Tunable adhesion and slip on a bio-mimetic sticky soft surface

Defect by design: Harnessing the "petal effect" for advanced hydrophobic surface applications

HYPOTHESIS

In the interfacial wetting boundary, the superhydrophobic surface is often damaged, and the anisotropic wettability of its surface has attracted many researchers' attention. The "petal effect" surface has typical anisotropic wettability. We predict that under the dual conditions of structural defects and high impact velocity, the "petal effect" becomes more adhesive on the surface.

EXPERIMENTS

This study refers to the droplet state on rose petals, structural defects were constructed on the superhydrophobic surface. This paper studies the influence of macro-structural defects on the wettability change from natural to bionic "lotus effect" to "petal effect" in both static and dynamic angles.

FINDINGS

Macro defects significantly change the static contact angle of the superhydrophobic surface. The higher the impact velocity of the droplet, the higher the energy dissipation of the "petal effect" surface (DSHS), which improves the adhesion of the surface to the droplet and prolongs the contact time. It is found that the defect structure and high impact velocity will directly affect the deposition and desorption of droplets on the superhydrophobic surface, and they are both essential. This wetting dynamic law is very likely to be helpful in the quantitative design of defect structure scale for dynamic desorption of droplets on superhydrophobic surfaces.

Can Microcavitated Slippery Surfaces Outperform Micropillared and Untextured?

Surface features' morphology is crucial in designing lubricant-infused slippery surfaces (LIS). Microcavities were hypothesized to provide lower physical pinning, reduced droplet normal adhesion, and superior lubricant retention as compared to micropillars and untextured surfaces. Micropillars and microcavities (h = 10 ± 3 μm, d = 8 ± 1 μm, p = 17 ± 3 μm, rw = 1.4 ± 0.2) were replicated on polydimethylsiloxane from Lotus leaf and were coated with 1000 cSt silicone oil films (530 nm-27 μm thick). Water wetting, water-oil thermodynamic stability, droplet's normal adhesion and oil shear drainage properties were investigated to evaluate the relative performance of microcavitated, micropillared and untextured LIS. For ≤7 μm thick oil films, cavitated and untextured LIS displayed superior slippery properties than micropillared LIS (16 ± 1°, 7 ± 1°, 30 ± 4° slide-off angles respectively). Also, normal adhesion is of the order: cavities < untextured < pillars, and smaller than their dry counterparts. Furthermore, the oil retention efficiency under the action of centrifugal forces and continuous shear flow of water is of the order: pillars > cavities > untextured. Thus, it can be concluded that microcavitated LIS can outperform micropillared and untextured LIS.

Bio-inspired microfluidics: A review

Biomicrofluidics, a subdomain of microfluidics, has been inspired by several ideas from nature. However, while the basic inspiration for the same may be drawn from the living world, the translation of all relevant essential functionalities to an artificially engineered framework does not remain trivial. Here, we review the recent progress in bio-inspired microfluidic systems via harnessing the integration of experimental and simulation tools delving into the interface of engineering and biology. Development of "on-chip" technologies as well as their multifarious applications is subsequently discussed, accompanying the relevant advancements in materials and fabrication technology. Pointers toward new directions in research, including an amalgamated fusion of data-driven modeling (such as artificial intelligence and machine learning) and physics-based paradigm, to come up with a human physiological replica on a synthetic bio-chip with due accounting of personalized features, are suggested. These are likely to facilitate physiologically replicating disease modeling on an artificially engineered biochip as well as advance drug development and screening in an expedited route with the minimization of animal and human trials.

Droplet Impact Dynamics on Biomimetic Replica of Yellow Rose Petals: Rebound to Micropinning Transition

