Research On Dynamic Characteristics Of Droplets Impacting Different Solid Surfaces And Thermocapillary Migration

The droplet impacting on the solid surfaces and the process of thermocapillary migration are ubiquitous in nature and industrial production.Its strong heat and mass transfer capacity makes it widely used in materials,chemicals,machinery and energy.Grasping the influence rule of technology parameters on the impact process,analyzing the factors affecting the oscillation and splash of droplets on different surfaces,systematically and deeply study the thermocapillary migration process of heterogeneous droplets,which have a very important meaning for designing the relevant industrial equipment and solving the problem in practical work.In view of different influential factors of dynamic behavior of droplet impacting on solid surfaces,the high-speed photography technology was used to capture oscillation processes and splash forms after seven kinds of physical properties droplets impacting on different roughness and temperature walls with different velocities and sizes.The phenomenon of thermocapillary migration of the heterogeneous droplets of paraffin oil and deionized water in different proportions placed on the metal substrate surface driven by the nonuniform temperature field was investigated,a theoretical model was set up.According to the“inclined plate method”,the characteristics of contact angle hysteresis of the droplets of different working fluid were tested,and the relationship between contact angle hysteresis and surface roughness was studied in multi-groups experiments.The specific research contents and main conclusions are as follows:Firstly,physical model was established,and the theoretical value correlation of maximum spreading factor was derived to compare the theoretical analysis with experimental results.The effects of viscosity,surface tension,impact velocity,diameter of droplet,roughness and temperature of the substrates on movement characteristics during the droplet impacting on the wall surface were investigated.As the research indicates,the rebound and oscillatory phenomena of the fluid become more obvious with an increase in surface tension,and viscous force restricts the spreading of droplets.The higher the impact velocity,the greater the spreading factor at the same time,and the more pronounced the splashing phenomenon will be.The growth rate of maximum spreading factor(?_max)increases at first and then decreases with increasing the initial diameter?d₀?of droplets.The smaller the d₀,the more consistent the experimental results with the analytical solutions.The equilibrium contact angle??_e?of the droplet increases with surface roughness?Ra?,whereas the surface wettability degrades.Besides,the influence of Ra on?_e decreases with the increase of Ra.?_e decreases with the rising of wall temperature;moreover,the higher the temperature is,the weaker the effect of Ra on?_e will be.The increase of Ra promotes the“finger-like edge”and the splash motion of droplets,and the critical velocity of splash declines with Ra.The optimum temperature?T_c?of droplet impacting the high temperature wall reduces with the decrease of Re?or the increase of viscosity?.Furthermore,the greater the difference between wall temperature and T_c is,the more significantly the?_max changes.Droplet spreading is hindered on the low temperature wall,and the lower the Re?or the higher the viscosity?is,the smaller the decrease amplitude of?_maxax with the wall temperature drops.Secondly,the morphological changes of the droplets driven by temperature gradient from the high temperature region to the low temperature region were recorded and analyzed by high-speed camera.The variation law of the viscosity with moisture content and temperature was researched.The results showed that the contact angle of heterogeneous droplets decreased with the rising of temperature and the wettability was enhanced.The migration velocity of heterogeneous droplets decreased with the increasing of viscosity and increased with the increasing of temperature gradient,and the difference of velocities under different viscosities increased with the increasing of temperature gradient.The variation trend of migration velocities with time decreased rapidly,and then slowly decreased to 0.The relationship between the migration velocity and the Marangoni number?Ma?was derived and verified,which indicated that the migration velocity increased markedly with the increasing of Ma.The greater the Ma was,the higher the matching degree between the experimental result and the theoretical analysis result were.Finally,as Ra increases,the advancing contact angle?_A increases and the receding contact angle?_R decreases,and the difference??between the two increases,that is,the contact angle hysteresis of different droplets increases.The contact angle hysteresis is related to the physical properties of the droplets.The larger the surface tension???of the droplets,the stronger the contact angle hysteresis.The critical inclined angle???of the substrates with contact angle hysteresis of the droplets of different working fluid decreases obviously with the increase of Ra,and the?decreases with the increase of?.The front and rear three-phase contact lines of the droplets on the hydrophilic and hydrophobic surfaces do not start moving at the same time,and the start of the movement are the front end and the rear end,respectively.