow in these gases for different liquids is measured using particle image velocimetry and found to be very different from each other. However, the width of the disturbed zone around the droplet is seen to be independent of the evaporating liquid and the size of the needle (for the range of needle diameters studied), and only depends on the ambient gas used. © 2019 Author(s).

Physics of Fluids

Evaporation-induced flow around a droplet in different gases

Optofluidic manipulation using lasers and colloidal systems have immense applications in microscale thermofluidic devices. The present study analyses the effect of laser as an external optical source in modulating the evaporation characteristics of pendent nanofluid microdroplets (which are free from substrate effects) so as to capture the physics behind the interfacial mass transport. The study analyses the effect of the power of laser, nature and concentration of the particle on evaporation rate of the complex fluids. Evidence of internal circulation was observed with Particle Image Velocimetry (PIV) technique in colloidal systems which together with the volumetric heat generation due to laser irradiation can be attributed to be the cause behind the enhanced evaporation rate. The nanoparticles are observed to efficiently convert the energy of radiant photons to heat while water is observed to show poor evaporative performance under laser irradiation. Theoretical analysis of the evaporation rate with the classical diffusion driven evaporation model is found to fall short in predicting the evaporation rate in colloidal systems, even in the absence of laser irradiation. Thermal Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found not potent enough to induce the circulation in such systems which could improve evaporation. Hence the observed weak internal circulation can be attributed to the solutal Marangoni arising out of the local concentration gradients of nanoparticles in the droplet and the enhanced Thermophoretic drift and Brownian dynamics of the nanoparticles. Also the enhancement in the diffusion coefficient and its strong dependence with the particle concentration is also contributing to the enhancement in enhanced evaporation rate in nanocolloidal systems. The augmentation in the evaporative process under laser radiation is found to be governed majorly by the optical heat generation with weak solutal Marangoni and a scaling model is able to accurately predict the experimental observations. The present findings could have implications in modulation of thermofluidic and species transport phenomena in microscale devices. © 2017 Elsevier Ltd

International Journal of Heat and Mass Transfer

Optical thermogeneration induced enhanced evaporation kinetics in pendant nanofluid droplets

Evaporation kinetics of pendant droplets is an area of immense importance in several applications, in addition to possessing rich fluid dynamics and thermal transport physics. This article experimentally and analytically sheds insight into the augmented evaporation dynamics of paramagnetic pendant droplets in the presence of a magnetic field stimulus. The literature provides information that solutal advection and the solutal Marangoni effect lead to enhanced evaporation in droplets with solvated ions. The main focus of this article is to modulate the thermosolutal advection with the aid of an external 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 pointing at the magnetophoretic and magnetosolutal advection. Additionally, flow visualization by particle image velocimetry illustrates that the internal advection currents within the droplet increase in magnitude and are distorted in orientation by the magnetic field. A mathematical formalism based on magnetothermal and magnetosolutal advection has been proposed via scaling analysis of the species and energy conservation equations. The formalism takes into account all major governing factors, viz., the magnetothermal and magnetosolutal Marangoni numbers, magneto-Prandtl and magneto-Schmidt numbers, and the Hartmann number. The modeling establishes the magnetosolutal advection to be the dominant factor behind the augmented evaporation dynamics. Accurate validation of the experimental internal circulation velocity is obtained from the proposed model. This study reveals rich insight into the magnetothermosolutal hydrodynamics in paramagnetic droplets. © 2018 American Physical Society.

Fulltext

Physical Review E

Magnetohydrodynamics- and magnetosolutal-transport-mediated evaporation dynamics in paramagnetic pendant droplets under field stimulus

The surface concentration on the liquid side of the interface of an evaporating multicomponent droplet could be different from the bulk concentration. In this work, surface tension is used as a means to measure surface concentration of an evaporating multicomponent droplet. Surface tension is measured using pendant droplet method that relies on the best fit between theoretical and experimental drop profiles. Surface tension is a surface property, and it exhibits a dependence on concentration. Hence, it is an ideal candidate to track the variation of surface concentration during the evaporation of a multicomponent droplet. This method is used to study the evaporation of ethanol-water and methanol-water droplets. The correctness and applicability of this technique are critically assessed, and important observations are made for single droplet evaporation for these binary mixtures. © 2010 Springer-Verlag.

Experiments in Fluids

Measurement of the surface concentration (liquid) of an evaporating multicomponent droplet using pendant droplet method

Journal	International Journal of Multiphase Flow
ISSN	03019322
Open Access	No