| Surface wettability has been an important factor that influences a wide range of areas such as surface science,fluid flow and heat transfer.Wetting of droplets on solid substrates has a wide range of applications in surface coating,oil recovery,and other fields that are common and critical in both nature and industry.It is essential to study the effect of solid-liquid interactions on the wetting of liquids on solid surfaces.However,although droplet wetting on solid substrates has been extensively studied,a fundamental understanding on how solid-liquid interactions control substrate hydrophobicity and wetting phenomena is still lacking,pollution of water resources has a negative impact on environmental safety,the problem of solid surface wettability in the field of oil-water separation also needs to be solved.Molecular dynamics simulation used Lennard-Jones(L–J)potential energy function and CVFF force field model(MD),The wetting behavior of nano oil droplets,water droplets and oil–water mixed droplets on smooth and rough polytetrafluoroethylene(PTFE)surfaces were investigated separately.The simulations were performed on the Large-scale atomic/Molecular Massively Parallel Simulator(LAMMPS)software.The results showed that the nano oil droplets were observed to spread on the smooth surfaces by changing the transection length and molecular number,with the contact angle of about 20?,the nano water droplets,on the other hand,were only slightly wetting on the surfaces with the contact angle greater than 90?.This indicated a large difference in the affinity of the PTFE surface for n-octane and water nanodroplets.The phase area fraction f and the roughness factor r were adjusted on rough surfaces mainly by changing the groove width and depth.The effects of substrate surface morphology and droplet chemistry on the wettability of nanodroplets were also investigated,and the wettability of two types of pure droplets on fence type,crater type,and convex column type rough PTFE were discussed in detail.The results indicated that on fence type rough surfaces,both n-octane and water nanodroplets were in the Wenzel state on the surfaces when r was less than 2.33,as r increased to 3.67,the nano oil droplets remained in the Wenzel state,while the nano water droplets then showed the Cassie state.On crater type rough surfaces,n-octane was in the Wenzel state when r=3.61,and water droplets were in the Cassie state at this time,when f=0.84,n-octane was still in the Wenzel state and the water droplets were in the Transition state,but only a relatively small number of water molecules were on the upper edge of the groove.On convex columnar type rough surfaces,when f=0.36 and r=3.61 were the two conditions,n-octane could completely saturate the grooves and the water droplets showed the Cassie state.This indicated that the specially wetted PTFE surface constructed in this paper had the potential to achieve oil–water separation.The simulation of pure component droplets on the rough surface provided the direction for the simulation of oil–water mixed droplet separation.For mixed oil–water droplets,with a separation efficiency of 75% on the smooth surfaces,this lipophilic hydrophobic surface showed the possibility of oil–water separation.The separation efficiency of 77% was achieved on the fence type rough surface with a large groove depth of f=0.41,which achieved the initial separation of n-octane-water mixture nanodroplets.The separation efficiency was98.7% when the surface morphology was changed to the crater type rough surface with the small groove width f=0.84.The best separation of oil and water droplets was obtained at the convex column type PTFE surface f=0.16 which could be completely separated,separation efficiency up to 99.8%.The surface was super lipophilic and super hydrophobic at this point.Therefore,the rough PTFE surface with special wettability enabled the n-octane-water mixture to be separated. |
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