We present a comprehensive experimental and theoretical investigation of the evaporation dynamics of freely levitated water droplets in an upward airstream under varying temperature and relative humidity conditions, using a custom-designed wind tunnel that replicates natural rainfall scenarios. A high-speed imaging system captures the temporal evolution of morphology, shape oscillations, and size reduction of the droplet undergoing evaporation. Our observations reveal that larger droplets exhibit persistent shape oscillations due to the interplay between inertia and surface tension in the presence of convective airflow, which significantly alters the evaporation rate compared to that of a stationary spherical droplet in quiescent air. To quantify the effects of air convection, complex morphology, and shape oscillations of the levitated droplet at different temperatures and humidity, we develop a modified evaporation model that extends the classical $d^2$-law. This model incorporates (i) a generalized Sherwood number that accounts for the variation in Reynolds number, Schmidt number, temperature, and relative humidity and (ii) a shape factor that captures the time-averaged surface area of oscillating droplets. The model is validated against experimental findings across a wide range of droplet sizes and environmental conditions, showing excellent agreement in predicting the temporal evolution of droplet diameter and total evaporation time. Furthermore, we construct a regime map showing the variation in the lifetime of the droplet in the temperature-humidity space. The present study establishes a framework that integrates convective transport and morphological deformation, offering new insights into the microphysics of raindrop evaporation.
The evaporation of droplets is a fundamental process encountered in a wide range of industrial applications, such as spray combustion, drying, and cooling technologies (Huang & Ayyaswamy 1990;Sazhin 2006;Pinheiro & Vedovoto 2019), biological systems (Mittal et al. 2020) and various natural phenomena (Houghton 1933;Best 1952;Caplan 1966;Abraham 1962;Tardif & Rasmussen 2010). During rainfall, evaporation alters the shape and size of falling raindrops through continuous heat and mass transfer with the surrounding air. As raindrops descend through the atmosphere, they encounter progressively warmer and drier conditions, leading to complex evaporation dynamics that significantly influence the evolution of raindrop size distributions. In addition to modulating rainfall, droplet evaporation has far-reaching implications for understanding cloud microphysics, quantifying precipitation intensity, and predicting the vertical redistribution of latent heat within the atmosphere. Thus, a detailed understanding of droplet-scale evaporation is essential to improve the accuracy of predictive models related to cloud development, raindrop formation, and atmospheric moisture transport (Beard & Pruppacher 1971;Beji & Merci 2018). The importance of evaporation in influencing both natural and engineering processes has been recognized for more than a century (Langmuir 1918;Apashev & Malov 1962;Beard & Pruppacher 1971), and it remains an active area of research even today (Schlottke & Weigand 2008;Tripathi & Sahu 2015;Pal & Biswas 2023;Sezen & Gungor 2023;Pal et al. 2024a,b).
A large volume of experimental investigations on evaporation have mainly focused on sessile droplet configurations, examining the influence of substrate wettability and surrounding ambient conditions (Birdi et al. 1989;Deegan et al. 1997;Shahidzadeh-Bonn et al. 2006;Sefiane & Bennacer 2011;Gurrala et al. 2019;Katre et al. 2021;Diddens et al. 2021). These studies have also contributed to the development of theoretical models for predicting evaporation rates by considering droplets under constant contact radius (CCR) and constant contact angle (CCA) modes, incorporating mechanisms such as pure diffusion, free convection, and passive vapor transport. Furthermore, several studies have examined the distribution of the vapor concentration, the evaporative flux above the droplet surface, and the influence of Marangoni flows on the evaporation dynamics (Picknett & Bexon 1977;Erbil 2012;Sáenz et al. 2017;Masoudi & Kuhlmann 2017). In addition to sessile droplets, a few studies have also investigated pendant droplet configurations (Picknett & Bexon 1977;Erbil 2012;Pandey et al. 2020). Due to their restricted dynamical behaviour, sessile and pendant droplets are relatively easier to investigate experimentally than falling droplets due to the simplicity of the experimental setup, better control over boundary conditions, and their suitability for detailed observation using imaging techniques.
In addition to sessile and pendant droplet configurations, several studies have investigated the evaporation of acoustically levitated droplets (Yarin et al. 1998(Yarin et al. , 1999(Yarin et al. , 2002b;;Brenn et al. 2007;Saha et al. 2010;Sasaki et al. 2019;Maruyama & Hasegawa 2020), albite in quiescent environments. Unlike sessile or pendant droplets, which are constrained by solid or supporting surfaces, acoustically levitated droplets are freely suspended in air at the pressure nodes of high-intensity sound waves. This contactless configuration eliminates surface-related effects such as heat conduction, contact-line dynamics, and surface contamination, thereby enabling a more intrinsic investigation of evaporation behaviour. Over the past few decades, numerous researchers have advanced the understanding of acoustically levitated droplet evaporation, progressing from fundamental studies of droplet deformation to detailed analyses of multicomponent and multiphase evaporation. Early theoretical and experimental works by Yarin et al. (1998Yarin et al. ( , 1999Yarin et al. ( , 2002a,b) ,b) established the influence of acoustic radiation pressure on droplet shape, heat transfer, and mass transfer, revealing that the acoustic field strongly governs evaporation dynamics. Subsequent investigations by Brenn et al. (2007) and Saha et al. (2010) incorporated convective effects and laser heating to examine the thermophysical evolution of acoustically levitated droplets. Later studies by Bjelobrk et al. (2012) and Sasaki et al. (2019) provided further insights into internal circulation and interfacial flow and their roles in evaporation dynamics. More recent research by Maruyama & Hasegawa (2020); Mitsuno et al. (2025); Wakata et al. (2024); Krishan et al. (2024) explored multicomponent evaporation and phase transitions through both experimental and numerical approaches. Collectively, these works have significantly deepened the understanding of evaporation under acoustic confinement. However,
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