Contact Line Dynamics Of Drop On Heated Substrate

The material,temperature,wettability of the substrate and the fluid characteristic can influence the dynamic characteristics of the drop by affecting the contact angle and the three-phase contact line.The research of the possible physical mechanism of the above factors on the drops movement is advantageous for regulating drop movement to improve the process and product quality.The present study adopts the method of numerical simulation to investigate the contact line dynamics of drop on heated substrate.Firstly,The dynamics of the evaporation of a sessile drop on a uniformly heated,horizontal,solid substrate are considered.Based on the lubrication theory and the Navier slip condition,an evolution equation for the drop height of the two-dimensional drop is established.The effects of evaporation and the dependence of liquid-solid,solid-gas,and liquid-gas surface tensions on temperature are analyzed.Secondly,the thermocapillary migration characteristics of a self-wetting drop without evaporating on the non-uniformly heated,horizontal substrate are investigated.The self-rewetting fluid has non-monotonic dependence of the surface tension on temperature with a well-defined minimum,and the position of the minimum corresponding to the temperature on the substrate is called the critical point.The effect of the relationship between the critical point and the drop position on drop dynamics is analyzed.Finally,the migration characteristics of the self-wetting drop on a uniformly heated,inclined substrate are investigated when the liquid-gas interfacial tension is positive,negative and in a quadratic functional relationship with temperature under the same wetting scenario.Then,the effects of the inclination angle,capillary number,Bond number and thermocapillary force on the drop migration are examined when the interfacial tension has a quadratic functional relation with temperature.Numerical results show that:(1)gravity exerts a promoting effect on drop spreading,while capillary force and thermocapillary force inhibit drop spreading.Under different temperature-sensitive coefficients of three interfaces,different drop evolution features are presented.It is more effective to manipulate the drop movement in the presence of evaporation by regulating the temperature-sensitive coefficient of the solid-gas interface.(2)Under the same wettability condition of the substrate,the drop always migrates toward the high interfacial tension region due to the thermocapillary force.In the presence of substrate wettability variations,the drop migrates toward the low temperature region no matter where the drop is placed relative to the critical point.This is due to the fact that the deterioration of substrate wettability on the right side of the drop prevents the drop from migrating toward the hot region.It is effective to manipulate theself-wetting drop movement by regulating the relationship between the critical point and the initial drop location.To inhibit the migration of the drop toward the cold region,the drop should be placed on the right side of the critical point.(3)Numerical results indicate that the direction of interfacial tension has a significant influence on drop spreading.When the conventional pure fluid drop and self-rewetting fluid drop have the same wettability,the self-rewetting drop spreads more rapidly.The enhancement of the inclination angle results in faster drop sliding velocity.The increase in Ca results in a delay of the contact line pinning state and prolonged pinning time;however,the increase in Bo leads to an advanced pinning state of the contact line and reduced pinning time.The drop profile tends to be flatter during the drop movement,owing to the strengthened effect of the thermocapillary force.