Evaporation Characteristics, Heat And Mass Transfer Mechanism Of Small Sessile Droplets On Substrates

The evaporation of small sessile droplets on substrates not only has wide and important applications in many traditional engineering fields,such as cooling of power devices,thermal control of nuclear reactors,heat exchange of chemical equipment,but also shows broad application prospects in the frontiers of science and technology including manufacture and cooling of microelectronic devices,fabrication of novel optical and electronic materials,micro total analysis systems??TAS?.Thus,it has attracted more and more attention from domestic and international researchers.A thorough understanding of the evaporation characteristics,heat and mass transfer mechanism of small sessile droplets on substrates is crucial for the development and improvement of these applications.With the expansion of application area,many fundamental problems need urgently to be studied and clarified.For this purpose,the evaporation characteristics,heat and mass transfer mechanism of small sessile droplets(10^-6 m<R<l_c,where R is the contact radius,l_c is the liquid capillary length)were thoroughly and systematically studied in this thesis.The evaporation rate of sessile droplets was first studied,then the evolution behaviors of evaporating sessile droplets in three typical evaporation modes were analyzed in detail.On this basis,the heat and mass transport model and the evolution behavior of small evaporating droplets on heated substrates were investigated.Moreover,the influence of Marangoni effect on the evaporation characteristics of sessile droplets was discussed.Since the surface temperature distribution of droplets plays a key role in Marangoni effect,it was manipulated through a microheating at the droplet bottom,and the characteristic of surface temperature distribution and the control mechanism of internal microflow of small sessile droplets under local heating were studied.The main contents and results are summarized as follows:?1?Evaporation rate of small sessile droplets on substrates.Based on a pure diffusion model,the vapor transport process surrounding evaporating droplets on substrates was studied,and the influence of droplet shape on the evaporation rate was analyzed.It was found that for droplets with the same surface area,the vapor concentration distributions surrounding the non-hemispheric droplets tend to be uniform in each direction and consistent with that of the hemispheric droplet as the distance from droplets increases.As compared to the contact angle and contact radius,the surface area of a droplet is the more essential geometry factor to determine the droplet evaporation rate.A new closed-form expression for predicting the evaporation rate of sessile droplets was proposed,which is simple and applicable for a wide range of contact angles?0<?<180o?.With this expression,the evaporation rate of sessile droplets was found to be proportional to the square root of droplet surface area.Finally,the obtained expression and conclusions were validated by the visualization experiment.?2?Evolution law of small sessile evaporating droplets.The evolution behaviors of droplet contact angle,contact radius and volume were theoretically studied for three typical evaporation modes:constant contact radius?CCR?,constant contact angle?CCA?and“stick-slip”?SS?modes.Simple closed-form expressions for the evolutions of contact angle and contact radius of droplets evaporating in three modes were proposed.With the proposed expressions,it was found that on hydrophilic surface??₀?90o?,the volume evolution of droplets in SS mode always occurs in a way between the CCR and CCA modes;however,on hydrophobic surface??₀>90o?,as the receding contact angle?_recec increases,the volume evolution of droplets in SS mode firstly departs from CCR mode,then approaches SS mode with?_rec=90o,and finally returns to CCA mode.The initially identical droplets on hydrophobic surfaces??₀>90o?with different?_rec can have approximately the same volume evolution behavior.Moreover,it was revealed that the evolution law of droplet volume with time is essentially dependent on the relationships between droplet surface area and evaporation rate,contact angle.?3?Heat and mass transport model and evolution behavior of small sessile evaporating droplets heated globally by substrates.A coupled heat and mass transfer model for simulating the evaporation process of droplets on heated substrates was developed.The transport mechanisms including vapor transport in air,heat transfer of substrate,droplet and air,buoyancy-induced convection in both liquid and air,evaporative cooling effect and Marangoni effect at liquid-air interface were comprehensively coupled in the proposed model.With this model,the evolution behavior of evaporating droplets on heated substrates was studied.It was found that the evolutions of contact angle and volume of pinned droplets follow the same universal laws,respectively,which are irrelevant with droplet base radius,substrate temperature and ambient humidity.From this basic point,the expressions for the evolutions of contact angle and volume of droplets on heated substrates were proposed,and further validated by the visualization experiment.?4?Marangoni effect and its influence on the evaporation characteristics of small sessile droplets.The influences of Marangoni effect on flow field,temperature field and evaporation rate of sessile droplets were numerically studied.It is shown that there exists a critical value of contact angle,above which the Marangoni effect enhances the heat transfer of the whole droplet,while under which the Marangoni effect deteriorates the heat transfer of droplet apex region due to the stagnation flow zone induced by Marangoni effect.With the Marangoni effect,the evaporation flux along the droplet surface becomes more uniform except the droplet apex area,which helps to better understand the suppression effect of Marangoni flow on the coffee-ring deposition pattern.The contribution of Marangoni effect to the evaporation rate increases as the contact angle and substrate temperature increase.When the contact angle is smaller than 60oand the substrate is at the room temperature,the influence of Marangoni effect on the droplet evaporation rate is negligible,but when the contact angle is 90oand the substrate temperature reaches 55oC,the Marangoni effect can increase the evaporation rate by30.3%.?5?Surface temperature distribution and control mechanism of internal microflow of small sessile droplets under local heating.The local heating at the bottom of droplets with contact radius of 1.6 mm was implemented successfully using a microheater.With the aid of infrared thermography technology,the influence of heating power on the surface temperature distribution of droplets was studied.Furthermore,the characteristics and the control mechanism of internal microflow of sessile droplets under local heating were numerically investigated.The experimental results show that local heating can easily alter the direction of surface temperature gradient of droplets,and the temperature difference on the surface of droplets under local heating can be much higher than those of droplets evaporating in nature condition or under global heating,which indicates that local heating is an efficient method to manipulate the surface temperature distribution,as well as the internal microflow of droplets.There exists an optimum range of local heating power for manipulating the surface temperature distribution of droplets,above which the variation of surface temperature gradient of droplets is relatively small.The simulation results show that,as compared to global heating method,the local heating can significantly enhance the internal convection of droplets.Moreover,the surface temperature distribution of droplets under local heating,which is warmer at apex and colder at edge,can induce circular convection inside droplets that is in the opposite direction of those under global heating,and it also can suppress the compensation flow inside droplets.This study provides important theoretical and technical support for manipulating microflow and enhancing the heat and mass transfer inside droplets.