Experimental Study Of Nanofluid Droplets Evaporation On Solid Heated Surface With The Electric Fields

Evaporation of complex fluid droplets has important application prospects for frontier technology field,such as biomedical testing,microelectronic chips and nanometer new materials preparation.Recognise and understand the kinetic behavior in the process of droplet evaporation is the key to achieving the above applications.Scholars mainly focus on the effects of surface wettability and surface temperature on ordinary fluid droplets evaporation under natural conditions.With the innovative applications of electrohydrodynamics in the fields of microfluidics technology,electrostatic spraying and additive manufacturing,the research on the evaporation behavior of complex fluid droplets in the electric field needs to be carried out.This paper proposes investigating the evaporation behavior of the nanofluid sessile droplets on the heated surface with the electric fields.Building a visualization experiment platform based on high-speed digital and delayed photography technology to capture the changes of droplet contact angle,contact diameter and transient phase interface during evaporation under the action of electric field.Based on diffusion theory and experimental assumptions,a prediction model for the dimensionless contact diameter from evaporation of droplets on superhydrophobic heated surface is proposed.And the influences of electrodes form and electric field strength for the evaporation patterns and evaporation time of pure water and graphene oxide nanofluid droplets on different wettable surfaces were investigated.Through a combination of experimental research and theoretical analysis,to explore the evaporation characteristics and laws of complex fluid droplets in the electric fields.To reveal the regulation mechanism of an electric field on the evaporation process of complex fluid droplets.Firstly,the research on the evaporation of pure water and graphene oxide nanofluid droplets on the superhydrophobic heated surface in the electric fields was experimental investigated.The calibration experiment of pure water droplet evaporation on the superhydrophobic heated surface is in good agreement with the theoretical prediction model.The study found that the droplets can stretch to ellipsoidal shaped along the direction of the potential gradient from the plate-plate electric field,which will regulate the size of droplet contact diameter and evaporation process.The reduction of the droplet contact diameter under the electric field will result in the evaporation time prolong.When the contraction ratio of droplet contact diameter decreases to a certain critical value,a new evaporation mode will appear: the limit evaporation mode in which both the contact angle and the contact diameter remain constant.As mass concentration increases from graphene oxide nanoparticles,the deformation of nanofluid in the electric field will be inhibited.And the evaporation modes of nanofluid droplets with different mass concentrations in the electric field will occur to significant differences.The strong electric field effect will induce the deposition morphology of nanoparticles tends to a spherical shape.In the needle-plate electric field,due to the effects of the ion wind and the induced pressure generated by the pin-plate electrode on the gas-liquid interface of the droplet,the droplet will undergo unsteady disturbing behavior.As the electric field strength increases,the amplitude of droplet disturbance and the period increases.The influence of the electric field on the motion of the surrounding external flow field accelerates the droplet convective heat exchanges so that promotes the evaporation of the droplet,but the electric field has a weak effect on the regulation of the droplet initial contact diameter size,evaporation process and nanoparticles deposition morphology.Again,the study for the evaporation and slip characteristics of graphene oxide nanofluid droplets on the hydrophobic heated surface under the absence of an electric field and an external electric field.It is shown that the contact line in a "pinned" state at the beginning of droplet evaporation on hydrophobic surfaces,and maintain constant contact radius mode evaporation.The addition of graphene oxide nanoparticles prolongs the evaporation time of droplets in this mode.However,the external electric field can regulate the "pinned" of contact line during droplet evaporation.In the plate-plate electric field,the droplet is stretched to a cone shape and the contact angle is reduced,which increases the tendency towards the droplet’s contact line to slip inward.The slip moment of the base liquid droplet contact line under the electric field occurs in advance,while the nanofluid droplets themselves strengthens the "pinning" resulting in the electric field is difficult to affect the movement of the contact line.When the contact angle of the droplet decreases of about 76o(retreat angle)as the electric field strength increase,the contact line slips and the droplet start to an unsteady jitter.The jitter frequency decreases as the mass concentration of graphene oxide nanoparticles increases.In contrast,the evaporation of the droplet in the constant contact radius mode under the action of the pin-plate electric field is prolonged and the total evaporation time is shortened.At the same time,the droplets will produce weak disturbance behavior in the electric field.The research based on the electric field regulating the evaporation behavior of complex fluid droplets will provide a research foundation and technical guidance for the development of new micro-control technologies,micro-nano material preparation,and biomedical testing.