Heat and mass transfer characteristics of binary droplets in acoustic levitation

Abstract

The complex relationships between the flow field and heat transfer phenomena of acoustically levitated droplets under evaporation were investigated. To explain these correlations, binary droplets of ethanol and water were used as test fluids. Immediately after droplet levitation, the droplet external flow field direction was toward the droplet, with a circulating vortex forming near the droplet surface. As evaporation progressed, the external flow transitioned toward the opposite direction, while the circulation vortex expanded. To better understand the transition process of the droplet thermal boundary layer, the heat transfer coefficient time series changes were calculated by assuming that the transitions of the ethanol and water binary droplets occurred in three stages: (1) preferential evaporation of ethanol, (2) transition (evaporation of ethanol and condensation of water), and (3) evaporation and condensation of water. Finally, by comparing the flow field and thermal boundary transitions, the transition mechanism for flow structures and heat transport phenomena of acoustically levitated droplets with evaporation was considered. Our experimental and analytical results provide deeper physical insights into noncontact fluid manipulation and suggest potential future applications, such as in acoustic tweezers and microreactors.

Evaporation and drying kinetics of water-NaCl droplets via acoustic levitation

“…The shapes are considered to be attributed to the inuence of the internal circulation driven by nonlinear sound waves. 24,27 The applied sound pressures at 10 wt%, 15 wt%, 20 wt%, and 25 wt% were 1.8 kPa, 1.7 kPa, 1.4 kPa, and 1.5 kPa, respectively. The maximum difference was 0.4 kPa.…”

“…Furthermore, in recent years, non-contact manipulation by using an ultrasonic phased array has also been investigated, thereby expanding the application of the ALM for non-contact manipulation. 21,22 However, the use of the ALM leads to the introduction of nonlinear dynamics on a levitated droplet, such as in the form of acoustic streaming [23][24][25][26][27][28] and dynamic behavior, 29,30 owing to the levitation of the sample by a nonlinear acoustic eld. These phenomena may affect the evaporation and precipitation of the levitated samples.…”

Theoretical and numerical analysis of the evaporation of mono- and multicomponent single fuel droplets

“…Experimental studies under different conditions have been used to study mono-and multicomponent droplets including suspended droplets from fibers (Nomura et al 1996;Ghassemi, Baek & Khan 2006;Chauveau et al 2008;Hallett & Beauchamp-Kiss 2010;Erbil 2012;Han et al 2015), levitating droplets (Gregson et al 2019;Niimura & Hasegawa 2019;Sasaki et al 2020), free falling droplets (Lee & Law 1992;Sirignano 2010;Hillenbrand & Brüggemann 2020;Muelas et al 2020), sessile droplets on heated and not-heated surfaces (Cazabat & Guena 2010;Erbil 2012) and droplets evaporating in heated air flows with elevated temperatures and pressures (Sirignano 2010). Various experimental techniques, each with advantages and disadvantages, have been instrumental in understanding complex phenomena such as puffing (Avulapati et al 2016;Shinjo et al 2016), particle deposition (Shmuylovich, Shen & Stone 2002;Sefiane, Tadrist & Douglas 2003), Marangoni currents (Gurrala et al 2019;Gao et al 2020) or the importance of surface hydrophobicity and wettability for the evaporation of drops and droplets (He, Liao & Qiu 2017).…”

“…2019; Niimura & Hasegawa 2019; Sasaki et al. 2020), free falling droplets (Lee & Law 1992; Sirignano 2010; Hillenbrand & Brüggemann 2020; Muelas et al. 2020), sessile droplets on heated and not-heated surfaces (Cazabat & Guena 2010; Erbil 2012) and droplets evaporating in heated air flows with elevated temperatures and pressures (Sirignano 2010).…”

Diffusion enhancement in a levitated droplet via oscillatory deformation

“…For instance, it is advantageous for the measurement of physical or chemical quantities with avoiding the pollution and the significant disturbance from chamber walls [1]. Thus levitateddroplet systems are studied not only under microgravity conditions in a spacecraft or during a parabolic flight but also in various manners, i.e., electrostatic levitation [2,3], magnetic levitation [4][5][6], acoustic levitation [7][8][9][10][11], aerodynamic levitation [12], and optical levitation [13]. For each levitation method, we have to design the droplet system for the stabilization of the levitating state, and thus it is somewhat challenging to apply additional manipulations.…”