Wetting Behaviors Of A Nano-droplet On A Rough Solid Substrate Under Electric Field

Wetting phenomenon can be seen everywhere in daily life.It has a great influence on the life activities of plants,animals and the human production activities.Electro-wetting is the external electric field controlling the wettability of droplets on the substrate surface.Wetting of droplets in an external applied electric field is an important phenomenon in many nano-optoelectronic and micro-electronic technologies.However,the influence of the polarity,roughness and the shape of the solid substrate on the wetting behavior of a droplet is still unknown.In this paper,Molecular dynamic simulation method was employed to investigate the electro-wetting behaviors of nano-droplet on rough solid substrates.Contact angle,obtained from fitting the density counter by ellipsoidal equation,was characterized the wetting properties.The following results were obtained by analyzing the static wetting process,dynamic wetting process and the water molecular dipole moment.Firstly,the effects of wall polarity and alternating electric field on the wetting behaviors of nano-droplet were studied,and a molecular dynamics simulation models of nano-droplet on polar silica solid substrate have been established.The results show that the molecular dipole orientations of water molecules experience a remarkable change from a random disordered distribution to an ordered profile because of the realignment of water molecules induced by electric fields,the polar solid surface has significant effects on the rearrangement of water molecules compared with a single droplet.The droplet spreads asymmetrically under the effect of the external applied field,and this asymmetry culminates to the maximum when the electric field equals to 0.45 V/nm.For an electric field of 1.0 V/nm,the difference between leading and trailing contact angles tends to increase first and then decrease with the increasing of the time.Ultimately,it tends to be about 15 degrees.The wetting asymmetry decreases by increasing the field frequency of the alternative electric field,When the field frequency increases up to 1000GHz,the average cos???values show a fluctuation around zero and do not change apparently with the field variation anymore.Then,a molecular dynamics simulation models of nano-droplet on difference pillar-shaped solid substrates have been established.It is shown that water droplets on rough pillar-shaped surfaces typically exhibit either a Wenzel state or a Cassie-Baxter state.Three parameters that may influence the transition are studied:the pillar height,the pillar width and the hollow size and the hollow size has the greatest influence.It was found that when the pillar height is less than the cut-off radius?1.5 nm?,the energy parameter?_si-o=0.19 kcal/mol can make the solid substrate from hydrophobicity to weak hydrophilicity.With the increase of the energy parameter,the hydrophilicity of the solid substrate is enhanced and the droplet is easier to expand on the solid substrate.In addition,a molecular dynamics simulation model of wetting was built to explore the wetting properties of nanoscale droplets on solid substrate with the different roughness factor and the different rough shape in the presence of electric field.The results show that when the electric field is 0 V/nm,the static contact angle tends to decrease slightly with the increasing of roughness factor.Under the perpendicular electric field,nano-droplet was elongated in the field direction.Keeping the roughness factor r?2.2 and changing the shape of the solid substrate,it was found that water droplets on rough solid substrate typically exhibit either a Wenzel state or a Cassie-Baxter state.When the electric field E_x=0.5 V/nm,nano-droplet was stretched to be strip-shaped on pillar solid substrate,It is indicated that the shape of the solid substrate plays an important role in the wetting behaviors of a droplet on a rough solid surface.