Droplet Morphology and Mobility on Lubricant-Impregnated Surfaces: A Molecular Dynamics Study

Abstract

Slippery liquid-infused porous surfaces (SLIPS) are gaining remarkable attention and have advanced performance in many fields. Although all SLIPS are related to lubricant-impregnation within nano/microstructures on a surface, they differ in many aspects, such as the morphology of droplets, the state of cloaking, the wetting edge, and the lubricant thickness. Requirements of the droplet morphology on SLIPS might change according to a specific application. A molecular-dynamics-based numerical model that can correctly simulate SLIPS is developed and is validated by comparing against the theoretical predictions for all possible stable states for a given droplet, lubricant, and solid surface. On the basis of this model, a detailed analysis of the equilibrium states is conducted. In particular, we discover that the four possible stable states on SLIPS predicted by theoretical studies can be extended to eight states by considering the effects of lubricant thickness and surface geometry in addition to the interfacial tension and surface wettability. These findings could be used to determine the conditions under which a thermodynamically stable state exists on SLIPS. The dynamic behavior of a nanodroplet on SLIPS is also studied, which provides insight into how a proper increase in the lubricant thickness might increase the sliding velocity. The above findings and developed model are expected to provide significant guidelines for designing SLIPS.

Wetting Dynamics on Solvophilic, Soft, Porous, and Responsive Surfaces

“…The detailed quantifications proposed in this study, for example, the equilibrium radius of the drop (which signifies the extent of wetting of the “surface” by the “liquid” in our hypothesized LIS), the time-dependent spreading and wicking behaviors (which signify how fast the fabrication of the LIS occurs), and the time-dependent response of the polymer molecules (which signify the “responsiveness” of the “responsive” surface), enable a nanoscopic quantification of designing an LIS or a coating. Earlier MD simulation studies have probed the dynamics of a liquid drop interacting with a pre-existing LIS; , however, these studies have not considered the nanoscale dynamics associated with the formation of a soft and responsive LIS, as has been considered here. Additionally, there are several other facets of this hypothesized LIS or coating that are worth (re)­emphasizing.…”

“…Studying the behavior of liquid drops on the surfaces of various complexities has been critical in designing and fabricating a wide variety of surfaces that demonstrate different wettabilities − which could be employed in various energy − and biomedical , applications as well as applications such as heat-transfer enhancement, − fabrication of anti-biofouling, − anti-corrosion, − anti-icing, − anti-bacterial, and anti-thrombotic surfaces, enhanced oil–water separation, , and many more. Advancement of computational techniques have enabled a nanoscale exploration of the drop–substrate interactions shedding light on fundamental mechanisms dictating these different interactions as well as enabling designing of surfaces with novel properties. − …”

Inner surface of Nepenthes slippery zone: ratchet effect of lunate cells causes anisotropic superhydrophobicity

“…According to a molecular dynamic investigation of droplet morphology on lubricant-impregnated surface [35], when an ultrapure-water droplet is sliding on the Nepenthes slippery zone towards pitcher up/bottom, the ultrapure-water droplet neither completely infiltrates into nor floats on the micro-nano scaled structures of the slippery zone. Instead, the ultrapure-water droplet partly infiltrates into the isotropic wax coverings and anisotropic lunate cells.…”

Anisotropic Ice Adhesion of Micro-Nano-Structured Metal Surface by a Femtosecond Laser

“…Ice coating on the surface of a metal has been a lot of trouble to many fields such as aviation, ship, and electricity. − Currently, mechanical or chemical deicing methods, which are commonly used, are often inefficient, energy-consuming, costly, or even harmful to the environment. In order to solve these problems, metal surfaces with passive icephobic characteristics of micro-nano structures have been widely studied in recent years. − An icephobic surface with hydrophobic wettability is considered to be one of the most attractive strategies for preparing icephobic materials. − For the low ice adhesion strength on the hydrophobic micro-nano-structured surface, the generally accepted explanation is that the water droplets freeze in the Cassie–Baxter state, and the air is retained under the droplets, which act as a stress concentration effect and reduce the actual contact area between the ice and the solid surface, and thus the ice adhesion strength is reduced . In recent years, researchers have found a correlation between ice adhesion strength and surface morphology and wettability from experiments and theoretical derivations. − Furthermore, a recent research study shows that such correlation in the polymer-based surface does not exist anymore when the ice adhesion is low, in particular, lower than 60 kPa .…”