Study On Evaporation Characteristics Of A Water Droplet Sessile On Heated Hydrophobic And Superhydrophobic Substrates

Evaporation of sessile droplets is an important fundamental phenomenon.Industrial applications based on droplet evaporation are widely used in high-tech fields such as microfluidics,microelectronics manufacturing and cooling,and biochemical analysis.A thorough understanding of the evaporation characteristics and fluid flow pattern is not only useful to optimize designs for practical droplet-evaporation-based applications,but also beneficial to further explore the potential value of droplet evaporation in the above fields.Based on the background,the present paper develops a three-dimensional numerical model describing droplet evaporation,in which various heat and mass transfer mechanisms are included.Numerical studies have been made to investigate the heat and mass transfer characteristics and droplet internal flow structure of the microliter droplet sessile on heated hydrophobic（contact angle:110°）and superhydrophobic（contact angle:160°）substrates.The main conclusions are as follows:（1）Evaporation characteristics of the droplet on heated hydrophobic substrates is axisymmetric,and an axisymmetric vortex circulation is observed inside the droplet;while the droplet on heated superhydrophobic substrates no longer has the axisymmetric evaporation characteristics,and the non-axisymmetric three-dimensional single-vortex rolling flow inside the droplet occupies the whole droplet.The flow velocity of asymmetric vortex is an order of magnitude higher than that of the axisymmetric flow.The dimensionless height of the vortex center in above two different flow modes is independent of the substrate temperature and droplet volume,and the dimensionless vertical distance is 0.420（axisymmetric flow）and 0.515（asymmetric vortex）,respectively.The internal velocity of droplets on both substrates is proportional to the square of the Ra number,but the convexity of the velocity v-Ra number curve is exactly the opposite.（2）The evaporative cooling along the droplet interface is obviously observed and the maximum temperature drop along the surface of the droplet is much greater for a superhydrophobic substrate.Affected by the internal flow on the temperature distribution,the coolest area appears on the droplet top for the hydrophobic surface while the coolest area appears on the downstream side of the flow direction instead of the droplet apex for the superhydrophobic surface.The evaporative cooling intensity for both substrates increases with an increase in substrate temperature.（3）Influence of the change in relative humidity on the evaporation is identical for both substrates.Both the instantaneous evaporation rate and the maximum temperature drop along the surface of the droplet decrease linearly with an increase in relative humidity.（4）In case of large contact angle,the axisymmetric flow inside the droplet is instable for a heated substrate in the present study.Any perturbation in the system can get enlarged and drive the instable axisymmetric flow to be a non-axisymmetric rolling flow.The flow bifurcation inside the droplet shows that,varying with the Bi number,the axisymmetric flow converts into asymmetric vortex if the Ra number exceeds the critical value Ra₁;while the asymmetric vortex can be degenerated into axisymmetric flow if the Ra number is lower than the critical value Ra₂.In the present study,Ra₁ is greater than Ra₂.