Rose petals exhibit a phenomenal wetting property of being sticky and superhydrophobic simultaneously. A recent study has shown that for short timescales, associated with drop impact phenomenon, lotus leaf and rose petal replicas exhibit similar wettability, thereby highlighting the difference between long and short time wettability. Also, short time wetting on rose petals of different colors remains completely unaddressed, as almost all existing study on wetting of rose petals have been performed with the classical red rose (Rosa chinensis). In this paper, we compare the drop impact studies on replicas of a yellow rose petal, with those on extensively studied red rose petal replicas and the lotus leaf over a wide range of Weber number (We), by varying the height of fall (h) from 10 to 375 mm. Our results reveal that over the replica of a yellow rose petal, the initial impact outcome varies from complete rebound to micro pinning and eventually complete pinning depending on the kinetic energy of the impacting drop, in contrast to that on red rose petal replica on which the droplet always pinned. Based on experimental finding, we present a comprehensive regime phase map of the post impact behavior of the drop on different surfaces as a function of impact height. We also present a simple scaling analysis to understand the combined effect of pattern height and periodicity on the critical h corresponding to wetting regime transition. Additionally, variation of maximum spreading diameter and spreading time with the h for the different surfaces is also discussed. The results highlight that the initial impact dynamics of a water drop over a topographically patterned substrate is a strong function of the topographical parameters and can be very different from the equilibrium wetting state.

Non-wetting Liquid-Infused Slippery Paper

Liquid-infused slippery surfaces have replaced structural superhydrophobic surfaces in a plethora of emerging applications, hallmarked by their favorable self-healing and liquid-repelling characteristics. Their ease of fabrication on different types of materials and increasing demand in various industrial applications have triggered research interests targeted toward developing an environmental-friendly, flexible, and frugal substrate as the underlying structural and functional backbone. Although many expensive polymers such as polytetrafluoroethylene have so far been used for their fabrication, these are constrained by their compromised flexibility and non-ecofriendliness due to the use of fluorine. Here, we explore the development and deployment of a biodegradable, recyclable, flexible, and an economically viable material in the form of a paper matrix for fabricating liquid-infused slippery interfaces for prolonged usage. We show by controlled experiments that a simple silanization followed by an oil infusion protocol imparts an inherent slipperiness (low contact angle hysteresis and low tilting angle for sliding) to the droplet motion on the paper substrate and provides favorable anti-icing characteristics, albeit keeping the paper microstructures unaltered. This ensures concomitant hydrophobicity, water adhesion, and capillarity for low surface tension fluids, such as mustard oil, with an implicit role played by the paper pore size distribution toward retaining a stable layer of the infused oil. With demonstrated supreme anti-icing characteristics, these results open up new possibilities of realizing high-throughput paper-based substrates for a wide variety of applications ranging from biomedical unit operations to droplet-based digital microfluidics.

Fully Repairable Slippery Organogel Surfaces with Reconfigurable Paraffin-Based Framework for Universal Antiadhesion

Constructing a slippery lubricant-infused surface (SLIS) whose internal microstructure and surface properties can be fully repaired helps to improve its property stability and extend technological implications but has presented a huge challenge. A class of fully repairable slippery organogel surfaces (SOSs), which uses microstructured paraffin as reconfigurable supporting structure and silicone oil as lubricant dispersion medium, is reported here. Attributed to nearly 90 wt % of silicone oil stored in the slippery organogel system and good compatibility with the paraffin-based framework, SOSs combine continuous lubricity and reliable lubricant storage stability. Furthermore, the thermally sensitive paraffin-based framework can quickly switch between solid supporting structure and liquid solution according to the ambient temperature, thereby achieving rapid regeneration of microstructure. This unique system consisting of reconfigurable framework and flowable lubricant derives two types of repairs aimed at varying degrees of damage. Significantly, the easy-to-prepare SOS, on the other hand, allows the adoption of various substrate surfaces for different purposes to form an antiadhesion coating and exhibits excellent antistain, antialgae, and anti-icing performance, thus greatly improving the flexibility of such materials in practical applications.

High Temperature Durability of Oleoplaned Slippery Copper Surfaces

Slippery surfaces, inspired by the functionality of trapping interfaces of specialized leaves of pitcher plants, have been widely used in self-cleaning, anti-icing, antifrost, and self-healing surfaces. They can be fabricated on metallic surfaces as well, presenting a more durable and low-maintenance anticorrosive surface on metals. However, the lack of studies on the durability of these slippery surfaces at high temperature prohibits their practical deployment in real industrial applications where thermal effects are critical and high temperature conditions are inevitable. We present here a unique fabrication technique of a copper-based oleoplaned slippery surface that has been tested for high temperature durability under repeated thermal cycles. Their slipperiness at high temperatures has also been tested in the absence of the Leidenfrost effect. Our findings suggest that these new substrates can be used for fabricating low maintenance surfaces for high temperature applications or even where the surface undergoes repeated thermal cycles like heat exchanger pipes, utensils, engine casings, and outdoor metallic structures.