Computational Simulation Studies Of Wettability And Dynamic Behaviors Of Nanodroplet On Slippery Lubricant-Infused Porous Surface

Slippery Lubricant-infused Porous Surfaces（SLIPS）are an advanced coating inspired by the predation of Nepenthes.Due to the excellent performance in selfcleaning,anti-icing,anti-biological corrosion,etc.,SLIPS have shown great application prospects in the fields of surface protection,condensation heat transfer,droplet transport,etc.,and are gaining remarkable attention.Although they all rely on the micronano rough structure to lock the lubricating oil to form a smooth surface,SLIPS can present varying droplet morphologies such as cloaking,wetting ridge,pinning,etc.Therefore,it is necessary to explore the factors affecting the droplet wetting state,which is particularly critical to improve the functionality and stability of SLIPS.Based on this,molecular dynamics were applied in this paper to study the effects of interfacial tension and lubricate thickness on the thermodynamic configuration of droplets.Moreover,the dynamic motion characteristics of droplets on SLIPS were also explored.Finally,the thermodynamic mechanisms of reversible transformation for droplets from Wenzel state to Cassie state were revealed.The main research contents of this paper are as follows:（1）The effect of the oil-droplet interface interaction energy ε_ow on the thermodynamic configuration of the droplet was studied.ε_ow affects the spreading coefficient S_ow（a）of droplets on the lubricant.When S_ow（a）<0,the droplet forms a non-cloaking state,while S_ow（a）>0,the droplet tends to form a cloaking state.（2）The effect of the oil-substrate interface interaction energy ε_os on the thermodynamic configuration and hydrophobic properties of SLIPS was studied.ε_os affects the contact angle θ_os（a）of lubricant in gas environment and the contact angleθ_os（w）in water environment.According to the relationship between θ_os（a）,θ_os（w）and the critical contact angle θ_c,the surface wettability of droplets or lubricant on solid surface is predicted.The solid-oil-air interface and the solid-oil-liquid interface can exhibit "encapsulated","emerged",and "impaled"（or "dry"）interface states.As for hydrophobic properties of SLIPS,ε_os shows weak influence.The apparent contact angle,shedding velocity and dynamic contact angle hysteresis change slightly with ε_os（3）The effect of the number of lubricant molecules N_m on the thermodynamic configuration and hydrophobic properties of SLIPS was studied.As N_m decreases,the thermodynamic configuration of the droplets on SLIPS shifts from "ideal SLIPS" to"emerged-encapsulated" and then to "emerged-emerged".Under the combined affect of N_m,sos and ε_ow,the thermodynamic phase diagrams of SLIPS system are drawn,and eight stable thermodynamic states can be obtained.Special configurations such as"slippery Cassie" and "slippery Wenzel" can be produced under the combination of specific thickness and interfacial tension.In terms of hydrophobicity,since the height of the wetting ridge increases with the thickness of lubricant,the size effect of the wetting ridge dramatically affects the apparent contact angle and shedding velocity of the droplet.The increase of N_m leads to the decrease of the apparent contact angle and shedding velocity.And the dynamic contact angle hysteresis increases with the thickness of the lubricant."Slippery Cassie" and superhydrophobic surfaces exhibit higher apparent contact angles and lower contact angle hysteresis.（4）The internal mechanism of droplet Wenzel-to-Cassie transition was studied.The state transition process of droplets on different superhydrophobic and "slippery Cassie" surfaces shows that the droplet cannot achieve the transition from Wenzel to Cassie state without adding lubricant,while the pinning state performs an extremely high energy barrier（1425 kcal/mol）.But the surface infused with lubricant can realize this conversion,and rolls forward smoothly after the transition,when the adsorption energy is only 333 kcal/mol.The difference indicates that lubricant can reduce the energy barrier of state transition and promote the occurrence of state transition.Based on the above research results,it is indicated that a stable and ideal configuration with good hydrophobicity can be obtained by rationally designing the surface wettability and lubricant thickness.In addition,the intrinsic mechanism of the reversible transition from Cassie to Wenzel state is revealed from the atomic/molecular aspects,providing profound guidance for the application of SLIPS.