Experimental Study On Characteristics Of Droplet Evaporation And Bounce On Superhydrophobic Surface Induced By Photothermal Effect

Droplet-based open microfluidics refers to a microfluidic technology which manipulates small volume discrete droplets on open microfluidic chips to achieve specific functions.Compared to the channel-based microfluidic systems,the transport,separation,mixing and section processes are realized in an open environment for the droplet based open microfluidics,which makes this technology more convenient for on-line analysis,and has good compatibility with most sensing,detection and control technologies.At the same time,it also has the advantages of good reconfiguration and avoiding cross contamination.In recent years,the combination of optics with the droplet microfluidic technology has created a new droplet manipulation technology of the optically manipulating droplets.In this technology,light is used as an excitation signal,which utilize the interactions between light and fluid to manipulate the droplet.It offers many advantages,such as diversification,high accuracy,targeted specifically and untouched control.As a result,the problems faced by previous control methods,such as the complex control system,the high requirements for operating environment and easy damage to system function,can be avoided.The optical manipulation method based on the photothermal effect is an important branch of the optical manipulation method,which has been widely applied to the droplet manipulation.Although promising,there are still a lot of scientific problems to be addressed.Especially in the droplet-based open microfluidics technology,the superhydrophobic substrate is often used to reduce the flow resistance,and the liquid-solid adhesion force in the control process.Moreover,the droplet evaporation is usually accompanied.However,the photothermal effect induced droplet evaporation on the superhydrophobic surface and the effect of the surface microstructure remain unclear.Hence,this thesis is devoted to the study on the characteristics of the photothermal effect induced droplet evaporation and motion on the superhydrophobic surface.In this thesis,the superhydrophobic surfaces were prepared by a variety of methods,and the hydrophobicity,reliability,applicability and controllability of the surfaces were tested and compared,by which the suitable surface was chosen as the substrate for the follow-up experiments.After that,the droplet evaporation on the superhydrophobic surface induced by photothermal effect was carried out on the selected substrate.The dynamic characteristics of the droplet evaporation was obtained by visual experiment.The effect of the microstructure was also investigated.Finally,based on the above experimental results,a superhydrophobic surface was designed,on which the bounce of the droplet caused by the photothermal effect was realized.And its mechanism was also analyzed.The main outcomes of this thesis are summarized as follows.(1)Various superhydrophobic surfaces were prepared by several methods,mainly including the superhydrophobic commercial coating method,PDMS combustion and deposition method,laser etching method,metal corrosion method and the method of constructing micro-pillars array structure.The hydrophobicity,reliability,applicability and controllability of each surface were analyzed and compared by measuring the contact angle and rolling angle,the surface microstructure and the mechanical strength.It was found that the hydrophobic surface fabricated by the method of constructing micro-pillars array structure on the PTFE surface showed the best comprehensive performance.As a result,this superhydrophobic surface was chosen as the experimental substrate in this thesis.(2)The photothermal effect induced droplet evaporation on the superhydrophobic PTFE surface with micro-pillars array was studied.Comparing with the smooth PTFE surface,the droplet on the superhydrophobic PTFE surface with micro-pillars array showed a more intense contact diameter expansion trend in the evaporation process.Caused by the “Pinning effect” of micro-pillars,the droplet contact diameter did not shrink in the process of evaporation.Influence of micro-pillars' geometrical parameters on the evaporation process and the interface behavior were also studied.It was found that reducing the length of the micro-pillars,increasing the height or distance of micro-pillars could lead to the increase of the air groove area ratio under the same heating condition so that the surface's wettability change by the condensation was weakened.The droplet's contact diameter expansion distance in the evaporation process decreased,and the droplet height,contact angle and surface temperature increased.(3)Based on the above experiments,a superhydrophobic PTFE substrate with the micro-pillars array was designed and fabricated,which was suitable for droplet-bounce.The hydrophobicity of the substrate was verified and improved.A circular cavity structure was constructed on the improved superhydrophobic substrate.It was found that when the droplet was located on the top of the cavity,the pressure in the cavity was accumulated due to the droplet evaporation caused by photothermal effect,which enabled the bounce of the droplet from the surface.By changing the size of the cavity,the effect of the diameter and depth of the cavity on the time required for the bouncing,the bouncing volume of the droplet and the bouncing velocity was achieved.Finally,the effect of the laser power on the bouncing behavior was studied.