Re‐Entrant Microstructures for Robust Liquid Repellent Surfaces

Abstract

nano-and microstructures on these surfaces. The anisotropic dewetting behavior of rice leaves and the self-cleaning ability of lotus leaves are two typical examples related to hydrophobicity. [3] In the past decades, a large number of biomimetic functional materials with controlled wettability has been explored for practical applications such as self-cleaning windows, [4] drag reduction, [5] surface treatment of medical devices, [6] waterproof clothes and textiles, [6] anti-corrosion, [7] anti-freezing, [8] heat transfer enhancement, [9] micro/ nanofluidic devices, [10] anti-biofouling surfaces, [11] and transparent self-cleaning surfaces for photovoltaics. [12] On the other hand, hydrophilic or water-loving surfaces are desired in some other applications, such as microfluidics, anti-fogging, liquid separation, or heat transfer. [13] Therefore, many efforts have been conducted to make controllable wetting structures. However, the stability of hydrophobic surface structures remains a significant difficulty, as they may become wet and lose the slip effect due to pollution, [14] air diffusion, [15] external pressure, [16] gravity, [17] vibrations, [16a,18] evaporation, [19] and impacts. [7b] Recently, it has been experimentally and theoretically discovered that the tip shape of pillars is a critical factor contributing to hydrophobic stability. Pillars with mushroom-like structures tend to improve this stability better than those with concave, spherical, or flat tips. [20] The underlying reason is that the net force directed upward competes with the internal pressure so that surfaces do not get wet, thanks to the tip formed on the top of the mushroom-like structure. [21] Oleophilic surfaces can attract oils or organic liquids. Oleophobicity refers to the property of surfaces with low affinity to oils. Oleophilic, oil-loving, surfaces have been known to have disastrous effects, for example, corrosion of oil pipelines, less self-cleaning ability of leaves, and swelling of elastomeric seals. [22] Therefore, we need omniphobic surface which could repel both polar and non-polar liquids. [23] Realizing the significance of superoleophobic surfaces, scientists developed surfaces with unusual patterns, such as re-entrant or negative-slope structures, to achieve a stable liquid-repellent state. [21a] The reentrant structure was first inspired by springtail, Figure 1. [24] The skin of this animal possesses an oil-repellent ability that protects its skin-breathing body against suffocation by complete Superhydrophobic surfaces have many interesting applications because of their self-cleaning, waterproof, anti-biofouling, anti-corrosion, and lowadhesion properties. Accordingly, numerous surfaces with hierarchical micro/ nanostructures are designed and engineered to achieve superhydrophobicity. However, these surfaces have two major problems. First, they lose superhydrophobic properties over time, primarily because of environmental conditions such as vibration, external pressure, evaporation, and pollution. Second, most su...

Anisotropic wettability manipulation via capturing architected liquid bridge shapes

“…Amid the plethora of micro/nanofabrication methods for fabricating re-entrant structures, 43 our approach stands out by virtue of its incorporation of the benets of prior template molding methods 43 and its unique strengths in the realm of morphological and compositional versatility (a comparison of our fabrication method with others is presented in Table S3, † which was reprinted from ref. 43 with minor revisions).…”

“…Amid the plethora of micro/nanofabrication methods for fabricating re-entrant structures, 43 our approach stands out by virtue of its incorporation of the benets of prior template molding methods 43 and its unique strengths in the realm of morphological and compositional versatility (a comparison of our fabrication method with others is presented in Table S3, † which was reprinted from ref. 43 with minor revisions). We contend that the myriad material and morphological combinations achievable via our technique harbor tremendous potential to serve as a platform for comprehensive surface engineering such as self-cleaning and smart liquid manipulation, representing a valuable alternative to conventional methodologies.…”

Bio-inspired Nanoarchitectonics of Re-entrant Geometries toward Transparent Omniphobic Surfaces

“…By pinning the three-phase contact line, omniphobic surfaces have a contact angle larger than 90° for liquids with a wide spectrum of surface tension. , Springtail, an insect dwelling in the soil, provides us with an impressive example of such omniphobic surfaces. , The skin of springtails consists of nanoscopic granules and interconnecting ridges, which retain their skin dry and through which they breathe. , Inspired by springtails, various omniphobic surfaces have been developed to realize their potential applications ranging from corrosion-free and self-cleaning materials to medical devices and gas-exchange membranes. , The essential feature of omniphobic surfaces rely on the so-called re-entrant or convex curvature topography. , …”

Drug Diffusion inside the Tapered Hydrogel Microneedle: A Numerical Study

“…Because of their small size, they do not reach the network of the nerves, and consequently, they cause no pain during insertion [4][5][6][7][8]. Developing local drug administration is of great interest to many fields [9][10][11][12][13][14]. Drug transport modeling can provide significant insights into MN drug delivery and promote this technology to achieve more desirable results [15].…”