Investigation on the droplet evaporation process on local heated substrates with different wettability
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ORIGINAL
Investigation on the droplet evaporation process on local heated substrates with different wettability Li Wang 1 & Zeyu Liu 1 & Xin Wang 1 & Yuying Yan 1,2 Received: 4 June 2020 / Accepted: 22 November 2020 # The Author(s) 2020
Abstract Marangoni effect is one of the critical factors in the droplet evaporation process, which is caused by surface tension gradient in the droplet interface. In this study, local heating is adopted to provide a more complicated temperature distribution on the droplet surface, and a detailed numerical investigation is carried out to address the effect of Marangoni flow on the droplet evaporation behaviour. Results show that asymmetric heat source position could result in the droplet morphology being asymmetric, especially for droplets on super-hydrophilic surfaces. The evaporation rate could be affected both by the heat source position and the droplet contact angle. When placed on a smooth substrate, the droplet will slip horizontally as a result of the asymmetric heating condition. The slipping behaviour is affected by both the heat source position and the surface wettability. Nomenclature Ma Marangoni Number T Temperature K l Length m fσ Distribution function for stream of the σthcomponent f eq Equilibrium distribution for stream of the σ th σ component gσ Distribution function for temperature of the σ th component geq Equilibrium distribution function for temperature of the σ σth component ei Lattice particle’s Microscopic speed m ⋅ s−1 c Reference lattice velocity u Velocity m ⋅ s−1 eq u Distribution velocity m ⋅ s−1 p Pressure Pa cp Thermal specific heat at constant pressure J ⋅ (kg ⋅ K)−1 cv Thermal specific heat at constant volume J ⋅ (kg ⋅ K)−1 h Droplet height m d Droplet contacting area length with substrate m g Gravity acceleration m ⋅ s−2 F Interactive force N
* Yuying Yan [email protected] 1
Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UK
2
Research Centre for Fluids and Thermal Engineering, University of Nottingham, Ningbo, China
G Lc
Coefficient for interaction forces Heat source location indicator
Greek symbols σSurface tension N/m ηDynamic viscosity kg ⋅ (m ⋅ s)−1 αThermal diffusivity m2/s τRelaxation time for stream field τTRelaxation time for temperature field ωiWeight coefficients in i direction φPhase change term ρDensity kg ⋅ m−3 λHeat conductivity W ⋅ (m ⋅ K)−1 υKinetic viscosity m2 ⋅ s−1. ψPseudo-potential of components. Super- and sub- scripts iLattice direction σLiquid/vapor component sSolid phase
1 Introduction Droplet evaporation has attracted increasing attention in recent decades, with its wide application in the industrial field. Commonly seen applications of droplet evaporation are Inkjet printing [1, 2], spray cooling [3], material processing [4], coffee-ring effect [5–8] and particle synthesis [9, 10]. In recent years, more medical applications have been developed for droplet evaporation, like DNA/RNA arrangement [11, 12] and medical diagnosis [13, 14]. Among the factors affecting the droplet evaporat
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