Effects of the Thickness of Boundary Layer on Droplet’s Evaporation Rate

Abstract

The evaporation rate of a droplet was explained in relation to the thickness of the boundary layer and the condition near the droplet’s surface. However, the number of results obtained from experiments is very limited. This study aims to investigate the thickness of the boundary layer of an ethanol–water mixture droplet and its effect on the evaporation rate by Z-type Schlieren visualization. Single and double droplets are tested and compared to identify the effect of the second droplet on the average and instantaneous evaporation rate. The double droplet’s lifetime is found to be longer than the single droplet’s lifetime. The formation of a larger vapor region on the top of the droplet indicates a higher instantaneous evaporation rate. The thickness of the boundary layer is found to increase with increase in ethanol concentration. Furthermore, a larger vapor distribution area is found in the case of higher ethanol concentration, which explains the faster evaporation rate at higher ethanol concentration.

Refractive index and mole fraction field of the vapor evaporated from ethanol-water mixture droplet

“…2. Except the humidity-controlled section and droplet releasing system that were newly added, other details of the experimental set up were similar to the previous report (Jonglearttrakull et al, 2020). An LED light source (Luminar Ace LA-HDF108A 20W) was used to generate white light (wavelength of 425 nm) which passed through the first convex lens with the focal length of 100 mm and diameter of 200 mm.…”

“…In our previous study (Jonglearttrakull et al, 2020), the thickness of the boundary layer of ethanol-water droplet was successfully acquired with the use of Z-type Schlieren imaging. However, the effect of ambient humidity was not considered.…”

“…where ∆𝑎 is light shifting distance at the knife-edge plain, and 𝑓 ) is the focal length of the second concave mirror. Equation ( 2) was used together with relation between light intensity and light shifted at the knife-edge as described in our previous report (Jonglearttrakull et al, 2020). Finally, refractive angle can be expressed as follow…”

“…nspecimen, vapor is the specimen refractive index in gas phase which is ethanol in our study (nethanol = 1.00088). Referring to the similar behavior between pure ethanol and high concentration of ethanol-water mixture (Jonglearttrakull et al, 2020), ethanol refractive index was used as nspecimen, vapor for ethanol-water mixture droplet of 75 vol% and 50 vol%.…”

“…Picture was started capturing 3 seconds before the droplet deposited onto the substrate in order to obtain the beginning image and the capturing continued until the droplet completely evaporated. Raw image results were processed and calculated using MATLAB with the same method explained in the previous report (Jonglearttrakull et al, 2020). Finally, refractive angle field (𝜀) was acquired.…”