The Effect of Nano-pillared Carbon Surfaces on the Wetting Transition of Liquid Metal

Author:Wang Jun

Supervisor:li hui


Degree Year:2019





Changes of the apparent contact angle on the rough surfaces are called wetting transitions,which may be spontaneous or caused by the external conditions such as the pressure or the vibration.Understanding the physical mechanism of wetting transitions is crucial to the design of highly stable superhydrophobic and multifunctional hydrophobic materials.It is of great scientific significance to study the factors influencing wettability and droplet morphological evolution to enrich the wetting theory,develop the interface material processing technology and advanced nanotechnology.The stretchable electronics achieved remarkable progress in soft robotics,flexible devices and especially in the biological field in recent years.Room temperature liquid metals have drawn increasing attention in these state-of-the-art applications because the desirable materials involved are intrinsically soft and remain functionally stable when their morphology changes.Gallium and its alloys being in a liquid state at room temperature with high thermal and electrical conductivity,as well as low toxicity and evaporation pressure,makes it an ideal candidate in these fields.And it is regarded as a promising alternative to the toxic mercury.Graphene with controllable stiffness performances,high electrical conductivity and low synthesis cost also possesses desirable deformability which can significantly support the gallium-based liquid metal as an electrically conductive and anticorrosive coating.There are few study to report the wettability of liquid Gallium on nanopillared graphne,as well as the formation behavior of liquid droplets on these carbon nanomaterials,especially the effects of surface structures on the wettability.In the present work,molecular dynamic simulations have been performed to investigate the wetting transition of pancake-like Ga nanofilm on types of carbon nanopillar-patterned graphene surfaces,with the aim of controlling the wetting pattern of liquid Ga and exploring the general rule on its wettability on the graphene-based substrate.The main contents are listed as follows:(1)Based on the Lennard-Jones(L-J)potential describing the solid-liquid interaction,the wettability of liquid gallium film on the smooth and rough graphene surfaces has been effectively investigated by mean-field theory.It is found that different regimes of the wetting are discovered by changing the depth of the L-J potential and the stable contact angle increases with the decrease of the Ga-C potential depth.The result showed that the equilibrium contact angle and the retraction velocities increase with a decrease of the L-J potential between the gallium and graphene,showing some distinct transitions from complete wetting to hydrophilic and to hydrophobic.The L-J potential depth obtained from the simulation results can be effectively employed to describe the interaction between the liquid gallium and the substrates because the resulted wetting angle is extremely close to the experimental value.When employing the most appropriate L-J potential,it is found that the initial retraction velocities increase with the proportional decrease of the thickness of the liquid Ga films.It means that the film thickness is not the crux for the change of the equilibrium contact angles and retraction velocities based on the similar conversion of potential energy to kinetic energy when it is in the wetting state.(2)Molecular dynamics(MD)simulation has been employed to study the wetting transitions of liquid gallium droplet on the graphene surfaces decorated by three types of carbon nanopillars.The simulation results showed that,at the beginning,the Ga film looks like an upside-down dish on the rough surface different with that on the smooth graphene surface,and its size is crucial to the final state of liquid.Ga droplets exhibit Cassie-Baxter state,Wenzel state,Mixed Wetting state,and dewetting state on the patterned surfaces by changing the distribution and morphology of nanopillars.Top morphology of nanopillars has a direct impact on the wetting transition of liquid Ga.Three transition states of Ga droplets are observed on the substrates decorated by two types of CNT,but two transition states are found on the substrates decorated by carbon nanocone(CNC).Furthermore,it is found that the substrates show high or low adhesion to the Ga droplet with the variation of their roughness and top morphology.With the roughness decreasing,the adhesion energy of the substrate bocomes lower.With the same roughness,the CNC/Graphene surface owns the lowest adhesion energy, followed by CNT/Graphene and capped CNT/Graphene surfaces.Our findings provide not only valid support to the previous works but also reveal new phenomenon of the wetting on the rough substrates.(3)We investigated the wetting states of liquid Ga films on four kinds of nanocone-patterned graphene by MD simulations.The adsorption strength of liquid on patterned substrates,compared with the smooth surface,is decreased not only due to the diminished surface energy along with dwindling roughness but also owing to the upside-down Ga dish in the initial stage which reduces its contact with the substrate.When the tip of nanocone is relatively sharp,as the roughness decreases,gallium droplet presents dewetting state and Wenzel state on the modified surface respectively,while Wenzel state appears on the other three kinds of surfaces.When the cone apex angle is larger,the isotropic effect of the substrate on the initial liquid film gradually disappears,and the adhesion performance between the droplet and the substrate also becomes lower,the droplet is easier to roll on the surface to select a suitable position to achieve the lowest energy state.Top morphology of nanopillars determines the wetting transition of liquid Ga by not only lessening the interaction between liquid and solid but also changing the movement pattern of liquid at the beginning.This work improves our understanding of wetting transitions and is expected to better facilitate the superhydrophobic development surfaces,such as self-cleaning nano-materials and the design of stretchable electronic devices.