Molecular Dynamics Of Hydrophilic And Hydrophobic Nanostructures On The Wetting Behavior And Condensation Heat Transfer Mechanism Of Droplets

Dropwise condensation is widely applied in industrial production and daily life.Due to the restriction of test techniques and theoretical patterns,when the characteristic scale of the droplet is at the nano level,some factors that can be ignored at the macro scale become impossible to ignore,such as Van der Waals forces between particles,wall slip effect of liquid flow,temperature jump in heat transfer process and so on.Therefore,at the nanoscale,the scale impact and interface impact of physical phenomena have increasingly got vital factors to discover the effect mechanism of macroscopic scale phenomena such as droplet dynamics and condensation heat transfer.In this work,molecular dynamics simulation is used to discuss the wetting behaviors and condensation process of droplet on the surface of nano-structures.The wetting behaviors on rough surface and the variation of the apparent contact angle are studied with different surface wettability and base angle of nano-structures;the influence mechanism of surface wettability,nano-structure pitch distance and surface subcooling on surface condensation mode is revealed;the impacts of interface and scale on steam nucleation,growth,coalescence,dynamic wetting and dewetting processes are discussed.Previous studies mainly focuse on droplet behaviors on nano-pillar structures.In this work,we discuss the wetting behaviors of droplet of argon atoms on a platinum surface,and the effects of geometry and wettability of nano-structures on the wetting modes and the mechanism of mode transition are studied firstly.Three types of nano-structures are studied with different longitudinal-section geometries,including base angles of 60°(inverted trapezoid),90°(rectangular),and 120°(regular trapezoid).The wettability is represented by the apparent contact angle ?e.It is shown that when ?e<118°,the wetting mode of the droplet on nano-structures is the Wenzel state,in which liquid wets the structure completely.When 118°<?e<145°,inverted trapezoidal nano-structure surface helps to keep the droplet in the Cassis state,in which liquid does not penetrate into the nano-structure,while regular trapezoidal nano-structure helps to keep the droplet in the partial Wenzel state,in which liquid penetrates and wets the nano-structure partially.It is shown that the wetting modes and their transition affected by nanostructure geometry satisfy the principle of the lowest surface free energy.Using molecular dynamics simulation method and using argon as the working fluid,the dynamic wetting characteristics of droplets on the surface of vertical(?=90°)and inverted trapezoidal(?=60°)nano-structures were compared and studied.It is found that the inverted trapezoidal nano-structure helps to drive the liquid molecules in the gap of the nano-structure to move to the surface,so that the nano-structure tends to remain dry,which is beneficial to realize the transition of Wenzel-Cassie state.The condensation heat transfer on the surface of the inverted trapezoidal(?=60°)nano-structures is further studied.The influences of different surface wettability,nano-structure pitch distance and surface subcooling on droplet dynamics and dynamic wettability during condensation are investigated.The results show that the droplet occurs dewetting process from Wenzel state to Cassie state by properly designing the geometrical size and surface wettability of the nanostructure.And the dewetting process is affected by the surface subcooling.At low and moderate surface subcooling,there is only one nucleation and a single time liquid dewetting from nano-structures in the condensation process.The de wetting mechanism is that a single droplet pulls out the liquid atoms in the nano-structure gap under the action of Laplace pressure difference.However,at high surface subcooling,the condensation process presents multiple nucleation and multiple dewetting processes.The dewetting mechanism can be divided into two categories:when only a single condensate droplet exists in the system,the mechanism is consistent with the low and moderate surface subcooling;when there are two or more condensate droplets in the system,the Laplace pressure difference as well as the surface energy released by droplet coalesce contributed to the dewetting process.