Nanodroplet wetting and electrowetting behavior on liquid-infused surfaces: A molecular simulation study

Due to the block of the lubricant on liquid-infused surfaces (LISs), the molecular processes near the liquid-liquid interface and the shape of droplet under electrowetting actuation are still difficult to observe with simple optical methods. Molecular dynamics (MD) simulation provides a visual method for observing the nanodroplet morphology on the LISs. In this paper, the impacts of the substrate char-acteristics and the lubricant thickness on the cloaking state, the wetting state and the diffusion diameter of nanodroplet were studied by the MD simulation. It is found the coating film of the nanodroplet increases with the lubricant layer thickness on the LISs of silicon nanopillar substrate, while the nan-odroplet is always in the not cloaking state on the LISs of copper nanopillar substrate, even with a very thick lubricant layer. Subsequently, the electrowetting behaviors of the nanodroplet under the external electric field in the parallel and perpendicular directions to the surface were studied. The results indicate the droplet changes from the Cassie state to the partial Wenzel state under the influence of the external electric field on the LISs of both substrates. Meanwhile, the coating film thickness increases with the elec-tric field intensity, the nanodroplet changes from a not cloaking state to a cloaking state on the LISs of copper substrate, while still is in a not cloaking state on the LISs of silicon substrate. Compared to the LISs of the silicon substrate, the electro-stretching is easier to happen on the LISs of the copper substrate. Finally, the impact of lubricant layer thickness on the nanodroplet electrowetting was studied on both copper and silicon substrates. It is found the reduction of nanodroplet apparent contact angle increases with the lubricant layer thickness under an external parallel electric field, while it has a maximum on the moderate lubricant thickness under an external electric field in a perpendicular upward or downward direction. Our results indicate the droplet deformation on LISs under the external electric field is con-tributed to the competition of the electrowetting, the electro-stretching, and the pinning effect.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

In this study, the effects of substrate characteristics and lubricant thickness on the cloaking state, wetting state, and diffusion diameter of nanodroplets were investigated through molecular dynamics simulation. The results showed that the presence of a thicker lubricant layer on silicon nanopillar substrates increased the coating film of nanodroplets, while nanodroplets on copper nanopillar substrates remained in a non-cloaking state, even with a thicker lubricant layer. Additionally, the electrowetting behaviors of nanodroplets under external electric fields were examined, and the results demonstrated a transition from Cassie state to partial Wenzel state on both substrates. Moreover, the impact of lubricant layer thickness on nanodroplet electrowetting was analyzed, revealing that the reduction of apparent contact angle increased with lubricant layer thickness under a parallel electric field, and had a maximum value for a moderate lubricant thickness under a perpendicular upward or downward electric field.